1 files changed, 2510 insertions, 0 deletions
diff --git a/block/qcow2-cluster.c b/block/qcow2-cluster.c
new file mode 100644
index 000000000..21884a1ab
--- /dev/null
+++ b/block/qcow2-cluster.c
@@ -0,0 +1,2510 @@
+/*
+ * Block driver for the QCOW version 2 format
+ *
+ * Copyright (c) 2004-2006 Fabrice Bellard
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL
+ * THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+
+#include "qemu/osdep.h"
+#include <zlib.h>
+
+#include "qapi/error.h"
+#include "qcow2.h"
+#include "qemu/bswap.h"
+#include "trace.h"
+
+int qcow2_shrink_l1_table(BlockDriverState *bs, uint64_t exact_size)
+{
+    BDRVQcow2State *s = bs->opaque;
+    int new_l1_size, i, ret;
+
+    if (exact_size >= s->l1_size) {
+        return 0;
+    }
+
+    new_l1_size = exact_size;
+
+#ifdef DEBUG_ALLOC2
+    fprintf(stderr, "shrink l1_table from %d to %d\n", s->l1_size, new_l1_size);
+#endif
+
+    BLKDBG_EVENT(bs->file, BLKDBG_L1_SHRINK_WRITE_TABLE);
+    ret = bdrv_pwrite_zeroes(bs->file, s->l1_table_offset +
+                                       new_l1_size * L1E_SIZE,
+                             (s->l1_size - new_l1_size) * L1E_SIZE, 0);
+    if (ret < 0) {
+        goto fail;
+    }
+
+    ret = bdrv_flush(bs->file->bs);
+    if (ret < 0) {
+        goto fail;
+    }
+
+    BLKDBG_EVENT(bs->file, BLKDBG_L1_SHRINK_FREE_L2_CLUSTERS);
+    for (i = s->l1_size - 1; i > new_l1_size - 1; i--) {
+        if ((s->l1_table[i] & L1E_OFFSET_MASK) == 0) {
+            continue;
+        }
+        qcow2_free_clusters(bs, s->l1_table[i] & L1E_OFFSET_MASK,
+                            s->cluster_size, QCOW2_DISCARD_ALWAYS);
+        s->l1_table[i] = 0;
+    }
+    return 0;
+
+fail:
+    /*
+     * If the write in the l1_table failed the image may contain a partially
+     * overwritten l1_table. In this case it would be better to clear the
+     * l1_table in memory to avoid possible image corruption.
+     */
+    memset(s->l1_table + new_l1_size, 0,
+           (s->l1_size - new_l1_size) * L1E_SIZE);
+    return ret;
+}
+
+int qcow2_grow_l1_table(BlockDriverState *bs, uint64_t min_size,
+                        bool exact_size)
+{
+    BDRVQcow2State *s = bs->opaque;
+    int new_l1_size2, ret, i;
+    uint64_t *new_l1_table;
+    int64_t old_l1_table_offset, old_l1_size;
+    int64_t new_l1_table_offset, new_l1_size;
+    uint8_t data[12];
+
+    if (min_size <= s->l1_size)
+        return 0;
+
+    /* Do a sanity check on min_size before trying to calculate new_l1_size
+     * (this prevents overflows during the while loop for the calculation of
+     * new_l1_size) */
+    if (min_size > INT_MAX / L1E_SIZE) {
+        return -EFBIG;
+    }
+
+    if (exact_size) {
+        new_l1_size = min_size;
+    } else {
+        /* Bump size up to reduce the number of times we have to grow */
+        new_l1_size = s->l1_size;
+        if (new_l1_size == 0) {
+            new_l1_size = 1;
+        }
+        while (min_size > new_l1_size) {
+            new_l1_size = DIV_ROUND_UP(new_l1_size * 3, 2);
+        }
+    }
+
+    QEMU_BUILD_BUG_ON(QCOW_MAX_L1_SIZE > INT_MAX);
+    if (new_l1_size > QCOW_MAX_L1_SIZE / L1E_SIZE) {
+        return -EFBIG;
+    }
+
+#ifdef DEBUG_ALLOC2
+    fprintf(stderr, "grow l1_table from %d to %" PRId64 "\n",
+            s->l1_size, new_l1_size);
+#endif
+
+    new_l1_size2 = L1E_SIZE * new_l1_size;
+    new_l1_table = qemu_try_blockalign(bs->file->bs, new_l1_size2);
+    if (new_l1_table == NULL) {
+        return -ENOMEM;
+    }
+    memset(new_l1_table, 0, new_l1_size2);
+
+    if (s->l1_size) {
+        memcpy(new_l1_table, s->l1_table, s->l1_size * L1E_SIZE);
+    }
+
+    /* write new table (align to cluster) */
+    BLKDBG_EVENT(bs->file, BLKDBG_L1_GROW_ALLOC_TABLE);
+    new_l1_table_offset = qcow2_alloc_clusters(bs, new_l1_size2);
+    if (new_l1_table_offset < 0) {
+        qemu_vfree(new_l1_table);
+        return new_l1_table_offset;
+    }
+
+    ret = qcow2_cache_flush(bs, s->refcount_block_cache);
+    if (ret < 0) {
+        goto fail;
+    }
+
+    /* the L1 position has not yet been updated, so these clusters must
+     * indeed be completely free */
+    ret = qcow2_pre_write_overlap_check(bs, 0, new_l1_table_offset,
+                                        new_l1_size2, false);
+    if (ret < 0) {
+        goto fail;
+    }
+
+    BLKDBG_EVENT(bs->file, BLKDBG_L1_GROW_WRITE_TABLE);
+    for(i = 0; i < s->l1_size; i++)
+        new_l1_table[i] = cpu_to_be64(new_l1_table[i]);
+    ret = bdrv_pwrite_sync(bs->file, new_l1_table_offset,
+                           new_l1_table, new_l1_size2);
+    if (ret < 0)
+        goto fail;
+    for(i = 0; i < s->l1_size; i++)
+        new_l1_table[i] = be64_to_cpu(new_l1_table[i]);
+
+    /* set new table */
+    BLKDBG_EVENT(bs->file, BLKDBG_L1_GROW_ACTIVATE_TABLE);
+    stl_be_p(data, new_l1_size);
+    stq_be_p(data + 4, new_l1_table_offset);
+    ret = bdrv_pwrite_sync(bs->file, offsetof(QCowHeader, l1_size),
+                           data, sizeof(data));
+    if (ret < 0) {
+        goto fail;
+    }
+    qemu_vfree(s->l1_table);
+    old_l1_table_offset = s->l1_table_offset;
+    s->l1_table_offset = new_l1_table_offset;
+    s->l1_table = new_l1_table;
+    old_l1_size = s->l1_size;
+    s->l1_size = new_l1_size;
+    qcow2_free_clusters(bs, old_l1_table_offset, old_l1_size * L1E_SIZE,
+                        QCOW2_DISCARD_OTHER);
+    return 0;
+ fail:
+    qemu_vfree(new_l1_table);
+    qcow2_free_clusters(bs, new_l1_table_offset, new_l1_size2,
+                        QCOW2_DISCARD_OTHER);
+    return ret;
+}
+
+/*
+ * l2_load
+ *
+ * @bs: The BlockDriverState
+ * @offset: A guest offset, used to calculate what slice of the L2
+ *          table to load.
+ * @l2_offset: Offset to the L2 table in the image file.
+ * @l2_slice: Location to store the pointer to the L2 slice.
+ *
+ * Loads a L2 slice into memory (L2 slices are the parts of L2 tables
+ * that are loaded by the qcow2 cache). If the slice is in the cache,
+ * the cache is used; otherwise the L2 slice is loaded from the image
+ * file.
+ */
+static int l2_load(BlockDriverState *bs, uint64_t offset,
+                   uint64_t l2_offset, uint64_t **l2_slice)
+{
+    BDRVQcow2State *s = bs->opaque;
+    int start_of_slice = l2_entry_size(s) *
+        (offset_to_l2_index(s, offset) - offset_to_l2_slice_index(s, offset));
+
+    return qcow2_cache_get(bs, s->l2_table_cache, l2_offset + start_of_slice,
+                           (void **)l2_slice);
+}
+
+/*
+ * Writes an L1 entry to disk (note that depending on the alignment
+ * requirements this function may write more that just one entry in
+ * order to prevent bdrv_pwrite from performing a read-modify-write)
+ */
+int qcow2_write_l1_entry(BlockDriverState *bs, int l1_index)
+{
+    BDRVQcow2State *s = bs->opaque;
+    int l1_start_index;
+    int i, ret;
+    int bufsize = MAX(L1E_SIZE,
+                      MIN(bs->file->bs->bl.request_alignment, s->cluster_size));
+    int nentries = bufsize / L1E_SIZE;
+    g_autofree uint64_t *buf = g_try_new0(uint64_t, nentries);
+
+    if (buf == NULL) {
+        return -ENOMEM;
+    }
+
+    l1_start_index = QEMU_ALIGN_DOWN(l1_index, nentries);
+    for (i = 0; i < MIN(nentries, s->l1_size - l1_start_index); i++) {
+        buf[i] = cpu_to_be64(s->l1_table[l1_start_index + i]);
+    }
+
+    ret = qcow2_pre_write_overlap_check(bs, QCOW2_OL_ACTIVE_L1,
+            s->l1_table_offset + L1E_SIZE * l1_start_index, bufsize, false);
+    if (ret < 0) {
+        return ret;
+    }
+
+    BLKDBG_EVENT(bs->file, BLKDBG_L1_UPDATE);
+    ret = bdrv_pwrite_sync(bs->file,
+                           s->l1_table_offset + L1E_SIZE * l1_start_index,
+                           buf, bufsize);
+    if (ret < 0) {
+        return ret;
+    }
+
+    return 0;
+}
+
+/*
+ * l2_allocate
+ *
+ * Allocate a new l2 entry in the file. If l1_index points to an already
+ * used entry in the L2 table (i.e. we are doing a copy on write for the L2
+ * table) copy the contents of the old L2 table into the newly allocated one.
+ * Otherwise the new table is initialized with zeros.
