Introduce Virtio-loopback epsilon release:

Epsilon release introduces a new compatibility layer which make virtio-loopback
design to work with QEMU and rust-vmm vhost-user backend without require any
changes.

Signed-off-by: Timos Ampelikiotis <t.ampelikiotis@virtualopensystems.com>
Change-Id: I52e57563e08a7d0bdc002f8e928ee61ba0c53dd9

1 files changed, 1936 insertions, 0 deletions

diff --git a/block/mirror.c b/block/mirror.c
new file mode 100644
index 000000000..efec2c767
--- /dev/null
+++ b/block/mirror.c
@@ -0,0 +1,1936 @@
+/*
+ * Image mirroring
+ *
+ * Copyright Red Hat, Inc. 2012
+ *
+ * Authors:
+ *  Paolo Bonzini  <pbonzini@redhat.com>
+ *
+ * This work is licensed under the terms of the GNU LGPL, version 2 or later.
+ * See the COPYING.LIB file in the top-level directory.
+ *
+ */
+
+#include "qemu/osdep.h"
+#include "qemu/cutils.h"
+#include "qemu/coroutine.h"
+#include "qemu/range.h"
+#include "trace.h"
+#include "block/blockjob_int.h"
+#include "block/block_int.h"
+#include "sysemu/block-backend.h"
+#include "qapi/error.h"
+#include "qapi/qmp/qerror.h"
+#include "qemu/ratelimit.h"
+#include "qemu/bitmap.h"
+
+#define MAX_IN_FLIGHT 16
+#define MAX_IO_BYTES (1 << 20) /* 1 Mb */
+#define DEFAULT_MIRROR_BUF_SIZE (MAX_IN_FLIGHT * MAX_IO_BYTES)
+
+/* The mirroring buffer is a list of granularity-sized chunks.
+ * Free chunks are organized in a list.
+ */
+typedef struct MirrorBuffer {
+    QSIMPLEQ_ENTRY(MirrorBuffer) next;
+} MirrorBuffer;
+
+typedef struct MirrorOp MirrorOp;
+
+typedef struct MirrorBlockJob {
+    BlockJob common;
+    BlockBackend *target;
+    BlockDriverState *mirror_top_bs;
+    BlockDriverState *base;
+    BlockDriverState *base_overlay;
+
+    /* The name of the graph node to replace */
+    char *replaces;
+    /* The BDS to replace */
+    BlockDriverState *to_replace;
+    /* Used to block operations on the drive-mirror-replace target */
+    Error *replace_blocker;
+    bool is_none_mode;
+    BlockMirrorBackingMode backing_mode;
+    /* Whether the target image requires explicit zero-initialization */
+    bool zero_target;
+    MirrorCopyMode copy_mode;
+    BlockdevOnError on_source_error, on_target_error;
+    /* Set when the target is synced (dirty bitmap is clean, nothing
+     * in flight) and the job is running in active mode */
+    bool actively_synced;
+    bool should_complete;
+    int64_t granularity;
+    size_t buf_size;
+    int64_t bdev_length;
+    unsigned long *cow_bitmap;
+    BdrvDirtyBitmap *dirty_bitmap;
+    BdrvDirtyBitmapIter *dbi;
+    uint8_t *buf;
+    QSIMPLEQ_HEAD(, MirrorBuffer) buf_free;
+    int buf_free_count;
+
+    uint64_t last_pause_ns;
+    unsigned long *in_flight_bitmap;
+    int in_flight;
+    int64_t bytes_in_flight;
+    QTAILQ_HEAD(, MirrorOp) ops_in_flight;
+    int ret;
+    bool unmap;
+    int target_cluster_size;
+    int max_iov;
+    bool initial_zeroing_ongoing;
+    int in_active_write_counter;
+    bool prepared;
+    bool in_drain;
+} MirrorBlockJob;
+
+typedef struct MirrorBDSOpaque {
+    MirrorBlockJob *job;
+    bool stop;
+    bool is_commit;
+} MirrorBDSOpaque;
+
+struct MirrorOp {
+    MirrorBlockJob *s;
+    QEMUIOVector qiov;
+    int64_t offset;
+    uint64_t bytes;
+
+    /* The pointee is set by mirror_co_read(), mirror_co_zero(), and
+     * mirror_co_discard() before yielding for the first time */
+    int64_t *bytes_handled;
+
+    bool is_pseudo_op;
+    bool is_active_write;
+    bool is_in_flight;
+    CoQueue waiting_requests;
+    Coroutine *co;
+    MirrorOp *waiting_for_op;
+
+    QTAILQ_ENTRY(MirrorOp) next;
+};
+
+typedef enum MirrorMethod {
+    MIRROR_METHOD_COPY,
+    MIRROR_METHOD_ZERO,
+    MIRROR_METHOD_DISCARD,
+} MirrorMethod;
+
+static BlockErrorAction mirror_error_action(MirrorBlockJob *s, bool read,
+                                            int error)
+{
+    s->actively_synced = false;
+    if (read) {
+        return block_job_error_action(&s->common, s->on_source_error,
+                                      true, error);
+    } else {
+        return block_job_error_action(&s->common, s->on_target_error,
+                                      false, error);
+    }
+}
+
+static void coroutine_fn mirror_wait_on_conflicts(MirrorOp *self,
+                                                  MirrorBlockJob *s,
+                                                  uint64_t offset,
+                                                  uint64_t bytes)
+{
+    uint64_t self_start_chunk = offset / s->granularity;
+    uint64_t self_end_chunk = DIV_ROUND_UP(offset + bytes, s->granularity);
+    uint64_t self_nb_chunks = self_end_chunk - self_start_chunk;
+
+    while (find_next_bit(s->in_flight_bitmap, self_end_chunk,
+                         self_start_chunk) < self_end_chunk &&
+           s->ret >= 0)
+    {
+        MirrorOp *op;
+
+        QTAILQ_FOREACH(op, &s->ops_in_flight, next) {
+            uint64_t op_start_chunk = op->offset / s->granularity;
+            uint64_t op_nb_chunks = DIV_ROUND_UP(op->offset + op->bytes,
+                                                 s->granularity) -
+                                    op_start_chunk;
+
+            if (op == self) {
+                continue;
+            }
+
+            if (ranges_overlap(self_start_chunk, self_nb_chunks,
+                               op_start_chunk, op_nb_chunks))
+            {
+                if (self) {
+                    /*
+                     * If the operation is already (indirectly) waiting for us,
+                     * or will wait for us as soon as it wakes up, then just go
+                     * on (instead of producing a deadlock in the former case).
+                     */
+                    if (op->waiting_for_op) {
+                        continue;
+                    }
+
+                    self->waiting_for_op = op;
+                }
+
+                qemu_co_queue_wait(&op->waiting_requests, NULL);
+
+                if (self) {
+                    self->waiting_for_op = NULL;
+                }
+
+                break;
+            }
+        }
+    }
+}
+
+static void coroutine_fn mirror_iteration_done(MirrorOp *op, int ret)
+{
+    MirrorBlockJob *s = op->s;
+    struct iovec *iov;
+    int64_t chunk_num;
+    int i, nb_chunks;
+
+    trace_mirror_iteration_done(s, op->offset, op->bytes, ret);
+
+    s->in_flight--;
+    s->bytes_in_flight -= op->bytes;
+    iov = op->qiov.iov;
+    for (i = 0; i < op->qiov.niov; i++) {
+        MirrorBuffer *buf = (MirrorBuffer *) iov[i].iov_base;
+        QSIMPLEQ_INSERT_TAIL(&s->buf_free, buf, next);
+        s->buf_free_count++;
+    }
+
+    chunk_num = op->offset / s->granularity;
+    nb_chunks = DIV_ROUND_UP(op->bytes, s->granularity);
+
+    bitmap_clear(s->in_flight_bitmap, chunk_num, nb_chunks);
+    QTAILQ_REMOVE(&s->ops_in_flight, op, next);
+    if (ret >= 0) {
+        if (s->cow_bitmap) {
+            bitmap_set(s->cow_bitmap, chunk_num, nb_chunks);
+        }
+        if (!s->initial_zeroing_ongoing) {
+            job_progress_update(&s->common.job, op->bytes);
+        }
+    }
+    qemu_iovec_destroy(&op->qiov);
+
+    qemu_co_queue_restart_all(&op->waiting_requests);
+    g_free(op);