+ *
+ */
+
+static int l2_allocate(BlockDriverState *bs, int l1_index)
+{
+    BDRVQcow2State *s = bs->opaque;
+    uint64_t old_l2_offset;
+    uint64_t *l2_slice = NULL;
+    unsigned slice, slice_size2, n_slices;
+    int64_t l2_offset;
+    int ret;
+
+    old_l2_offset = s->l1_table[l1_index];
+
+    trace_qcow2_l2_allocate(bs, l1_index);
+
+    /* allocate a new l2 entry */
+
+    l2_offset = qcow2_alloc_clusters(bs, s->l2_size * l2_entry_size(s));
+    if (l2_offset < 0) {
+        ret = l2_offset;
+        goto fail;
+    }
+
+    /* The offset must fit in the offset field of the L1 table entry */
+    assert((l2_offset & L1E_OFFSET_MASK) == l2_offset);
+
+    /* If we're allocating the table at offset 0 then something is wrong */
+    if (l2_offset == 0) {
+        qcow2_signal_corruption(bs, true, -1, -1, "Preventing invalid "
+                                "allocation of L2 table at offset 0");
+        ret = -EIO;
+        goto fail;
+    }
+
+    ret = qcow2_cache_flush(bs, s->refcount_block_cache);
+    if (ret < 0) {
+        goto fail;
+    }
+
+    /* allocate a new entry in the l2 cache */
+
+    slice_size2 = s->l2_slice_size * l2_entry_size(s);
+    n_slices = s->cluster_size / slice_size2;
+
+    trace_qcow2_l2_allocate_get_empty(bs, l1_index);
+    for (slice = 0; slice < n_slices; slice++) {
+        ret = qcow2_cache_get_empty(bs, s->l2_table_cache,
+                                    l2_offset + slice * slice_size2,
+                                    (void **) &l2_slice);
+        if (ret < 0) {
+            goto fail;
+        }
+
+        if ((old_l2_offset & L1E_OFFSET_MASK) == 0) {
+            /* if there was no old l2 table, clear the new slice */
+            memset(l2_slice, 0, slice_size2);
+        } else {
+            uint64_t *old_slice;
+            uint64_t old_l2_slice_offset =
+                (old_l2_offset & L1E_OFFSET_MASK) + slice * slice_size2;
+
+            /* if there was an old l2 table, read a slice from the disk */
+            BLKDBG_EVENT(bs->file, BLKDBG_L2_ALLOC_COW_READ);
+            ret = qcow2_cache_get(bs, s->l2_table_cache, old_l2_slice_offset,
+                                  (void **) &old_slice);
+            if (ret < 0) {
+                goto fail;
+            }
+
+            memcpy(l2_slice, old_slice, slice_size2);
+
+            qcow2_cache_put(s->l2_table_cache, (void **) &old_slice);
+        }
+
+        /* write the l2 slice to the file */
+        BLKDBG_EVENT(bs->file, BLKDBG_L2_ALLOC_WRITE);
+
+        trace_qcow2_l2_allocate_write_l2(bs, l1_index);
+        qcow2_cache_entry_mark_dirty(s->l2_table_cache, l2_slice);
+        qcow2_cache_put(s->l2_table_cache, (void **) &l2_slice);
+    }
+
+    ret = qcow2_cache_flush(bs, s->l2_table_cache);
+    if (ret < 0) {
+        goto fail;
+    }
+
+    /* update the L1 entry */
+    trace_qcow2_l2_allocate_write_l1(bs, l1_index);
+    s->l1_table[l1_index] = l2_offset | QCOW_OFLAG_COPIED;
+    ret = qcow2_write_l1_entry(bs, l1_index);
+    if (ret < 0) {
+        goto fail;
+    }
+
+    trace_qcow2_l2_allocate_done(bs, l1_index, 0);
+    return 0;
+
+fail:
+    trace_qcow2_l2_allocate_done(bs, l1_index, ret);
+    if (l2_slice != NULL) {
+        qcow2_cache_put(s->l2_table_cache, (void **) &l2_slice);
+    }
+    s->l1_table[l1_index] = old_l2_offset;
+    if (l2_offset > 0) {
+        qcow2_free_clusters(bs, l2_offset, s->l2_size * l2_entry_size(s),
+                            QCOW2_DISCARD_ALWAYS);
+    }
+    return ret;
+}
+
+/*
+ * For a given L2 entry, count the number of contiguous subclusters of
+ * the same type starting from @sc_from. Compressed clusters are
+ * treated as if they were divided into subclusters of size
+ * s->subcluster_size.
+ *
+ * Return the number of contiguous subclusters and set @type to the
+ * subcluster type.
+ *
+ * If the L2 entry is invalid return -errno and set @type to
+ * QCOW2_SUBCLUSTER_INVALID.
+ */
+static int qcow2_get_subcluster_range_type(BlockDriverState *bs,
+                                           uint64_t l2_entry,
+                                           uint64_t l2_bitmap,
+                                           unsigned sc_from,
+                                           QCow2SubclusterType *type)
+{
+    BDRVQcow2State *s = bs->opaque;
+    uint32_t val;
+
+    *type = qcow2_get_subcluster_type(bs, l2_entry, l2_bitmap, sc_from);
+
+    if (*type == QCOW2_SUBCLUSTER_INVALID) {
+        return -EINVAL;
+    } else if (!has_subclusters(s) || *type == QCOW2_SUBCLUSTER_COMPRESSED) {
+        return s->subclusters_per_cluster - sc_from;
+    }
+
+    switch (*type) {
+    case QCOW2_SUBCLUSTER_NORMAL:
+        val = l2_bitmap | QCOW_OFLAG_SUB_ALLOC_RANGE(0, sc_from);
+        return cto32(val) - sc_from;
+
+    case QCOW2_SUBCLUSTER_ZERO_PLAIN:
+    case QCOW2_SUBCLUSTER_ZERO_ALLOC:
+        val = (l2_bitmap | QCOW_OFLAG_SUB_ZERO_RANGE(0, sc_from)) >> 32;
+        return cto32(val) - sc_from;
+
+    case QCOW2_SUBCLUSTER_UNALLOCATED_PLAIN:
+    case QCOW2_SUBCLUSTER_UNALLOCATED_ALLOC:
+        val = ((l2_bitmap >> 32) | l2_bitmap)
+            & ~QCOW_OFLAG_SUB_ALLOC_RANGE(0, sc_from);
+        return ctz32(val) - sc_from;
+
+    default:
+        g_assert_not_reached();
+    }
+}
+
+/*
+ * Return the number of contiguous subclusters of the exact same type
+ * in a given L2 slice, starting from cluster @l2_index, subcluster
+ * @sc_index. Allocated subclusters are required to be contiguous in
+ * the image file.
+ * At most @nb_clusters are checked (note that this means clusters,
+ * not subclusters).
+ * Compressed clusters are always processed one by one but for the
+ * purpose of this count they are treated as if they were divided into
+ * subclusters of size s->subcluster_size.
+ * On failure return -errno and update @l2_index to point to the
+ * invalid entry.
+ */
+static int count_contiguous_subclusters(BlockDriverState *bs, int nb_clusters,
+                                        unsigned sc_index, uint64_t *l2_slice,
+                                        unsigned *l2_index)
+{
+    BDRVQcow2State *s = bs->opaque;
+    int i, count = 0;
+    bool check_offset = false;
+    uint64_t expected_offset = 0;
+    QCow2SubclusterType expected_type = QCOW2_SUBCLUSTER_NORMAL, type;
+
+    assert(*l2_index + nb_clusters <= s->l2_slice_size);
+
+    for (i = 0; i < nb_clusters; i++) {
+        unsigned first_sc = (i == 0) ? sc_index : 0;
+        uint64_t l2_entry = get_l2_entry(s, l2_slice, *l2_index + i);
+        uint64_t l2_bitmap = get_l2_bitmap(s, l2_slice, *l2_index + i);
+        int ret = qcow2_get_subcluster_range_type(bs, l2_entry, l2_bitmap,
+                                                  first_sc, &type);
+        if (ret < 0) {
+            *l2_index += i; /* Point to the invalid entry */
+            return -EIO;
+        }
+        if (i == 0) {
+            if (type == QCOW2_SUBCLUSTER_COMPRESSED) {
+                /* Compressed clusters are always processed one by one */
+                return ret;
+            }
+            expected_type = type;
+            expected_offset = l2_entry & L2E_OFFSET_MASK;
+            check_offset = (type == QCOW2_SUBCLUSTER_NORMAL ||
+                            type == QCOW2_SUBCLUSTER_ZERO_ALLOC ||
+                            type == QCOW2_SUBCLUSTER_UNALLOCATED_ALLOC);
+        } else if (type != expected_type) {
+            break;
+        } else if (check_offset) {
+            expected_offset += s->cluster_size;
+            if (expected_offset != (l2_entry & L2E_OFFSET_MASK)) {
+                break;
+            }
+        }
+        count += ret;
+        /* Stop if there are type changes before the end of the cluster */
+        if (first_sc + ret < s->subclusters_per_cluster) {
+            break;
+        }
+    }
+
+    return count;
+}
+
+static int coroutine_fn do_perform_cow_read(BlockDriverState *bs,
+                                            uint64_t src_cluster_offset,
+                                            unsigned offset_in_cluster,
+                                            QEMUIOVector *qiov)
+{
+    int ret;
+
+    if (qiov->size == 0) {
+        return 0;
+    }
+
+    BLKDBG_EVENT(bs->file, BLKDBG_COW_READ);
+
+    if (!bs->drv) {
+        return -ENOMEDIUM;
+    }
+
+    /*
+     * We never deal with requests that don't satisfy
+     * bdrv_check_qiov_request(), and aligning requests to clusters never
+     * breaks this condition. So, do some assertions before calling
+     * bs->drv->bdrv_co_preadv_part() which has int64_t arguments.
+     */
+    assert(src_cluster_offset <= INT64_MAX);
+    assert(src_cluster_offset + offset_in_cluster <= INT64_MAX);
+    /* Cast qiov->size to uint64_t to silence a compiler warning on -m32 */
+    assert((uint64_t)qiov->size <= INT64_MAX);
+    bdrv_check_qiov_request(src_cluster_offset + offset_in_cluster, qiov->size,
+                            qiov, 0, &error_abort);
+    /*
+     * Call .bdrv_co_readv() directly instead of using the public block-layer
+     * interface.  This avoids double I/O throttling and request tracking,
+     * which can lead to deadlock when block layer copy-on-read is enabled.
+     */
+    ret = bs->drv->bdrv_co_preadv_part(bs,
+                                       src_cluster_offset + offset_in_cluster,
+                                       qiov->size, qiov, 0, 0);
+    if (ret < 0) {
+        return ret;
+    }
+
+    return 0;
+}
+
+static int coroutine_fn do_perform_cow_write(BlockDriverState *bs,
+                                             uint64_t cluster_offset,
+                                             unsigned offset_in_cluster,
+                                             QEMUIOVector *qiov)
+{
+    BDRVQcow2State *s = bs->opaque;
+    int ret;
+
+    if (qiov->size == 0) {
+        return 0;
+    }
+
+    ret = qcow2_pre_write_overlap_check(bs, 0,
+            cluster_offset + offset_in_cluster, qiov->size, true);
+    if (ret < 0) {
+        return ret;
+    }
+
+    BLKDBG_EVENT(bs->file, BLKDBG_COW_WRITE);
+    ret = bdrv_co_pwritev(s->data_file, cluster_offset + offset_in_cluster,
+                          qiov->size, qiov, 0);
+    if (ret < 0) {
+        return ret;
+    }
+
+    return 0;
+}
+
+
+/*
+ * get_host_offset
+ *
+ * For a given offset of the virtual disk find the equivalent host
+ * offset in the qcow2 file and store it in *host_offset. Neither
+ * offset needs to be aligned to a cluster boundary.
+ *
+ * If the cluster is unallocated then *host_offset will be 0.
+ * If the cluster is compressed then *host_offset will contain the l2 entry.
+ *
+ * On entry, *bytes is the maximum number of contiguous bytes starting at
+ * offset that we are interested in.
+ *
+ * On exit, *bytes is the number of bytes starting at offset that have the same
+ * subcluster type and (if applicable) are stored contiguously in the image
+ * file. The subcluster type is stored in *subcluster_type.
+ * Compressed clusters are always processed one by one.
+ *
+ * Returns 0 on success, -errno in error cases.