+}
+
+static void coroutine_fn mirror_write_complete(MirrorOp *op, int ret)
+{
+    MirrorBlockJob *s = op->s;
+
+    if (ret < 0) {
+        BlockErrorAction action;
+
+        bdrv_set_dirty_bitmap(s->dirty_bitmap, op->offset, op->bytes);
+        action = mirror_error_action(s, false, -ret);
+        if (action == BLOCK_ERROR_ACTION_REPORT && s->ret >= 0) {
+            s->ret = ret;
+        }
+    }
+
+    mirror_iteration_done(op, ret);
+}
+
+static void coroutine_fn mirror_read_complete(MirrorOp *op, int ret)
+{
+    MirrorBlockJob *s = op->s;
+
+    if (ret < 0) {
+        BlockErrorAction action;
+
+        bdrv_set_dirty_bitmap(s->dirty_bitmap, op->offset, op->bytes);
+        action = mirror_error_action(s, true, -ret);
+        if (action == BLOCK_ERROR_ACTION_REPORT && s->ret >= 0) {
+            s->ret = ret;
+        }
+
+        mirror_iteration_done(op, ret);
+        return;
+    }
+
+    ret = blk_co_pwritev(s->target, op->offset, op->qiov.size, &op->qiov, 0);
+    mirror_write_complete(op, ret);
+}
+
+/* Clip bytes relative to offset to not exceed end-of-file */
+static inline int64_t mirror_clip_bytes(MirrorBlockJob *s,
+                                        int64_t offset,
+                                        int64_t bytes)
+{
+    return MIN(bytes, s->bdev_length - offset);
+}
+
+/* Round offset and/or bytes to target cluster if COW is needed, and
+ * return the offset of the adjusted tail against original. */
+static int mirror_cow_align(MirrorBlockJob *s, int64_t *offset,
+                            uint64_t *bytes)
+{
+    bool need_cow;
+    int ret = 0;
+    int64_t align_offset = *offset;
+    int64_t align_bytes = *bytes;
+    int max_bytes = s->granularity * s->max_iov;
+
+    need_cow = !test_bit(*offset / s->granularity, s->cow_bitmap);
+    need_cow |= !test_bit((*offset + *bytes - 1) / s->granularity,
+                          s->cow_bitmap);
+    if (need_cow) {
+        bdrv_round_to_clusters(blk_bs(s->target), *offset, *bytes,
+                               &align_offset, &align_bytes);
+    }
+
+    if (align_bytes > max_bytes) {
+        align_bytes = max_bytes;
+        if (need_cow) {
+            align_bytes = QEMU_ALIGN_DOWN(align_bytes, s->target_cluster_size);
+        }
+    }
+    /* Clipping may result in align_bytes unaligned to chunk boundary, but
+     * that doesn't matter because it's already the end of source image. */
+    align_bytes = mirror_clip_bytes(s, align_offset, align_bytes);
+
+    ret = align_offset + align_bytes - (*offset + *bytes);
+    *offset = align_offset;
+    *bytes = align_bytes;
+    assert(ret >= 0);
+    return ret;
+}
+
+static inline void coroutine_fn
+mirror_wait_for_any_operation(MirrorBlockJob *s, bool active)
+{
+    MirrorOp *op;
+
+    QTAILQ_FOREACH(op, &s->ops_in_flight, next) {
+        /* Do not wait on pseudo ops, because it may in turn wait on
+         * some other operation to start, which may in fact be the
+         * caller of this function.  Since there is only one pseudo op
+         * at any given time, we will always find some real operation
+         * to wait on. */
+        if (!op->is_pseudo_op && op->is_in_flight &&
+            op->is_active_write == active)
+        {
+            qemu_co_queue_wait(&op->waiting_requests, NULL);
+            return;
+        }
+    }
+    abort();
+}
+
+static inline void coroutine_fn
+mirror_wait_for_free_in_flight_slot(MirrorBlockJob *s)
+{
+    /* Only non-active operations use up in-flight slots */
+    mirror_wait_for_any_operation(s, false);
+}
+
+/* Perform a mirror copy operation.
+ *
+ * *op->bytes_handled is set to the number of bytes copied after and
+ * including offset, excluding any bytes copied prior to offset due
+ * to alignment.  This will be op->bytes if no alignment is necessary,
+ * or (new_end - op->offset) if the tail is rounded up or down due to
+ * alignment or buffer limit.
+ */
+static void coroutine_fn mirror_co_read(void *opaque)
+{
+    MirrorOp *op = opaque;
+    MirrorBlockJob *s = op->s;
+    int nb_chunks;
+    uint64_t ret;
+    uint64_t max_bytes;
+
+    max_bytes = s->granularity * s->max_iov;
+
+    /* We can only handle as much as buf_size at a time. */
+    op->bytes = MIN(s->buf_size, MIN(max_bytes, op->bytes));
+    assert(op->bytes);
+    assert(op->bytes < BDRV_REQUEST_MAX_BYTES);
+    *op->bytes_handled = op->bytes;
+
+    if (s->cow_bitmap) {
+        *op->bytes_handled += mirror_cow_align(s, &op->offset, &op->bytes);
+    }
+    /* Cannot exceed BDRV_REQUEST_MAX_BYTES + INT_MAX */
+    assert(*op->bytes_handled <= UINT_MAX);
+    assert(op->bytes <= s->buf_size);
+    /* The offset is granularity-aligned because:
+     * 1) Caller passes in aligned values;
+     * 2) mirror_cow_align is used only when target cluster is larger. */
+    assert(QEMU_IS_ALIGNED(op->offset, s->granularity));
+    /* The range is sector-aligned, since bdrv_getlength() rounds up. */
+    assert(QEMU_IS_ALIGNED(op->bytes, BDRV_SECTOR_SIZE));
+    nb_chunks = DIV_ROUND_UP(op->bytes, s->granularity);
+
+    while (s->buf_free_count < nb_chunks) {
+        trace_mirror_yield_in_flight(s, op->offset, s->in_flight);
+        mirror_wait_for_free_in_flight_slot(s);
+    }
+
+    /* Now make a QEMUIOVector taking enough granularity-sized chunks
+     * from s->buf_free.