+ */
+int qcow2_get_host_offset(BlockDriverState *bs, uint64_t offset,
+                          unsigned int *bytes, uint64_t *host_offset,
+                          QCow2SubclusterType *subcluster_type)
+{
+    BDRVQcow2State *s = bs->opaque;
+    unsigned int l2_index, sc_index;
+    uint64_t l1_index, l2_offset, *l2_slice, l2_entry, l2_bitmap;
+    int sc;
+    unsigned int offset_in_cluster;
+    uint64_t bytes_available, bytes_needed, nb_clusters;
+    QCow2SubclusterType type;
+    int ret;
+
+    offset_in_cluster = offset_into_cluster(s, offset);
+    bytes_needed = (uint64_t) *bytes + offset_in_cluster;
+
+    /* compute how many bytes there are between the start of the cluster
+     * containing offset and the end of the l2 slice that contains
+     * the entry pointing to it */
+    bytes_available =
+        ((uint64_t) (s->l2_slice_size - offset_to_l2_slice_index(s, offset)))
+        << s->cluster_bits;
+
+    if (bytes_needed > bytes_available) {
+        bytes_needed = bytes_available;
+    }
+
+    *host_offset = 0;
+
+    /* seek to the l2 offset in the l1 table */
+
+    l1_index = offset_to_l1_index(s, offset);
+    if (l1_index >= s->l1_size) {
+        type = QCOW2_SUBCLUSTER_UNALLOCATED_PLAIN;
+        goto out;
+    }
+
+    l2_offset = s->l1_table[l1_index] & L1E_OFFSET_MASK;
+    if (!l2_offset) {
+        type = QCOW2_SUBCLUSTER_UNALLOCATED_PLAIN;
+        goto out;
+    }
+
+    if (offset_into_cluster(s, l2_offset)) {
+        qcow2_signal_corruption(bs, true, -1, -1, "L2 table offset %#" PRIx64
+                                " unaligned (L1 index: %#" PRIx64 ")",
+                                l2_offset, l1_index);
+        return -EIO;
+    }
+
+    /* load the l2 slice in memory */
+
+    ret = l2_load(bs, offset, l2_offset, &l2_slice);
+    if (ret < 0) {
+        return ret;
+    }
+
+    /* find the cluster offset for the given disk offset */
+
+    l2_index = offset_to_l2_slice_index(s, offset);
+    sc_index = offset_to_sc_index(s, offset);
+    l2_entry = get_l2_entry(s, l2_slice, l2_index);
+    l2_bitmap = get_l2_bitmap(s, l2_slice, l2_index);
+
+    nb_clusters = size_to_clusters(s, bytes_needed);
+    /* bytes_needed <= *bytes + offset_in_cluster, both of which are unsigned
+     * integers; the minimum cluster size is 512, so this assertion is always
+     * true */
+    assert(nb_clusters <= INT_MAX);
+
+    type = qcow2_get_subcluster_type(bs, l2_entry, l2_bitmap, sc_index);
+    if (s->qcow_version < 3 && (type == QCOW2_SUBCLUSTER_ZERO_PLAIN ||
+                                type == QCOW2_SUBCLUSTER_ZERO_ALLOC)) {
+        qcow2_signal_corruption(bs, true, -1, -1, "Zero cluster entry found"
+                                " in pre-v3 image (L2 offset: %#" PRIx64
+                                ", L2 index: %#x)", l2_offset, l2_index);
+        ret = -EIO;
+        goto fail;
+    }
+    switch (type) {
+    case QCOW2_SUBCLUSTER_INVALID:
+        break; /* This is handled by count_contiguous_subclusters() below */
+    case QCOW2_SUBCLUSTER_COMPRESSED:
+        if (has_data_file(bs)) {
+            qcow2_signal_corruption(bs, true, -1, -1, "Compressed cluster "
+                                    "entry found in image with external data "
+                                    "file (L2 offset: %#" PRIx64 ", L2 index: "
+                                    "%#x)", l2_offset, l2_index);
+            ret = -EIO;
+            goto fail;
+        }
+        *host_offset = l2_entry;
+        break;
+    case QCOW2_SUBCLUSTER_ZERO_PLAIN:
+    case QCOW2_SUBCLUSTER_UNALLOCATED_PLAIN:
+        break;
+    case QCOW2_SUBCLUSTER_ZERO_ALLOC:
+    case QCOW2_SUBCLUSTER_NORMAL:
+    case QCOW2_SUBCLUSTER_UNALLOCATED_ALLOC: {
+        uint64_t host_cluster_offset = l2_entry & L2E_OFFSET_MASK;
+        *host_offset = host_cluster_offset + offset_in_cluster;
+        if (offset_into_cluster(s, host_cluster_offset)) {
+            qcow2_signal_corruption(bs, true, -1, -1,
+                                    "Cluster allocation offset %#"
+                                    PRIx64 " unaligned (L2 offset: %#" PRIx64
+                                    ", L2 index: %#x)", host_cluster_offset,
+                                    l2_offset, l2_index);
+            ret = -EIO;
+            goto fail;
+        }
+        if (has_data_file(bs) && *host_offset != offset) {
+            qcow2_signal_corruption(bs, true, -1, -1,
+                                    "External data file host cluster offset %#"
+                                    PRIx64 " does not match guest cluster "
+                                    "offset: %#" PRIx64
+                                    ", L2 index: %#x)", host_cluster_offset,
+                                    offset - offset_in_cluster, l2_index);
+            ret = -EIO;
+            goto fail;
+        }
+        break;
+    }
+    default:
+        abort();
+    }
+
+    sc = count_contiguous_subclusters(bs, nb_clusters, sc_index,
+                                      l2_slice, &l2_index);
+    if (sc < 0) {
+        qcow2_signal_corruption(bs, true, -1, -1, "Invalid cluster entry found "
+                                " (L2 offset: %#" PRIx64 ", L2 index: %#x)",
+                                l2_offset, l2_index);
+        ret = -EIO;
+        goto fail;
+    }
+    qcow2_cache_put(s->l2_table_cache, (void **) &l2_slice);
+
+    bytes_available = ((int64_t)sc + sc_index) << s->subcluster_bits;
+
+out:
+    if (bytes_available > bytes_needed) {
+        bytes_available = bytes_needed;
+    }
+
+    /* bytes_available <= bytes_needed <= *bytes + offset_in_cluster;
+     * subtracting offset_in_cluster will therefore definitely yield something
+     * not exceeding UINT_MAX */
+    assert(bytes_available - offset_in_cluster <= UINT_MAX);
+    *bytes = bytes_available - offset_in_cluster;
+
+    *subcluster_type = type;
+
+    return 0;
+
+fail:
+    qcow2_cache_put(s->l2_table_cache, (void **)&l2_slice);
+    return ret;
+}
+
+/*
+ * get_cluster_table
+ *
+ * for a given disk offset, load (and allocate if needed)
+ * the appropriate slice of its l2 table.
+ *
+ * the cluster index in the l2 slice is given to the caller.
+ *
+ * Returns 0 on success, -errno in failure case
+ */
+static int get_cluster_table(BlockDriverState *bs, uint64_t offset,
+                             uint64_t **new_l2_slice,
+                             int *new_l2_index)
+{
+    BDRVQcow2State *s = bs->opaque;
+    unsigned int l2_index;
+    uint64_t l1_index, l2_offset;
+    uint64_t *l2_slice = NULL;
+    int ret;
+
+    /* seek to the l2 offset in the l1 table */
+
+    l1_index = offset_to_l1_index(s, offset);
+    if (l1_index >= s->l1_size) {
+        ret = qcow2_grow_l1_table(bs, l1_index + 1, false);
+        if (ret < 0) {
+            return ret;
+        }
+    }
+
+    assert(l1_index < s->l1_size);
+    l2_offset = s->l1_table[l1_index] & L1E_OFFSET_MASK;
+    if (offset_into_cluster(s, l2_offset)) {
+        qcow2_signal_corruption(bs, true, -1, -1, "L2 table offset %#" PRIx64
+                                " unaligned (L1 index: %#" PRIx64 ")",
+                                l2_offset, l1_index);
+        return -EIO;
+    }
+
+    if (!(s->l1_table[l1_index] & QCOW_OFLAG_COPIED)) {
+        /* First allocate a new L2 table (and do COW if needed) */
+        ret = l2_allocate(bs, l1_index);
+        if (ret < 0) {
+            return ret;
+        }
+
+        /* Then decrease the refcount of the old table */
+        if (l2_offset) {
+            qcow2_free_clusters(bs, l2_offset, s->l2_size * l2_entry_size(s),
+                                QCOW2_DISCARD_OTHER);
+        }
+
+        /* Get the offset of the newly-allocated l2 table */
+        l2_offset = s->l1_table[l1_index] & L1E_OFFSET_MASK;
+        assert(offset_into_cluster(s, l2_offset) == 0);
+    }
+
+    /* load the l2 slice in memory */
+    ret = l2_load(bs, offset, l2_offset, &l2_slice);
+    if (ret < 0) {
+        return ret;
+    }
+
+    /* find the cluster offset for the given disk offset */
+
+    l2_index = offset_to_l2_slice_index(s, offset);
+
+    *new_l2_slice = l2_slice;
+    *new_l2_index = l2_index;
+
+    return 0;
+}
+
+/*
+ * alloc_compressed_cluster_offset
+ *
+ * For a given offset on the virtual disk, allocate a new compressed cluster
+ * and put the host offset of the cluster into *host_offset. If a cluster is
+ * already allocated at the offset, return an error.
+ *
+ * Return 0 on success and -errno in error cases
+ */
+int qcow2_alloc_compressed_cluster_offset(BlockDriverState *bs,
+                                          uint64_t offset,
+                                          int compressed_size,
+                                          uint64_t *host_offset)
+{
+    BDRVQcow2State *s = bs->opaque;
+    int l2_index, ret;
+    uint64_t *l2_slice;
+    int64_t cluster_offset;
+    int nb_csectors;
+
+    if (has_data_file(bs)) {
+        return 0;
+    }
+
+    ret = get_cluster_table(bs, offset, &l2_slice, &l2_index);
+    if (ret < 0) {
+        return ret;
+    }
+
+    /* Compression can't overwrite anything. Fail if the cluster was already
+     * allocated. */
+    cluster_offset = get_l2_entry(s, l2_slice, l2_index);
+    if (cluster_offset & L2E_OFFSET_MASK) {
+        qcow2_cache_put(s->l2_table_cache, (void **) &l2_slice);
+        return -EIO;
+    }
+
+    cluster_offset = qcow2_alloc_bytes(bs, compressed_size);
+    if (cluster_offset < 0) {
+        qcow2_cache_put(s->l2_table_cache, (void **) &l2_slice);
+        return cluster_offset;
+    }
+
+    nb_csectors =
+        (cluster_offset + compressed_size - 1) / QCOW2_COMPRESSED_SECTOR_SIZE -
+        (cluster_offset / QCOW2_COMPRESSED_SECTOR_SIZE);
+
+    /* The offset and size must fit in their fields of the L2 table entry */
+    assert((cluster_offset & s->cluster_offset_mask) == cluster_offset);
+    assert((nb_csectors & s->csize_mask) == nb_csectors);
+
+    cluster_offset |= QCOW_OFLAG_COMPRESSED |
+                      ((uint64_t)nb_csectors << s->csize_shift);
+
+    /* update L2 table */
+
+    /* compressed clusters never have the copied flag */
+
+    BLKDBG_EVENT(bs->file, BLKDBG_L2_UPDATE_COMPRESSED);
+    qcow2_cache_entry_mark_dirty(s->l2_table_cache, l2_slice);
+    set_l2_entry(s, l2_slice, l2_index, cluster_offset);
+    if (has_subclusters(s)) {
+        set_l2_bitmap(s, l2_slice, l2_index, 0);
+    }
+    qcow2_cache_put(s->l2_table_cache, (void **) &l2_slice);
+
+    *host_offset = cluster_offset & s->cluster_offset_mask;
+    return 0;
+}
+
+static int perform_cow(BlockDriverState *bs, QCowL2Meta *m)
+{
+    BDRVQcow2State *s = bs->opaque;
+    Qcow2COWRegion *start = &m->cow_start;
+    Qcow2COWRegion *end = &m->cow_end;
+    unsigned buffer_size;
+    unsigned data_bytes = end->offset - (start->offset + start->nb_bytes);
+    bool merge_reads;
+    uint8_t *start_buffer, *end_buffer;
+    QEMUIOVector qiov;
+    int ret;
+
+    assert(start->nb_bytes <= UINT_MAX - end->nb_bytes);
+    assert(start->nb_bytes + end->nb_bytes <= UINT_MAX - data_bytes);
+    assert(start->offset + start->nb_bytes <= end->offset);
+
+    if ((start->nb_bytes == 0 && end->nb_bytes == 0) || m->skip_cow) {
+        return 0;
+    }
+
+    /* If we have to read both the start and end COW regions and the
+     * middle region is not too large then perform just one read
+     * operation */
+    merge_reads = start->nb_bytes && end->nb_bytes && data_bytes <= 16384;
+    if (merge_reads) {
+        buffer_size = start->nb_bytes + data_bytes + end->nb_bytes;
+    } else {
+        /* If we have to do two reads, add some padding in the middle
+         * if necessary to make sure that the end region is optimally
+         * aligned. */
+        size_t align = bdrv_opt_mem_align(bs);
+        assert(align > 0 && align <= UINT_MAX);
+        assert(QEMU_ALIGN_UP(start->nb_bytes, align) <=
+               UINT_MAX - end->nb_bytes);
+        buffer_size = QEMU_ALIGN_UP(start->nb_bytes, align) + end->nb_bytes;
+    }
+
+    /* Reserve a buffer large enough to store all the data that we're
+     * going to read */
+    start_buffer = qemu_try_blockalign(bs, buffer_size);
+    if (start_buffer == NULL) {
+        return -ENOMEM;
+    }
+    /* The part of the buffer where the end region is located */
+    end_buffer = start_buffer + buffer_size - end->nb_bytes;
+
+    qemu_iovec_init(&qiov, 2 + (m->data_qiov ?