+     */
+    qemu_iovec_init(&op->qiov, nb_chunks);
+    while (nb_chunks-- > 0) {
+        MirrorBuffer *buf = QSIMPLEQ_FIRST(&s->buf_free);
+        size_t remaining = op->bytes - op->qiov.size;
+
+        QSIMPLEQ_REMOVE_HEAD(&s->buf_free, next);
+        s->buf_free_count--;
+        qemu_iovec_add(&op->qiov, buf, MIN(s->granularity, remaining));
+    }
+
+    /* Copy the dirty cluster.  */
+    s->in_flight++;
+    s->bytes_in_flight += op->bytes;
+    op->is_in_flight = true;
+    trace_mirror_one_iteration(s, op->offset, op->bytes);
+
+    ret = bdrv_co_preadv(s->mirror_top_bs->backing, op->offset, op->bytes,
+                         &op->qiov, 0);
+    mirror_read_complete(op, ret);
+}
+
+static void coroutine_fn mirror_co_zero(void *opaque)
+{
+    MirrorOp *op = opaque;
+    int ret;
+
+    op->s->in_flight++;
+    op->s->bytes_in_flight += op->bytes;
+    *op->bytes_handled = op->bytes;
+    op->is_in_flight = true;
+
+    ret = blk_co_pwrite_zeroes(op->s->target, op->offset, op->bytes,
+                               op->s->unmap ? BDRV_REQ_MAY_UNMAP : 0);
+    mirror_write_complete(op, ret);
+}
+
+static void coroutine_fn mirror_co_discard(void *opaque)
+{
+    MirrorOp *op = opaque;
+    int ret;
+
+    op->s->in_flight++;
+    op->s->bytes_in_flight += op->bytes;
+    *op->bytes_handled = op->bytes;
+    op->is_in_flight = true;
+
+    ret = blk_co_pdiscard(op->s->target, op->offset, op->bytes);
+    mirror_write_complete(op, ret);
+}
+
+static unsigned mirror_perform(MirrorBlockJob *s, int64_t offset,
+                               unsigned bytes, MirrorMethod mirror_method)
+{
+    MirrorOp *op;
+    Coroutine *co;
+    int64_t bytes_handled = -1;
+
+    op = g_new(MirrorOp, 1);
+    *op = (MirrorOp){
+        .s              = s,
+        .offset         = offset,
+        .bytes          = bytes,
+        .bytes_handled  = &bytes_handled,
+    };
+    qemu_co_queue_init(&op->waiting_requests);
+
+    switch (mirror_method) {
+    case MIRROR_METHOD_COPY:
+        co = qemu_coroutine_create(mirror_co_read, op);
+        break;
+    case MIRROR_METHOD_ZERO:
+        co = qemu_coroutine_create(mirror_co_zero, op);
+        break;
+    case MIRROR_METHOD_DISCARD:
+        co = qemu_coroutine_create(mirror_co_discard, op);
+        break;
+    default:
+        abort();
+    }
+    op->co = co;
+
+    QTAILQ_INSERT_TAIL(&s->ops_in_flight, op, next);
+    qemu_coroutine_enter(co);
+    /* At this point, ownership of op has been moved to the coroutine
+     * and the object may already be freed */
+
+    /* Assert that this value has been set */
+    assert(bytes_handled >= 0);
+
+    /* Same assertion as in mirror_co_read() (and for mirror_co_read()
+     * and mirror_co_discard(), bytes_handled == op->bytes, which
+     * is the @bytes parameter given to this function) */
+    assert(bytes_handled <= UINT_MAX);
+    return bytes_handled;
+}
+
+static uint64_t coroutine_fn mirror_iteration(MirrorBlockJob *s)
+{
+    BlockDriverState *source = s->mirror_top_bs->backing->bs;
+    MirrorOp *pseudo_op;
+    int64_t offset;
+    uint64_t delay_ns = 0, ret = 0;
+    /* At least the first dirty chunk is mirrored in one iteration. */
+    int nb_chunks = 1;
+    bool write_zeroes_ok = bdrv_can_write_zeroes_with_unmap(blk_bs(s->target));
+    int max_io_bytes = MAX(s->buf_size / MAX_IN_FLIGHT, MAX_IO_BYTES);
+
+    bdrv_dirty_bitmap_lock(s->dirty_bitmap);
+    offset = bdrv_dirty_iter_next(s->dbi);
+    if (offset < 0) {
+        bdrv_set_dirty_iter(s->dbi, 0);
+        offset = bdrv_dirty_iter_next(s->dbi);