+                                qemu_iovec_subvec_niov(m->data_qiov,
+                                                       m->data_qiov_offset,
+                                                       data_bytes)
+                                : 0));
+
+    qemu_co_mutex_unlock(&s->lock);
+    /* First we read the existing data from both COW regions. We
+     * either read the whole region in one go, or the start and end
+     * regions separately. */
+    if (merge_reads) {
+        qemu_iovec_add(&qiov, start_buffer, buffer_size);
+        ret = do_perform_cow_read(bs, m->offset, start->offset, &qiov);
+    } else {
+        qemu_iovec_add(&qiov, start_buffer, start->nb_bytes);
+        ret = do_perform_cow_read(bs, m->offset, start->offset, &qiov);
+        if (ret < 0) {
+            goto fail;
+        }
+
+        qemu_iovec_reset(&qiov);
+        qemu_iovec_add(&qiov, end_buffer, end->nb_bytes);
+        ret = do_perform_cow_read(bs, m->offset, end->offset, &qiov);
+    }
+    if (ret < 0) {
+        goto fail;
+    }
+
+    /* Encrypt the data if necessary before writing it */
+    if (bs->encrypted) {
+        ret = qcow2_co_encrypt(bs,
+                               m->alloc_offset + start->offset,
+                               m->offset + start->offset,
+                               start_buffer, start->nb_bytes);
+        if (ret < 0) {
+            goto fail;
+        }
+
+        ret = qcow2_co_encrypt(bs,
+                               m->alloc_offset + end->offset,
+                               m->offset + end->offset,
+                               end_buffer, end->nb_bytes);
+        if (ret < 0) {
+            goto fail;
+        }
+    }
+
+    /* And now we can write everything. If we have the guest data we
+     * can write everything in one single operation */
+    if (m->data_qiov) {
+        qemu_iovec_reset(&qiov);
+        if (start->nb_bytes) {
+            qemu_iovec_add(&qiov, start_buffer, start->nb_bytes);
+        }
+        qemu_iovec_concat(&qiov, m->data_qiov, m->data_qiov_offset, data_bytes);
+        if (end->nb_bytes) {
+            qemu_iovec_add(&qiov, end_buffer, end->nb_bytes);
+        }
+        /* NOTE: we have a write_aio blkdebug event here followed by
+         * a cow_write one in do_perform_cow_write(), but there's only
+         * one single I/O operation */
+        BLKDBG_EVENT(bs->file, BLKDBG_WRITE_AIO);
+        ret = do_perform_cow_write(bs, m->alloc_offset, start->offset, &qiov);
+    } else {
+        /* If there's no guest data then write both COW regions separately */
+        qemu_iovec_reset(&qiov);
+        qemu_iovec_add(&qiov, start_buffer, start->nb_bytes);
+        ret = do_perform_cow_write(bs, m->alloc_offset, start->offset, &qiov);
+        if (ret < 0) {
+            goto fail;
+        }
+
+        qemu_iovec_reset(&qiov);
+        qemu_iovec_add(&qiov, end_buffer, end->nb_bytes);
+        ret = do_perform_cow_write(bs, m->alloc_offset, end->offset, &qiov);
+    }
+
+fail:
+    qemu_co_mutex_lock(&s->lock);
+
+    /*
+     * Before we update the L2 table to actually point to the new cluster, we
+     * need to be sure that the refcounts have been increased and COW was
+     * handled.
+     */
+    if (ret == 0) {
+        qcow2_cache_depends_on_flush(s->l2_table_cache);
+    }
+
+    qemu_vfree(start_buffer);
+    qemu_iovec_destroy(&qiov);
+    return ret;
+}
+
+int qcow2_alloc_cluster_link_l2(BlockDriverState *bs, QCowL2Meta *m)
+{
+    BDRVQcow2State *s = bs->opaque;
+    int i, j = 0, l2_index, ret;
+    uint64_t *old_cluster, *l2_slice;
+    uint64_t cluster_offset = m->alloc_offset;
+
+    trace_qcow2_cluster_link_l2(qemu_coroutine_self(), m->nb_clusters);
+    assert(m->nb_clusters > 0);
+
+    old_cluster = g_try_new(uint64_t, m->nb_clusters);
+    if (old_cluster == NULL) {
+        ret = -ENOMEM;
+        goto err;
+    }
+
+    /* copy content of unmodified sectors */
+    ret = perform_cow(bs, m);
+    if (ret < 0) {
+        goto err;
+    }
+
+    /* Update L2 table. */
+    if (s->use_lazy_refcounts) {
+        qcow2_mark_dirty(bs);
+    }
+    if (qcow2_need_accurate_refcounts(s)) {
+        qcow2_cache_set_dependency(bs, s->l2_table_cache,
+                                   s->refcount_block_cache);
+    }
+
+    ret = get_cluster_table(bs, m->offset, &l2_slice, &l2_index);
+    if (ret < 0) {
+        goto err;
+    }
+    qcow2_cache_entry_mark_dirty(s->l2_table_cache, l2_slice);
+
+    assert(l2_index + m->nb_clusters <= s->l2_slice_size);
+    assert(m->cow_end.offset + m->cow_end.nb_bytes <=
+           m->nb_clusters << s->cluster_bits);
+    for (i = 0; i < m->nb_clusters; i++) {
+        uint64_t offset = cluster_offset + ((uint64_t)i << s->cluster_bits);
+        /* if two concurrent writes happen to the same unallocated cluster
+         * each write allocates separate cluster and writes data concurrently.
+         * The first one to complete updates l2 table with pointer to its
+         * cluster the second one has to do RMW (which is done above by
+         * perform_cow()), update l2 table with its cluster pointer and free
+         * old cluster. This is what this loop does */
+        if (get_l2_entry(s, l2_slice, l2_index + i) != 0) {
+            old_cluster[j++] = get_l2_entry(s, l2_slice, l2_index + i);
+        }
+
+        /* The offset must fit in the offset field of the L2 table entry */
+        assert((offset & L2E_OFFSET_MASK) == offset);
+
+        set_l2_entry(s, l2_slice, l2_index + i, offset | QCOW_OFLAG_COPIED);
+
+        /* Update bitmap with the subclusters that were just written */
+        if (has_subclusters(s) && !m->prealloc) {
+            uint64_t l2_bitmap = get_l2_bitmap(s, l2_slice, l2_index + i);
+            unsigned written_from = m->cow_start.offset;
+            unsigned written_to = m->cow_end.offset + m->cow_end.nb_bytes;
+            int first_sc, last_sc;
+            /* Narrow written_from and written_to down to the current cluster */
+            written_from = MAX(written_from, i << s->cluster_bits);
+            written_to   = MIN(written_to, (i + 1) << s->cluster_bits);
+            assert(written_from < written_to);
+            first_sc = offset_to_sc_index(s, written_from);
+            last_sc  = offset_to_sc_index(s, written_to - 1);
+            l2_bitmap |= QCOW_OFLAG_SUB_ALLOC_RANGE(first_sc, last_sc + 1);
+            l2_bitmap &= ~QCOW_OFLAG_SUB_ZERO_RANGE(first_sc, last_sc + 1);
+            set_l2_bitmap(s, l2_slice, l2_index + i, l2_bitmap);
+        }
+     }
+
+
+    qcow2_cache_put(s->l2_table_cache, (void **) &l2_slice);
+
+    /*
+     * If this was a COW, we need to decrease the refcount of the old cluster.
+     *
+     * Don't discard clusters that reach a refcount of 0 (e.g. compressed
+     * clusters), the next write will reuse them anyway.
+     */
+    if (!m->keep_old_clusters && j != 0) {
+        for (i = 0; i < j; i++) {
+            qcow2_free_any_cluster(bs, old_cluster[i], QCOW2_DISCARD_NEVER);
+        }
+    }
+
+    ret = 0;
+err:
+    g_free(old_cluster);
+    return ret;
+ }
+
+/**
+ * Frees the allocated clusters because the request failed and they won't
+ * actually be linked.
+ */
+void qcow2_alloc_cluster_abort(BlockDriverState *bs, QCowL2Meta *m)
+{
+    BDRVQcow2State *s = bs->opaque;
+    if (!has_data_file(bs) && !m->keep_old_clusters) {
+        qcow2_free_clusters(bs, m->alloc_offset,
+                            m->nb_clusters << s->cluster_bits,
+                            QCOW2_DISCARD_NEVER);
+    }
+}
+
+/*
+ * For a given write request, create a new QCowL2Meta structure, add
+ * it to @m and the BDRVQcow2State.cluster_allocs list. If the write
+ * request does not need copy-on-write or changes to the L2 metadata
+ * then this function does nothing.
+ *
+ * @host_cluster_offset points to the beginning of the first cluster.
+ *
+ * @guest_offset and @bytes indicate the offset and length of the
+ * request.
+ *
+ * @l2_slice contains the L2 entries of all clusters involved in this
+ * write request.
+ *
+ * If @keep_old is true it means that the clusters were already
+ * allocated and will be overwritten. If false then the clusters are
+ * new and we have to decrease the reference count of the old ones.
+ *
+ * Returns 0 on success, -errno on failure.
+ */
+static int calculate_l2_meta(BlockDriverState *bs, uint64_t host_cluster_offset,
+                             uint64_t guest_offset, unsigned bytes,
+                             uint64_t *l2_slice, QCowL2Meta **m, bool keep_old)
+{
+    BDRVQcow2State *s = bs->opaque;
+    int sc_index, l2_index = offset_to_l2_slice_index(s, guest_offset);
+    uint64_t l2_entry, l2_bitmap;
+    unsigned cow_start_from, cow_end_to;
+    unsigned cow_start_to = offset_into_cluster(s, guest_offset);
+    unsigned cow_end_from = cow_start_to + bytes;
+    unsigned nb_clusters = size_to_clusters(s, cow_end_from);
+    QCowL2Meta *old_m = *m;
+    QCow2SubclusterType type;
+    int i;
+    bool skip_cow = keep_old;
+
+    assert(nb_clusters <= s->l2_slice_size - l2_index);
+
+    /* Check the type of all affected subclusters */
+    for (i = 0; i < nb_clusters; i++) {
+        l2_entry = get_l2_entry(s, l2_slice, l2_index + i);
+        l2_bitmap = get_l2_bitmap(s, l2_slice, l2_index + i);
+        if (skip_cow) {
+            unsigned write_from = MAX(cow_start_to, i << s->cluster_bits);
+            unsigned write_to = MIN(cow_end_from, (i + 1) << s->cluster_bits);
+            int first_sc = offset_to_sc_index(s, write_from);
+            int last_sc = offset_to_sc_index(s, write_to - 1);
+            int cnt = qcow2_get_subcluster_range_type(bs, l2_entry, l2_bitmap,
+                                                      first_sc, &type);
+            /* Is any of the subclusters of type != QCOW2_SUBCLUSTER_NORMAL ? */
+            if (type != QCOW2_SUBCLUSTER_NORMAL || first_sc + cnt <= last_sc) {
+                skip_cow = false;
+            }
+        } else {
+            /* If we can't skip the cow we can still look for invalid entries */
+            type = qcow2_get_subcluster_type(bs, l2_entry, l2_bitmap, 0);
+        }
+        if (type == QCOW2_SUBCLUSTER_INVALID) {
+            int l1_index = offset_to_l1_index(s, guest_offset);
+            uint64_t l2_offset = s->l1_table[l1_index] & L1E_OFFSET_MASK;
+            qcow2_signal_corruption(bs, true, -1, -1, "Invalid cluster "
+                                    "entry found (L2 offset: %#" PRIx64
+                                    ", L2 index: %#x)",
+                                    l2_offset, l2_index + i);
+            return -EIO;
+        }
+    }
+
+    if (skip_cow) {
+        return 0;
+    }
+
+    /* Get the L2 entry of the first cluster */
+    l2_entry = get_l2_entry(s, l2_slice, l2_index);
+    l2_bitmap = get_l2_bitmap(s, l2_slice, l2_index);
+    sc_index = offset_to_sc_index(s, guest_offset);
+    type = qcow2_get_subcluster_type(bs, l2_entry, l2_bitmap, sc_index);
+
+    if (!keep_old) {
+        switch (type) {
+        case QCOW2_SUBCLUSTER_COMPRESSED:
+            cow_start_from = 0;
+            break;
+        case QCOW2_SUBCLUSTER_NORMAL:
+        case QCOW2_SUBCLUSTER_ZERO_ALLOC:
+        case QCOW2_SUBCLUSTER_UNALLOCATED_ALLOC:
+            if (has_subclusters(s)) {
+                /* Skip all leading zero and unallocated subclusters */
+                uint32_t alloc_bitmap = l2_bitmap & QCOW_L2_BITMAP_ALL_ALLOC;
+                cow_start_from =
+                    MIN(sc_index, ctz32(alloc_bitmap)) << s->subcluster_bits;
+            } else {
+                cow_start_from = 0;
+            }
+            break;
+        case QCOW2_SUBCLUSTER_ZERO_PLAIN:
+        case QCOW2_SUBCLUSTER_UNALLOCATED_PLAIN:
+            cow_start_from = sc_index << s->subcluster_bits;
+            break;
+        default:
+            g_assert_not_reached();
+        }
+    } else {
+        switch (type) {
+        case QCOW2_SUBCLUSTER_NORMAL:
+            cow_start_from = cow_start_to;
+            break;
+        case QCOW2_SUBCLUSTER_ZERO_ALLOC:
+        case QCOW2_SUBCLUSTER_UNALLOCATED_ALLOC:
+            cow_start_from = sc_index << s->subcluster_bits;
+            break;
+        default:
+            g_assert_not_reached();
+        }
+    }
+
+    /* Get the L2 entry of the last cluster */
+    l2_index += nb_clusters - 1;
+    l2_entry = get_l2_entry(s, l2_slice, l2_index);
+    l2_bitmap = get_l2_bitmap(s, l2_slice, l2_index);
+    sc_index = offset_to_sc_index(s, guest_offset + bytes - 1);
+    type = qcow2_get_subcluster_type(bs, l2_entry, l2_bitmap, sc_index);
+
+    if (!keep_old) {
+        switch (type) {
+        case QCOW2_SUBCLUSTER_COMPRESSED:
+            cow_end_to = ROUND_UP(cow_end_from, s->cluster_size);
+            break;
+        case QCOW2_SUBCLUSTER_NORMAL:
+        case QCOW2_SUBCLUSTER_ZERO_ALLOC:
+        case QCOW2_SUBCLUSTER_UNALLOCATED_ALLOC:
+            cow_end_to = ROUND_UP(cow_end_from, s->cluster_size);
+            if (has_subclusters(s)) {
+                /* Skip all trailing zero and unallocated subclusters */
+                uint32_t alloc_bitmap = l2_bitmap & QCOW_L2_BITMAP_ALL_ALLOC;
+                cow_end_to -=
+                    MIN(s->subclusters_per_cluster - sc_index - 1,
+                        clz32(alloc_bitmap)) << s->subcluster_bits;
+            }
+            break;
+        case QCOW2_SUBCLUSTER_ZERO_PLAIN:
+        case QCOW2_SUBCLUSTER_UNALLOCATED_PLAIN:
+            cow_end_to = ROUND_UP(cow_end_from, s->subcluster_size);
+            break;
+        default:
+            g_assert_not_reached();
+        }
+    } else {
+        switch (type) {
+        case QCOW2_SUBCLUSTER_NORMAL:
+            cow_end_to = cow_end_from;
+            break;
+        case QCOW2_SUBCLUSTER_ZERO_ALLOC:
+        case QCOW2_SUBCLUSTER_UNALLOCATED_ALLOC:
+            cow_end_to = ROUND_UP(cow_end_from, s->subcluster_size);
+            break;
+        default:
+            g_assert_not_reached();
+        }
+    }
+
+    *m = g_malloc0(sizeof(**m));
+    **m = (QCowL2Meta) {
+        .next           = old_m,
+
+        .alloc_offset   = host_cluster_offset,
+        .offset         = start_of_cluster(s, guest_offset),
+        .nb_clusters    = nb_clusters,
+
+        .keep_old_clusters = keep_old,
+
+        .cow_start = {
+            .offset     = cow_start_from,
+            .nb_bytes   = cow_start_to - cow_start_from,
+        },
+        .cow_end = {
+            .offset     = cow_end_from,
+            .nb_bytes   = cow_end_to - cow_end_from,
+        },
+    };
+
+    qemu_co_queue_init(&(*m)->dependent_requests);
+    QLIST_INSERT_HEAD(&s->cluster_allocs, *m, next_in_flight);
+
+    return 0;
+}
+
+/*
+ * Returns true if writing to the cluster pointed to by @l2_entry
+ * requires a new allocation (that is, if the cluster is unallocated
+ * or has refcount > 1 and therefore cannot be written in-place).
+ */
+static bool cluster_needs_new_alloc(BlockDriverState *bs, uint64_t l2_entry)
+{
+    switch (qcow2_get_cluster_type(bs, l2_entry)) {
+    case QCOW2_CLUSTER_NORMAL:
+    case QCOW2_CLUSTER_ZERO_ALLOC:
+        if (l2_entry & QCOW_OFLAG_COPIED) {
+            return false;
+        }
+        /* fallthrough */
+    case QCOW2_CLUSTER_UNALLOCATED:
+    case QCOW2_CLUSTER_COMPRESSED:
+    case QCOW2_CLUSTER_ZERO_PLAIN:
+        return true;
+    default:
+        abort();
+    }
+}
+
+/*
+ * Returns the number of contiguous clusters that can be written to
+ * using one single write request, starting from @l2_index.
+ * At most @nb_clusters are checked.
+ *
+ * If @new_alloc is true this counts clusters that are either
+ * unallocated, or allocated but with refcount > 1 (so they need to be
+ * newly allocated and COWed).
+ *
+ * If @new_alloc is false this counts clusters that are already
+ * allocated and can be overwritten in-place (this includes clusters
+ * of type QCOW2_CLUSTER_ZERO_ALLOC).
+ */
+static int count_single_write_clusters(BlockDriverState *bs, int nb_clusters,
+                                       uint64_t *l2_slice, int l2_index,
+                                       bool new_alloc)
+{
+    BDRVQcow2State *s = bs->opaque;
+    uint64_t l2_entry = get_l2_entry(s, l2_slice, l2_index);
+    uint64_t expected_offset = l2_entry & L2E_OFFSET_MASK;
+    int i;
+
+    for (i = 0; i < nb_clusters; i++) {
+        l2_entry = get_l2_entry(s, l2_slice, l2_index + i);
+        if (cluster_needs_new_alloc(bs, l2_entry) != new_alloc) {
+            break;
+        }
+        if (!new_alloc) {
+            if (expected_offset != (l2_entry & L2E_OFFSET_MASK)) {
+                break;
+            }
+            expected_offset += s->cluster_size;
+        }
+    }
+
+    assert(i <= nb_clusters);
+    return i;
+}
+
+/*
+ * Check if there already is an AIO write request in flight which allocates
+ * the same cluster. In this case we need to wait until the previous
+ * request has completed and updated the L2 table accordingly.
+ *
+ * Returns:
+ *   0       if there was no dependency. *cur_bytes indicates the number of
+ *           bytes from guest_offset that can be read before the next
+ *           dependency must be processed (or the request is complete)
+ *
+ *   -EAGAIN if we had to wait for another request, previously gathered
+ *           information on cluster allocation may be invalid now. The caller
+ *           must start over anyway, so consider *cur_bytes undefined.
+ */
+static int handle_dependencies(BlockDriverState *bs, uint64_t guest_offset,
+    uint64_t *cur_bytes, QCowL2Meta **m)
+{
+    BDRVQcow2State *s = bs->opaque;
+    QCowL2Meta *old_alloc;
+    uint64_t bytes = *cur_bytes;
+
+    QLIST_FOREACH(old_alloc, &s->cluster_allocs, next_in_flight) {
+
+        uint64_t start = guest_offset;
+        uint64_t end = start + bytes;
+        uint64_t old_start = start_of_cluster(s, l2meta_cow_start(old_alloc));
+        uint64_t old_end = ROUND_UP(l2meta_cow_end(old_alloc), s->cluster_size);
+
+        if (end <= old_start || start >= old_end) {
+            /* No intersection */
+            continue;
+        }
+
+        if (old_alloc->keep_old_clusters &&
+            (end <= l2meta_cow_start(old_alloc) ||
+             start >= l2meta_cow_end(old_alloc)))
+        {
+            /*
+             * Clusters intersect but COW areas don't. And cluster itself is
+             * already allocated. So, there is no actual conflict.
+             */
+            continue;
+        }
+
+        /* Conflict */
+
+        if (start < old_start) {
+            /* Stop at the start of a running allocation */
+            bytes = old_start - start;
+        } else {
+            bytes = 0;
+        }
+
+        /*
+         * Stop if an l2meta already exists. After yielding, it wouldn't
+         * be valid any more, so we'd have to clean up the old L2Metas
+         * and deal with requests depending on them before starting to
+         * gather new ones. Not worth the trouble.
+         */
+        if (bytes == 0 && *m) {
+            *cur_bytes = 0;
+            return 0;
+        }
+
+        if (bytes == 0) {
+            /*
+             * Wait for the dependency to complete. We need to recheck
+             * the free/allocated clusters when we continue.
+             */
+            qemu_co_queue_wait(&old_alloc->dependent_requests, &s->lock);
+            return -EAGAIN;
+        }
+    }
+
+    /* Make sure that existing clusters and new allocations are only used up to
+     * the next dependency if we shortened the request above */
+    *cur_bytes = bytes;
+
+    return 0;
+}
+
+/*
+ * Checks how many already allocated clusters that don't require a new
+ * allocation there are at the given guest_offset (up to *bytes).
+ * If *host_offset is not INV_OFFSET, only physically contiguous clusters
+ * beginning at this host offset are counted.
+ *
+ * Note that guest_offset may not be cluster aligned. In this case, the
+ * returned *host_offset points to exact byte referenced by guest_offset and
+ * therefore isn't cluster aligned as well.
+ *
+ * Returns:
+ *   0:     if no allocated clusters are available at the given offset.
+ *          *bytes is normally unchanged. It is set to 0 if the cluster
+ *          is allocated and can be overwritten in-place but doesn't have
+ *          the right physical offset.
+ *
+ *   1:     if allocated clusters that can be overwritten in place are
+ *          available at the requested offset. *bytes may have decreased
+ *          and describes the length of the area that can be written to.
+ *
+ *  -errno: in error cases
+ */
+static int handle_copied(BlockDriverState *bs, uint64_t guest_offset,
+    uint64_t *host_offset, uint64_t *bytes, QCowL2Meta **m)
+{
+    BDRVQcow2State *s = bs->opaque;
+    int l2_index;
+    uint64_t l2_entry, cluster_offset;
+    uint64_t *l2_slice;
+    uint64_t nb_clusters;
+    unsigned int keep_clusters;
+    int ret;
+
+    trace_qcow2_handle_copied(qemu_coroutine_self(), guest_offset, *host_offset,
+                              *bytes);
+
+    assert(*host_offset == INV_OFFSET || offset_into_cluster(s, guest_offset)
+                                      == offset_into_cluster(s, *host_offset));
+
+    /*
+     * Calculate the number of clusters to look for. We stop at L2 slice
+     * boundaries to keep things simple.
+     */
+    nb_clusters =
+        size_to_clusters(s, offset_into_cluster(s, guest_offset) + *bytes);
+
+    l2_index = offset_to_l2_slice_index(s, guest_offset);
+    nb_clusters = MIN(nb_clusters, s->l2_slice_size - l2_index);
+    /* Limit total byte count to BDRV_REQUEST_MAX_BYTES */
+    nb_clusters = MIN(nb_clusters, BDRV_REQUEST_MAX_BYTES >> s->cluster_bits);
+
+    /* Find L2 entry for the first involved cluster */
+    ret = get_cluster_table(bs, guest_offset, &l2_slice, &l2_index);
+    if (ret < 0) {
+        return ret;
+    }
+
+    l2_entry = get_l2_entry(s, l2_slice, l2_index);
+    cluster_offset = l2_entry & L2E_OFFSET_MASK;
+
+    if (!cluster_needs_new_alloc(bs, l2_entry)) {
+        if (offset_into_cluster(s, cluster_offset)) {
+            qcow2_signal_corruption(bs, true, -1, -1, "%s cluster offset "
+                                    "%#" PRIx64 " unaligned (guest offset: %#"
+                                    PRIx64 ")", l2_entry & QCOW_OFLAG_ZERO ?