+        trace_mirror_restart_iter(s, bdrv_get_dirty_count(s->dirty_bitmap));
+        assert(offset >= 0);
+    }
+    bdrv_dirty_bitmap_unlock(s->dirty_bitmap);
+
+    mirror_wait_on_conflicts(NULL, s, offset, 1);
+
+    job_pause_point(&s->common.job);
+
+    /* Find the number of consective dirty chunks following the first dirty
+     * one, and wait for in flight requests in them. */
+    bdrv_dirty_bitmap_lock(s->dirty_bitmap);
+    while (nb_chunks * s->granularity < s->buf_size) {
+        int64_t next_dirty;
+        int64_t next_offset = offset + nb_chunks * s->granularity;
+        int64_t next_chunk = next_offset / s->granularity;
+        if (next_offset >= s->bdev_length ||
+            !bdrv_dirty_bitmap_get_locked(s->dirty_bitmap, next_offset)) {
+            break;
+        }
+        if (test_bit(next_chunk, s->in_flight_bitmap)) {
+            break;
+        }
+
+        next_dirty = bdrv_dirty_iter_next(s->dbi);
+        if (next_dirty > next_offset || next_dirty < 0) {
+            /* The bitmap iterator's cache is stale, refresh it */
+            bdrv_set_dirty_iter(s->dbi, next_offset);
+            next_dirty = bdrv_dirty_iter_next(s->dbi);
+        }
+        assert(next_dirty == next_offset);
+        nb_chunks++;
+    }
+
+    /* Clear dirty bits before querying the block status, because
+     * calling bdrv_block_status_above could yield - if some blocks are
+     * marked dirty in this window, we need to know.
+     */
+    bdrv_reset_dirty_bitmap_locked(s->dirty_bitmap, offset,
+                                   nb_chunks * s->granularity);
+    bdrv_dirty_bitmap_unlock(s->dirty_bitmap);
+
+    /* Before claiming an area in the in-flight bitmap, we have to
+     * create a MirrorOp for it so that conflicting requests can wait
+     * for it.  mirror_perform() will create the real MirrorOps later,
+     * for now we just create a pseudo operation that will wake up all
+     * conflicting requests once all real operations have been
+     * launched. */
+    pseudo_op = g_new(MirrorOp, 1);
+    *pseudo_op = (MirrorOp){
+        .offset         = offset,
+        .bytes          = nb_chunks * s->granularity,
+        .is_pseudo_op   = true,
+    };
+    qemu_co_queue_init(&pseudo_op->waiting_requests);
+    QTAILQ_INSERT_TAIL(&s->ops_in_flight, pseudo_op, next);
+
+    bitmap_set(s->in_flight_bitmap, offset / s->granularity, nb_chunks);
+    while (nb_chunks > 0 && offset < s->bdev_length) {
+        int ret;
+        int64_t io_bytes;
+        int64_t io_bytes_acct;
+        MirrorMethod mirror_method = MIRROR_METHOD_COPY;
+
+        assert(!(offset % s->granularity));
+        ret = bdrv_block_status_above(source, NULL, offset,
+                                      nb_chunks * s->granularity,
+                                      &io_bytes, NULL, NULL);
+        if (ret < 0) {
+            io_bytes = MIN(nb_chunks * s->granularity, max_io_bytes);
+        } else if (ret & BDRV_BLOCK_DATA) {
+            io_bytes = MIN(io_bytes, max_io_bytes);
+        }
+
+        io_bytes -= io_bytes % s->granularity;
+        if (io_bytes < s->granularity) {
+            io_bytes = s->granularity;
+        } else if (ret >= 0 && !(ret & BDRV_BLOCK_DATA)) {
+            int64_t target_offset;
+            int64_t target_bytes;
+            bdrv_round_to_clusters(blk_bs(s->target), offset, io_bytes,
+                                   &target_offset, &target_bytes);
+            if (target_offset == offset &&
+                target_bytes == io_bytes) {
+                mirror_method = ret & BDRV_BLOCK_ZERO ?