+                                    "Preallocated zero" : "Data",
+                                    cluster_offset, guest_offset);
+            ret = -EIO;
+            goto out;
+        }
+
+        /* If a specific host_offset is required, check it */
+        if (*host_offset != INV_OFFSET && cluster_offset != *host_offset) {
+            *bytes = 0;
+            ret = 0;
+            goto out;
+        }
+
+        /* We keep all QCOW_OFLAG_COPIED clusters */
+        keep_clusters = count_single_write_clusters(bs, nb_clusters, l2_slice,
+                                                    l2_index, false);
+        assert(keep_clusters <= nb_clusters);
+
+        *bytes = MIN(*bytes,
+                 keep_clusters * s->cluster_size
+                 - offset_into_cluster(s, guest_offset));
+        assert(*bytes != 0);
+
+        ret = calculate_l2_meta(bs, cluster_offset, guest_offset,
+                                *bytes, l2_slice, m, true);
+        if (ret < 0) {
+            goto out;
+        }
+
+        ret = 1;
+    } else {
+        ret = 0;
+    }
+
+    /* Cleanup */
+out:
+    qcow2_cache_put(s->l2_table_cache, (void **) &l2_slice);
+
+    /* Only return a host offset if we actually made progress. Otherwise we
+     * would make requirements for handle_alloc() that it can't fulfill */
+    if (ret > 0) {
+        *host_offset = cluster_offset + offset_into_cluster(s, guest_offset);
+    }
+
+    return ret;
+}
+
+/*
+ * Allocates new clusters for the given guest_offset.
+ *
+ * At most *nb_clusters are allocated, and on return *nb_clusters is updated to
+ * contain the number of clusters that have been allocated and are contiguous
+ * in the image file.
+ *
+ * If *host_offset is not INV_OFFSET, it specifies the offset in the image file
+ * at which the new clusters must start. *nb_clusters can be 0 on return in
+ * this case if the cluster at host_offset is already in use. If *host_offset
+ * is INV_OFFSET, the clusters can be allocated anywhere in the image file.
+ *
+ * *host_offset is updated to contain the offset into the image file at which
+ * the first allocated cluster starts.
+ *
+ * Return 0 on success and -errno in error cases. -EAGAIN means that the
+ * function has been waiting for another request and the allocation must be
+ * restarted, but the whole request should not be failed.
+ */
+static int do_alloc_cluster_offset(BlockDriverState *bs, uint64_t guest_offset,
+                                   uint64_t *host_offset, uint64_t *nb_clusters)
+{
+    BDRVQcow2State *s = bs->opaque;
+
+    trace_qcow2_do_alloc_clusters_offset(qemu_coroutine_self(), guest_offset,
+                                         *host_offset, *nb_clusters);
+
+    if (has_data_file(bs)) {
+        assert(*host_offset == INV_OFFSET ||
+               *host_offset == start_of_cluster(s, guest_offset));
+        *host_offset = start_of_cluster(s, guest_offset);
+        return 0;
+    }
+
+    /* Allocate new clusters */
+    trace_qcow2_cluster_alloc_phys(qemu_coroutine_self());
+    if (*host_offset == INV_OFFSET) {
+        int64_t cluster_offset =
+            qcow2_alloc_clusters(bs, *nb_clusters * s->cluster_size);
+        if (cluster_offset < 0) {
+            return cluster_offset;
+        }
+        *host_offset = cluster_offset;
+        return 0;
+    } else {
+        int64_t ret = qcow2_alloc_clusters_at(bs, *host_offset, *nb_clusters);
+        if (ret < 0) {
+            return ret;
+        }
+        *nb_clusters = ret;
+        return 0;
+    }
+}
+
+/*
+ * Allocates new clusters for an area that is either still unallocated or
+ * cannot be overwritten in-place. If *host_offset is not INV_OFFSET,
+ * clusters are only allocated if the new allocation can match the specified
+ * host offset.
+ *
+ * Note that guest_offset may not be cluster aligned. In this case, the
+ * returned *host_offset points to exact byte referenced by guest_offset and
+ * therefore isn't cluster aligned as well.
+ *
+ * Returns:
+ *   0:     if no clusters could be allocated. *bytes is set to 0,
+ *          *host_offset is left unchanged.
+ *
+ *   1:     if new clusters were allocated. *bytes may be decreased if the
+ *          new allocation doesn't cover all of the requested area.
+ *          *host_offset is updated to contain the host offset of the first
+ *          newly allocated cluster.
+ *
+ *  -errno: in error cases
+ */
+static int handle_alloc(BlockDriverState *bs, uint64_t guest_offset,
+    uint64_t *host_offset, uint64_t *bytes, QCowL2Meta **m)
+{
+    BDRVQcow2State *s = bs->opaque;
+    int l2_index;
+    uint64_t *l2_slice;
+    uint64_t nb_clusters;
+    int ret;
+
+    uint64_t alloc_cluster_offset;
+
+    trace_qcow2_handle_alloc(qemu_coroutine_self(), guest_offset, *host_offset,
+                             *bytes);
+    assert(*bytes > 0);
+
+    /*
+     * Calculate the number of clusters to look for. We stop at L2 slice
+     * boundaries to keep things simple.
+     */
+    nb_clusters =
+        size_to_clusters(s, offset_into_cluster(s, guest_offset) + *bytes);
+
+    l2_index = offset_to_l2_slice_index(s, guest_offset);
+    nb_clusters = MIN(nb_clusters, s->l2_slice_size - l2_index);
+    /* Limit total allocation byte count to BDRV_REQUEST_MAX_BYTES */
+    nb_clusters = MIN(nb_clusters, BDRV_REQUEST_MAX_BYTES >> s->cluster_bits);
+
+    /* Find L2 entry for the first involved cluster */
+    ret = get_cluster_table(bs, guest_offset, &l2_slice, &l2_index);
+    if (ret < 0) {
+        return ret;
+    }
+
+    nb_clusters = count_single_write_clusters(bs, nb_clusters,
+                                              l2_slice, l2_index, true);
+
+    /* This function is only called when there were no non-COW clusters, so if
+     * we can't find any unallocated or COW clusters either, something is
+     * wrong with our code. */
+    assert(nb_clusters > 0);
+
+    /* Allocate at a given offset in the image file */
+    alloc_cluster_offset = *host_offset == INV_OFFSET ? INV_OFFSET :
+        start_of_cluster(s, *host_offset);
+    ret = do_alloc_cluster_offset(bs, guest_offset, &alloc_cluster_offset,
+                                  &nb_clusters);
+    if (ret < 0) {
+        goto out;
+    }
+
+    /* Can't extend contiguous allocation */
+    if (nb_clusters == 0) {
+        *bytes = 0;
+        ret = 0;
+        goto out;
+    }
+
+    assert(alloc_cluster_offset != INV_OFFSET);
+
+    /*
+     * Save info needed for meta data update.
+     *
+     * requested_bytes: Number of bytes from the start of the first
+     * newly allocated cluster to the end of the (possibly shortened
+     * before) write request.
+     *
+     * avail_bytes: Number of bytes from the start of the first
+     * newly allocated to the end of the last newly allocated cluster.
+     *
+     * nb_bytes: The number of bytes from the start of the first
+     * newly allocated cluster to the end of the area that the write
+     * request actually writes to (excluding COW at the end)
+     */
+    uint64_t requested_bytes = *bytes + offset_into_cluster(s, guest_offset);
+    int avail_bytes = nb_clusters << s->cluster_bits;
+    int nb_bytes = MIN(requested_bytes, avail_bytes);
+
+    *host_offset = alloc_cluster_offset + offset_into_cluster(s, guest_offset);
+    *bytes = MIN(*bytes, nb_bytes - offset_into_cluster(s, guest_offset));
+    assert(*bytes != 0);
+
+    ret = calculate_l2_meta(bs, alloc_cluster_offset, guest_offset, *bytes,
+                            l2_slice, m, false);
+    if (ret < 0) {
+        goto out;
+    }
+
+    ret = 1;
+
+out:
+    qcow2_cache_put(s->l2_table_cache, (void **) &l2_slice);
+    return ret;
+}
+
+/*
+ * For a given area on the virtual disk defined by @offset and @bytes,
+ * find the corresponding area on the qcow2 image, allocating new
+ * clusters (or subclusters) if necessary. The result can span a
+ * combination of allocated and previously unallocated clusters.
+ *
+ * Note that offset may not be cluster aligned. In this case, the returned
+ * *host_offset points to exact byte referenced by offset and therefore
+ * isn't cluster aligned as well.
+ *
+ * On return, @host_offset is set to the beginning of the requested
+ * area. This area is guaranteed to be contiguous on the qcow2 file
+ * but it can be smaller than initially requested. In this case @bytes
+ * is updated with the actual size.
+ *
+ * If any clusters or subclusters were allocated then @m contains a
+ * list with the information of all the affected regions. Note that
+ * this can happen regardless of whether this function succeeds or
+ * not. The caller is responsible for updating the L2 metadata of the
+ * allocated clusters (on success) or freeing them (on failure), and
+ * for clearing the contents of @m afterwards in both cases.
+ *
+ * If the request conflicts with another write request in flight, the coroutine
+ * is queued and will be reentered when the dependency has completed.
+ *
+ * Return 0 on success and -errno in error cases
+ */
+int qcow2_alloc_host_offset(BlockDriverState *bs, uint64_t offset,
+                            unsigned int *bytes, uint64_t *host_offset,
+                            QCowL2Meta **m)
+{
+    BDRVQcow2State *s = bs->opaque;
+    uint64_t start, remaining;
+    uint64_t cluster_offset;
+    uint64_t cur_bytes;
+    int ret;
+
+    trace_qcow2_alloc_clusters_offset(qemu_coroutine_self(), offset, *bytes);
+
+again:
+    start = offset;
+    remaining = *bytes;
+    cluster_offset = INV_OFFSET;
+    *host_offset = INV_OFFSET;
+    cur_bytes = 0;
+    *m = NULL;
+
+    while (true) {
+
+        if (*host_offset == INV_OFFSET && cluster_offset != INV_OFFSET) {
+            *host_offset = cluster_offset;
+        }
+
+        assert(remaining >= cur_bytes);
+
+        start           += cur_bytes;
+        remaining       -= cur_bytes;
+
+        if (cluster_offset != INV_OFFSET) {
+            cluster_offset += cur_bytes;
+        }
+
+        if (remaining == 0) {
+            break;
+        }
+
+        cur_bytes = remaining;
+
+        /*
+         * Now start gathering as many contiguous clusters as possible:
+         *
+         * 1. Check for overlaps with in-flight allocations
+         *
+         *      a) Overlap not in the first cluster -> shorten this request and
+         *         let the caller handle the rest in its next loop iteration.
+         *
+         *      b) Real overlaps of two requests. Yield and restart the search
+         *         for contiguous clusters (the situation could have changed
+         *         while we were sleeping)
+         *
+         *      c) TODO: Request starts in the same cluster as the in-flight
+         *         allocation ends. Shorten the COW of the in-fight allocation,
+         *         set cluster_offset to write to the same cluster and set up
+         *         the right synchronisation between the in-flight request and
+         *         the new one.
+         */
+        ret = handle_dependencies(bs, start, &cur_bytes, m);
+        if (ret == -EAGAIN) {
+            /* Currently handle_dependencies() doesn't yield if we already had
+             * an allocation. If it did, we would have to clean up the L2Meta
+             * structs before starting over. */
+            assert(*m == NULL);
+            goto again;
+        } else if (ret < 0) {
+            return ret;
+        } else if (cur_bytes == 0) {
+            break;
+        } else {
+            /* handle_dependencies() may have decreased cur_bytes (shortened
+             * the allocations below) so that the next dependency is processed
+             * correctly during the next loop iteration. */
+        }
+
+        /*
+         * 2. Count contiguous COPIED clusters.
+         */
+        ret = handle_copied(bs, start, &cluster_offset, &cur_bytes, m);
+        if (ret < 0) {
+            return ret;
+        } else if (ret) {
+            continue;
+        } else if (cur_bytes == 0) {
+            break;
+        }
+
+        /*
+         * 3. If the request still hasn't completed, allocate new clusters,
+         *    considering any cluster_offset of steps 1c or 2.
+         */
+        ret = handle_alloc(bs, start, &cluster_offset, &cur_bytes, m);
+        if (ret < 0) {
+            return ret;
+        } else if (ret) {
+            continue;
+        } else {
+            assert(cur_bytes == 0);
+            break;
+        }
+    }
+
+    *bytes -= remaining;
+    assert(*bytes > 0);
+    assert(*host_offset != INV_OFFSET);
+    assert(offset_into_cluster(s, *host_offset) ==
+           offset_into_cluster(s, offset));
+
+    return 0;
+}
+
+/*
+ * This discards as many clusters of nb_clusters as possible at once (i.e.
+ * all clusters in the same L2 slice) and returns the number of discarded
+ * clusters.
+ */
+static int discard_in_l2_slice(BlockDriverState *bs, uint64_t offset,
+                               uint64_t nb_clusters,
+                               enum qcow2_discard_type type, bool full_discard)
+{
+    BDRVQcow2State *s = bs->opaque;
+    uint64_t *l2_slice;
+    int l2_index;
+    int ret;
+    int i;
+
+    ret = get_cluster_table(bs, offset, &l2_slice, &l2_index);
+    if (ret < 0) {
+        return ret;
+    }
+
+    /* Limit nb_clusters to one L2 slice */
+    nb_clusters = MIN(nb_clusters, s->l2_slice_size - l2_index);
+    assert(nb_clusters <= INT_MAX);
+
+    for (i = 0; i < nb_clusters; i++) {
+        uint64_t old_l2_entry = get_l2_entry(s, l2_slice, l2_index + i);
+        uint64_t old_l2_bitmap = get_l2_bitmap(s, l2_slice, l2_index + i);
+        uint64_t new_l2_entry = old_l2_entry;
+        uint64_t new_l2_bitmap = old_l2_bitmap;
+        QCow2ClusterType cluster_type =
+            qcow2_get_cluster_type(bs, old_l2_entry);
+
+        /*
+         * If full_discard is true, the cluster should not read back as zeroes,
+         * but rather fall through to the backing file.
+         *
+         * If full_discard is false, make sure that a discarded area reads back
+         * as zeroes for v3 images (we cannot do it for v2 without actually
+         * writing a zero-filled buffer). We can skip the operation if the
+         * cluster is already marked as zero, or if it's unallocated and we
+         * don't have a backing file.
+         *
+         * TODO We might want to use bdrv_block_status(bs) here, but we're
+         * holding s->lock, so that doesn't work today.
+         */
+        if (full_discard) {
+            new_l2_entry = new_l2_bitmap = 0;
+        } else if (bs->backing || qcow2_cluster_is_allocated(cluster_type)) {
+            if (has_subclusters(s)) {
+                new_l2_entry = 0;
+                new_l2_bitmap = QCOW_L2_BITMAP_ALL_ZEROES;
+            } else {
+                new_l2_entry = s->qcow_version >= 3 ? QCOW_OFLAG_ZERO : 0;
+            }
+        }
+
+        if (old_l2_entry == new_l2_entry && old_l2_bitmap == new_l2_bitmap) {
+            continue;
+        }
+
+        /* First remove L2 entries */
+        qcow2_cache_entry_mark_dirty(s->l2_table_cache, l2_slice);
+        set_l2_entry(s, l2_slice, l2_index + i, new_l2_entry);
+        if (has_subclusters(s)) {
+            set_l2_bitmap(s, l2_slice, l2_index + i, new_l2_bitmap);
+        }
+        /* Then decrease the refcount */
+        qcow2_free_any_cluster(bs, old_l2_entry, type);
+    }
+
+    qcow2_cache_put(s->l2_table_cache, (void **) &l2_slice);
+
+    return nb_clusters;
+}
+
+int qcow2_cluster_discard(BlockDriverState *bs, uint64_t offset,
+                          uint64_t bytes, enum qcow2_discard_type type,
+                          bool full_discard)
+{
+    BDRVQcow2State *s = bs->opaque;
+    uint64_t end_offset = offset + bytes;
+    uint64_t nb_clusters;
+    int64_t cleared;
+    int ret;
+
+    /* Caller must pass aligned values, except at image end */
+    assert(QEMU_IS_ALIGNED(offset, s->cluster_size));
+    assert(QEMU_IS_ALIGNED(end_offset, s->cluster_size) ||
+           end_offset == bs->total_sectors << BDRV_SECTOR_BITS);
+
+    nb_clusters = size_to_clusters(s, bytes);
+
+    s->cache_discards = true;
+
+    /* Each L2 slice is handled by its own loop iteration */
+    while (nb_clusters > 0) {
+        cleared = discard_in_l2_slice(bs, offset, nb_clusters, type,
+                                      full_discard);
+        if (cleared < 0) {
+            ret = cleared;
+            goto fail;
+        }
+
+        nb_clusters -= cleared;
+        offset += (cleared * s->cluster_size);
+    }
+
+    ret = 0;
+fail:
+    s->cache_discards = false;
+    qcow2_process_discards(bs, ret);
+
+    return ret;
+}
+
+/*
+ * This zeroes as many clusters of nb_clusters as possible at once (i.e.
+ * all clusters in the same L2 slice) and returns the number of zeroed
+ * clusters.
+ */
+static int zero_in_l2_slice(BlockDriverState *bs, uint64_t offset,
+                            uint64_t nb_clusters, int flags)
+{
+    BDRVQcow2State *s = bs->opaque;
+    uint64_t *l2_slice;
+    int l2_index;
+    int ret;
+    int i;
+
+    ret = get_cluster_table(bs, offset, &l2_slice, &l2_index);
+    if (ret < 0) {
+        return ret;
+    }
+
+    /* Limit nb_clusters to one L2 slice */
+    nb_clusters = MIN(nb_clusters, s->l2_slice_size - l2_index);
+    assert(nb_clusters <= INT_MAX);
+
+    for (i = 0; i < nb_clusters; i++) {
+        uint64_t old_l2_entry = get_l2_entry(s, l2_slice, l2_index + i);
+        uint64_t old_l2_bitmap = get_l2_bitmap(s, l2_slice, l2_index + i);
+        QCow2ClusterType type = qcow2_get_cluster_type(bs, old_l2_entry);
+        bool unmap = (type == QCOW2_CLUSTER_COMPRESSED) ||
+            ((flags & BDRV_REQ_MAY_UNMAP) && qcow2_cluster_is_allocated(type));
+        uint64_t new_l2_entry = unmap ? 0 : old_l2_entry;
+        uint64_t new_l2_bitmap = old_l2_bitmap;
+
+        if (has_subclusters(s)) {
+            new_l2_bitmap = QCOW_L2_BITMAP_ALL_ZEROES;
+        } else {
+            new_l2_entry |= QCOW_OFLAG_ZERO;
+        }
+
+        if (old_l2_entry == new_l2_entry && old_l2_bitmap == new_l2_bitmap) {
+            continue;
+        }
+
+        /* First update L2 entries */
+        qcow2_cache_entry_mark_dirty(s->l2_table_cache, l2_slice);
+        set_l2_entry(s, l2_slice, l2_index + i, new_l2_entry);
+        if (has_subclusters(s)) {
+            set_l2_bitmap(s, l2_slice, l2_index + i, new_l2_bitmap);
+        }
+
+        /* Then decrease the refcount */
+        if (unmap) {
+            qcow2_free_any_cluster(bs, old_l2_entry, QCOW2_DISCARD_REQUEST);
+        }
+    }
+
+    qcow2_cache_put(s->l2_table_cache, (void **) &l2_slice);
+
+    return nb_clusters;
+}
+
+static int zero_l2_subclusters(BlockDriverState *bs, uint64_t offset,
+                               unsigned nb_subclusters)
+{
+    BDRVQcow2State *s = bs->opaque;
+    uint64_t *l2_slice;
+    uint64_t old_l2_bitmap, l2_bitmap;
+    int l2_index, ret, sc = offset_to_sc_index(s, offset);
+
+    /* For full clusters use zero_in_l2_slice() instead */
+    assert(nb_subclusters > 0 && nb_subclusters < s->subclusters_per_cluster);
+    assert(sc + nb_subclusters <= s->subclusters_per_cluster);
+    assert(offset_into_subcluster(s, offset) == 0);
+
+    ret = get_cluster_table(bs, offset, &l2_slice, &l2_index);
+    if (ret < 0) {
+        return ret;
+    }
+
+    switch (qcow2_get_cluster_type(bs, get_l2_entry(s, l2_slice, l2_index))) {
+    case QCOW2_CLUSTER_COMPRESSED:
+        ret = -ENOTSUP; /* We cannot partially zeroize compressed clusters */
+        goto out;
+    case QCOW2_CLUSTER_NORMAL:
+    case QCOW2_CLUSTER_UNALLOCATED:
+        break;
+    default:
+        g_assert_not_reached();
+    }
+
+    old_l2_bitmap = l2_bitmap = get_l2_bitmap(s, l2_slice, l2_index);
+
+    l2_bitmap |=  QCOW_OFLAG_SUB_ZERO_RANGE(sc, sc + nb_subclusters);
+    l2_bitmap &= ~QCOW_OFLAG_SUB_ALLOC_RANGE(sc, sc + nb_subclusters);
+
+    if (old_l2_bitmap != l2_bitmap) {
+        set_l2_bitmap(s, l2_slice, l2_index, l2_bitmap);
+        qcow2_cache_entry_mark_dirty(s->l2_table_cache, l2_slice);
+    }
+
+    ret = 0;
+out:
+    qcow2_cache_put(s->l2_table_cache, (void **) &l2_slice);
+
+    return ret;
+}
+
+int qcow2_subcluster_zeroize(BlockDriverState *bs, uint64_t offset,
+                             uint64_t bytes, int flags)
+{
+    BDRVQcow2State *s = bs->opaque;
+    uint64_t end_offset = offset + bytes;
+    uint64_t nb_clusters;
+    unsigned head, tail;
+    int64_t cleared;
+    int ret;
+
+    /* If we have to stay in sync with an external data file, zero out
+     * s->data_file first. */
+    if (data_file_is_raw(bs)) {
+        assert(has_data_file(bs));
+        ret = bdrv_co_pwrite_zeroes(s->data_file, offset, bytes, flags);
+        if (ret < 0) {
+            return ret;
+        }
+    }
+
+    /* Caller must pass aligned values, except at image end */
+    assert(offset_into_subcluster(s, offset) == 0);
+    assert(offset_into_subcluster(s, end_offset) == 0 ||
+           end_offset >= bs->total_sectors << BDRV_SECTOR_BITS);
+
+    /*
+     * The zero flag is only supported by version 3 and newer. However, if we
+     * have no backing file, we can resort to discard in version 2.
+     */
+    if (s->qcow_version < 3) {
+        if (!bs->backing) {
+            return qcow2_cluster_discard(bs, offset, bytes,
+                                         QCOW2_DISCARD_REQUEST, false);
+        }
+        return -ENOTSUP;
+    }
+
+    head = MIN(end_offset, ROUND_UP(offset, s->cluster_size)) - offset;
+    offset += head;
+
+    tail = (end_offset >= bs->total_sectors << BDRV_SECTOR_BITS) ? 0 :
+        end_offset - MAX(offset, start_of_cluster(s, end_offset));
+    end_offset -= tail;
+
+    s->cache_discards = true;
+
+    if (head) {
+        ret = zero_l2_subclusters(bs, offset - head,
+                                  size_to_subclusters(s, head));
+        if (ret < 0) {
+            goto fail;
+        }
+    }
+
+    /* Each L2 slice is handled by its own loop iteration */
+    nb_clusters = size_to_clusters(s, end_offset - offset);
+
+    while (nb_clusters > 0) {
+        cleared = zero_in_l2_slice(bs, offset, nb_clusters, flags);
+        if (cleared < 0) {
+            ret = cleared;
+            goto fail;
+        }
+
+        nb_clusters -= cleared;
+        offset += (cleared * s->cluster_size);
+    }
+
+    if (tail) {
+        ret = zero_l2_subclusters(bs, end_offset, size_to_subclusters(s, tail));
+        if (ret < 0) {
+            goto fail;
+        }
+    }
+
+    ret = 0;
+fail:
+    s->cache_discards = false;
+    qcow2_process_discards(bs, ret);
+
+    return ret;
+}
+
+/*
+ * Expands all zero clusters in a specific L1 table (or deallocates them, for
+ * non-backed non-pre-allocated zero clusters).