+                                    MIRROR_METHOD_ZERO :
+                                    MIRROR_METHOD_DISCARD;
+            }
+        }
+
+        while (s->in_flight >= MAX_IN_FLIGHT) {
+            trace_mirror_yield_in_flight(s, offset, s->in_flight);
+            mirror_wait_for_free_in_flight_slot(s);
+        }
+
+        if (s->ret < 0) {
+            ret = 0;
+            goto fail;
+        }
+
+        io_bytes = mirror_clip_bytes(s, offset, io_bytes);
+        io_bytes = mirror_perform(s, offset, io_bytes, mirror_method);
+        if (mirror_method != MIRROR_METHOD_COPY && write_zeroes_ok) {
+            io_bytes_acct = 0;
+        } else {
+            io_bytes_acct = io_bytes;
+        }
+        assert(io_bytes);
+        offset += io_bytes;
+        nb_chunks -= DIV_ROUND_UP(io_bytes, s->granularity);
+        delay_ns = block_job_ratelimit_get_delay(&s->common, io_bytes_acct);
+    }
+
+    ret = delay_ns;
+fail:
+    QTAILQ_REMOVE(&s->ops_in_flight, pseudo_op, next);
+    qemu_co_queue_restart_all(&pseudo_op->waiting_requests);
+    g_free(pseudo_op);
+
+    return ret;
+}
+
+static void mirror_free_init(MirrorBlockJob *s)
+{
+    int granularity = s->granularity;
+    size_t buf_size = s->buf_size;
+    uint8_t *buf = s->buf;
+
+    assert(s->buf_free_count == 0);
+    QSIMPLEQ_INIT(&s->buf_free);
+    while (buf_size != 0) {
+        MirrorBuffer *cur = (MirrorBuffer *)buf;
+        QSIMPLEQ_INSERT_TAIL(&s->buf_free, cur, next);
+        s->buf_free_count++;
+        buf_size -= granularity;
+        buf += granularity;
+    }
+}
+
+/* This is also used for the .pause callback. There is no matching
+ * mirror_resume() because mirror_run() will begin iterating again
+ * when the job is resumed.
+ */
+static void coroutine_fn mirror_wait_for_all_io(MirrorBlockJob *s)
+{
+    while (s->in_flight > 0) {
+        mirror_wait_for_free_in_flight_slot(s);
+    }
+}
+
+/**
+ * mirror_exit_common: handle both abort() and prepare() cases.
+ * for .prepare, returns 0 on success and -errno on failure.
+ * for .abort cases, denoted by abort = true, MUST return 0.
+ */
+static int mirror_exit_common(Job *job)
+{
+    MirrorBlockJob *s = container_of(job, MirrorBlockJob, common.job);
+    BlockJob *bjob = &s->common;
+    MirrorBDSOpaque *bs_opaque;
+    AioContext *replace_aio_context = NULL;
+    BlockDriverState *src;
+    BlockDriverState *target_bs;
+    BlockDriverState *mirror_top_bs;
+    Error *local_err = NULL;
+    bool abort = job->ret < 0;
+    int ret = 0;
+
+    if (s->prepared) {
+        return 0;
+    }
+    s->prepared = true;
+
+    mirror_top_bs = s->mirror_top_bs;
+    bs_opaque = mirror_top_bs->opaque;
+    src = mirror_top_bs->backing->bs;
+    target_bs = blk_bs(s->target);
+
+    if (bdrv_chain_contains(src, target_bs)) {
+        bdrv_unfreeze_backing_chain(mirror_top_bs, target_bs);
+    }
+
+    bdrv_release_dirty_bitmap(s->dirty_bitmap);
+
+    /* Make sure that the source BDS doesn't go away during bdrv_replace_node,
+     * before we can call bdrv_drained_end */
+    bdrv_ref(src);
+    bdrv_ref(mirror_top_bs);
+    bdrv_ref(target_bs);
+
+    /*
+     * Remove target parent that still uses BLK_PERM_WRITE/RESIZE before
+     * inserting target_bs at s->to_replace, where we might not be able to get
+     * these permissions.