+ *
+ * l1_entries and *visited_l1_entries are used to keep track of progress for
+ * status_cb(). l1_entries contains the total number of L1 entries and
+ * *visited_l1_entries counts all visited L1 entries.
+ */
+static int expand_zero_clusters_in_l1(BlockDriverState *bs, uint64_t *l1_table,
+                                      int l1_size, int64_t *visited_l1_entries,
+                                      int64_t l1_entries,
+                                      BlockDriverAmendStatusCB *status_cb,
+                                      void *cb_opaque)
+{
+    BDRVQcow2State *s = bs->opaque;
+    bool is_active_l1 = (l1_table == s->l1_table);
+    uint64_t *l2_slice = NULL;
+    unsigned slice, slice_size2, n_slices;
+    int ret;
+    int i, j;
+
+    /* qcow2_downgrade() is not allowed in images with subclusters */
+    assert(!has_subclusters(s));
+
+    slice_size2 = s->l2_slice_size * l2_entry_size(s);
+    n_slices = s->cluster_size / slice_size2;
+
+    if (!is_active_l1) {
+        /* inactive L2 tables require a buffer to be stored in when loading
+         * them from disk */
+        l2_slice = qemu_try_blockalign(bs->file->bs, slice_size2);
+        if (l2_slice == NULL) {
+            return -ENOMEM;
+        }
+    }
+
+    for (i = 0; i < l1_size; i++) {
+        uint64_t l2_offset = l1_table[i] & L1E_OFFSET_MASK;
+        uint64_t l2_refcount;
+
+        if (!l2_offset) {
+            /* unallocated */
+            (*visited_l1_entries)++;
+            if (status_cb) {
+                status_cb(bs, *visited_l1_entries, l1_entries, cb_opaque);
+            }
+            continue;
+        }
+
+        if (offset_into_cluster(s, l2_offset)) {
+            qcow2_signal_corruption(bs, true, -1, -1, "L2 table offset %#"
+                                    PRIx64 " unaligned (L1 index: %#x)",
+                                    l2_offset, i);
+            ret = -EIO;
+            goto fail;
+        }
+
+        ret = qcow2_get_refcount(bs, l2_offset >> s->cluster_bits,
+                                 &l2_refcount);
+        if (ret < 0) {
+            goto fail;
+        }
+
+        for (slice = 0; slice < n_slices; slice++) {
+            uint64_t slice_offset = l2_offset + slice * slice_size2;
+            bool l2_dirty = false;
+            if (is_active_l1) {
+                /* get active L2 tables from cache */
+                ret = qcow2_cache_get(bs, s->l2_table_cache, slice_offset,
+                                      (void **)&l2_slice);
+            } else {
+                /* load inactive L2 tables from disk */
+                ret = bdrv_pread(bs->file, slice_offset, l2_slice, slice_size2);
+            }
+            if (ret < 0) {
+                goto fail;
+            }
+
+            for (j = 0; j < s->l2_slice_size; j++) {
+                uint64_t l2_entry = get_l2_entry(s, l2_slice, j);
+                int64_t offset = l2_entry & L2E_OFFSET_MASK;
+                QCow2ClusterType cluster_type =
+                    qcow2_get_cluster_type(bs, l2_entry);
+
+                if (cluster_type != QCOW2_CLUSTER_ZERO_PLAIN &&
+                    cluster_type != QCOW2_CLUSTER_ZERO_ALLOC) {
+                    continue;
+                }
+
+                if (cluster_type == QCOW2_CLUSTER_ZERO_PLAIN) {
+                    if (!bs->backing) {
+                        /*
+                         * not backed; therefore we can simply deallocate the
+                         * cluster. No need to call set_l2_bitmap(), this
+                         * function doesn't support images with subclusters.
+                         */
+                        set_l2_entry(s, l2_slice, j, 0);
+                        l2_dirty = true;
+                        continue;
+                    }
+
+                    offset = qcow2_alloc_clusters(bs, s->cluster_size);
+                    if (offset < 0) {
+                        ret = offset;
+                        goto fail;
+                    }
+
+                    /* The offset must fit in the offset field */
+                    assert((offset & L2E_OFFSET_MASK) == offset);
+
+                    if (l2_refcount > 1) {
+                        /* For shared L2 tables, set the refcount accordingly
+                         * (it is already 1 and needs to be l2_refcount) */
+                        ret = qcow2_update_cluster_refcount(
+                            bs, offset >> s->cluster_bits,
+                            refcount_diff(1, l2_refcount), false,
+                            QCOW2_DISCARD_OTHER);
+                        if (ret < 0) {
+                            qcow2_free_clusters(bs, offset, s->cluster_size,
+                                                QCOW2_DISCARD_OTHER);
+                            goto fail;
+                        }
+                    }
+                }
+
+                if (offset_into_cluster(s, offset)) {
+                    int l2_index = slice * s->l2_slice_size + j;
+                    qcow2_signal_corruption(
+                        bs, true, -1, -1,
+                        "Cluster allocation offset "
+                        "%#" PRIx64 " unaligned (L2 offset: %#"
+                        PRIx64 ", L2 index: %#x)", offset,
+                        l2_offset, l2_index);
+                    if (cluster_type == QCOW2_CLUSTER_ZERO_PLAIN) {
+                        qcow2_free_clusters(bs, offset, s->cluster_size,
+                                            QCOW2_DISCARD_ALWAYS);
+                    }
+                    ret = -EIO;
+                    goto fail;
+                }
+
+                ret = qcow2_pre_write_overlap_check(bs, 0, offset,
+                                                    s->cluster_size, true);
+                if (ret < 0) {
+                    if (cluster_type == QCOW2_CLUSTER_ZERO_PLAIN) {
+                        qcow2_free_clusters(bs, offset, s->cluster_size,
+                                            QCOW2_DISCARD_ALWAYS);
+                    }
+                    goto fail;
+                }
+
+                ret = bdrv_pwrite_zeroes(s->data_file, offset,
+                                         s->cluster_size, 0);
+                if (ret < 0) {
+                    if (cluster_type == QCOW2_CLUSTER_ZERO_PLAIN) {
+                        qcow2_free_clusters(bs, offset, s->cluster_size,
+                                            QCOW2_DISCARD_ALWAYS);
+                    }
+                    goto fail;
+                }
+
+                if (l2_refcount == 1) {
+                    set_l2_entry(s, l2_slice, j, offset | QCOW_OFLAG_COPIED);
+                } else {
+                    set_l2_entry(s, l2_slice, j, offset);
+                }
+                /*
+                 * No need to call set_l2_bitmap() after set_l2_entry() because
+                 * this function doesn't support images with subclusters.
+                 */
+                l2_dirty = true;
+            }
+
+            if (is_active_l1) {
+                if (l2_dirty) {
+                    qcow2_cache_entry_mark_dirty(s->l2_table_cache, l2_slice);
+                    qcow2_cache_depends_on_flush(s->l2_table_cache);
+                }
+                qcow2_cache_put(s->l2_table_cache, (void **) &l2_slice);
+            } else {
+                if (l2_dirty) {
+                    ret = qcow2_pre_write_overlap_check(
+                        bs, QCOW2_OL_INACTIVE_L2 | QCOW2_OL_ACTIVE_L2,
+                        slice_offset, slice_size2, false);
+                    if (ret < 0) {
+                        goto fail;
+                    }
+
+                    ret = bdrv_pwrite(bs->file, slice_offset,
+                                      l2_slice, slice_size2);
+                    if (ret < 0) {
+                        goto fail;
+                    }
+                }
+            }
+        }
+
+        (*visited_l1_entries)++;
+        if (status_cb) {
+            status_cb(bs, *visited_l1_entries, l1_entries, cb_opaque);
+        }
+    }
+
+    ret = 0;
+
+fail:
+    if (l2_slice) {
+        if (!is_active_l1) {
+            qemu_vfree(l2_slice);
+        } else {
+            qcow2_cache_put(s->l2_table_cache, (void **) &l2_slice);
+        }
+    }
+    return ret;
+}
+
+/*
+ * For backed images, expands all zero clusters on the image. For non-backed
+ * images, deallocates all non-pre-allocated zero clusters (and claims the
+ * allocation for pre-allocated ones). This is important for downgrading to a
+ * qcow2 version which doesn't yet support metadata zero clusters.
+ */
+int qcow2_expand_zero_clusters(BlockDriverState *bs,
+                               BlockDriverAmendStatusCB *status_cb,
+                               void *cb_opaque)
+{
+    BDRVQcow2State *s = bs->opaque;
+    uint64_t *l1_table = NULL;
+    int64_t l1_entries = 0, visited_l1_entries = 0;
+    int ret;
+    int i, j;
+
+    if (status_cb) {
+        l1_entries = s->l1_size;
+        for (i = 0; i < s->nb_snapshots; i++) {
+            l1_entries += s->snapshots[i].l1_size;
+        }
+    }
+
+    ret = expand_zero_clusters_in_l1(bs, s->l1_table, s->l1_size,
+                                     &visited_l1_entries, l1_entries,
+                                     status_cb, cb_opaque);
+    if (ret < 0) {
+        goto fail;
+    }
+
+    /* Inactive L1 tables may point to active L2 tables - therefore it is
+     * necessary to flush the L2 table cache before trying to access the L2
+     * tables pointed to by inactive L1 entries (else we might try to expand
+     * zero clusters that have already been expanded); furthermore, it is also
+     * necessary to empty the L2 table cache, since it may contain tables which
+     * are now going to be modified directly on disk, bypassing the cache.
+     * qcow2_cache_empty() does both for us. */
+    ret = qcow2_cache_empty(bs, s->l2_table_cache);
+    if (ret < 0) {
+        goto fail;
+    }
+
+    for (i = 0; i < s->nb_snapshots; i++) {
+        int l1_size2;
+        uint64_t *new_l1_table;
+        Error *local_err = NULL;
+
+        ret = qcow2_validate_table(bs, s->snapshots[i].l1_table_offset,
+                                   s->snapshots[i].l1_size, L1E_SIZE,
+                                   QCOW_MAX_L1_SIZE, "Snapshot L1 table",
+                                   &local_err);
+        if (ret < 0) {
+            error_report_err(local_err);
+            goto fail;
+        }
+
+        l1_size2 = s->snapshots[i].l1_size * L1E_SIZE;
+        new_l1_table = g_try_realloc(l1_table, l1_size2);
+
+        if (!new_l1_table) {
+            ret = -ENOMEM;
+            goto fail;
+        }
+
+        l1_table = new_l1_table;
+
+        ret = bdrv_pread(bs->file, s->snapshots[i].l1_table_offset,
+                         l1_table, l1_size2);
+        if (ret < 0) {
+            goto fail;
+        }
+
+        for (j = 0; j < s->snapshots[i].l1_size; j++) {
+            be64_to_cpus(&l1_table[j]);
+        }
+
+        ret = expand_zero_clusters_in_l1(bs, l1_table, s->snapshots[i].l1_size,
+                                         &visited_l1_entries, l1_entries,
+                                         status_cb, cb_opaque);
+        if (ret < 0) {
+            goto fail;
+        }
+    }
+
+    ret = 0;
+
+fail:
+    g_free(l1_table);
+    return ret;
+}
+
+void qcow2_parse_compressed_l2_entry(BlockDriverState *bs, uint64_t l2_entry,
+                                     uint64_t *coffset, int *csize)
+{
+    BDRVQcow2State *s = bs->opaque;
+    int nb_csectors;
+
+    assert(qcow2_get_cluster_type(bs, l2_entry) == QCOW2_CLUSTER_COMPRESSED);
+
+    *coffset = l2_entry & s->cluster_offset_mask;
+
+    nb_csectors = ((l2_entry >> s->csize_shift) & s->csize_mask) + 1;
+    *csize = nb_csectors * QCOW2_COMPRESSED_SECTOR_SIZE -
+        (*coffset & (QCOW2_COMPRESSED_SECTOR_SIZE - 1));
+}