+     */
+    blk_unref(s->target);
+    s->target = NULL;
+
+    /* We don't access the source any more. Dropping any WRITE/RESIZE is
+     * required before it could become a backing file of target_bs. Not having
+     * these permissions any more means that we can't allow any new requests on
+     * mirror_top_bs from now on, so keep it drained. */
+    bdrv_drained_begin(mirror_top_bs);
+    bs_opaque->stop = true;
+    bdrv_child_refresh_perms(mirror_top_bs, mirror_top_bs->backing,
+                             &error_abort);
+    if (!abort && s->backing_mode == MIRROR_SOURCE_BACKING_CHAIN) {
+        BlockDriverState *backing = s->is_none_mode ? src : s->base;
+        BlockDriverState *unfiltered_target = bdrv_skip_filters(target_bs);
+
+        if (bdrv_cow_bs(unfiltered_target) != backing) {
+            bdrv_set_backing_hd(unfiltered_target, backing, &local_err);
+            if (local_err) {
+                error_report_err(local_err);
+                local_err = NULL;
+                ret = -EPERM;
+            }
+        }
+    } else if (!abort && s->backing_mode == MIRROR_OPEN_BACKING_CHAIN) {
+        assert(!bdrv_backing_chain_next(target_bs));
+        ret = bdrv_open_backing_file(bdrv_skip_filters(target_bs), NULL,
+                                     "backing", &local_err);
+        if (ret < 0) {
+            error_report_err(local_err);
+            local_err = NULL;
+        }
+    }
+
+    if (s->to_replace) {
+        replace_aio_context = bdrv_get_aio_context(s->to_replace);
+        aio_context_acquire(replace_aio_context);
+    }
+
+    if (s->should_complete && !abort) {
+        BlockDriverState *to_replace = s->to_replace ?: src;
+        bool ro = bdrv_is_read_only(to_replace);
+
+        if (ro != bdrv_is_read_only(target_bs)) {
+            bdrv_reopen_set_read_only(target_bs, ro, NULL);
+        }
+
+        /* The mirror job has no requests in flight any more, but we need to
+         * drain potential other users of the BDS before changing the graph. */
+        assert(s->in_drain);
+        bdrv_drained_begin(target_bs);
+        /*
+         * Cannot use check_to_replace_node() here, because that would
+         * check for an op blocker on @to_replace, and we have our own
+         * there.
+         */
+        if (bdrv_recurse_can_replace(src, to_replace)) {
+            bdrv_replace_node(to_replace, target_bs, &local_err);
+        } else {
+            error_setg(&local_err, "Can no longer replace '%s' by '%s', "
+                       "because it can no longer be guaranteed that doing so "
+                       "would not lead to an abrupt change of visible data",
+                       to_replace->node_name, target_bs->node_name);
+        }
+        bdrv_drained_end(target_bs);
+        if (local_err) {
+            error_report_err(local_err);
+            ret = -EPERM;
+        }
+    }
+    if (s->to_replace) {
+        bdrv_op_unblock_all(s->to_replace, s->replace_blocker);
+        error_free(s->replace_blocker);
+        bdrv_unref(s->to_replace);
+    }
+    if (replace_aio_context) {
+        aio_context_release(replace_aio_context);
+    }
+    g_free(s->replaces);
+    bdrv_unref(target_bs);
+
+    /*
+     * Remove the mirror filter driver from the graph. Before this, get rid of
+     * the blockers on the intermediate nodes so that the resulting state is
+     * valid.
+     */
+    block_job_remove_all_bdrv(bjob);
+    bdrv_replace_node(mirror_top_bs, mirror_top_bs->backing->bs, &error_abort);
+
+    /* We just changed the BDS the job BB refers to (with either or both of the
+     * bdrv_replace_node() calls), so switch the BB back so the cleanup does
+     * the right thing. We don't need any permissions any more now. */
+    blk_remove_bs(bjob->blk);
+    blk_set_perm(bjob->blk, 0, BLK_PERM_ALL, &error_abort);
+    blk_insert_bs(bjob->blk, mirror_top_bs, &error_abort);
+
+    bs_opaque->job = NULL;
+
+    bdrv_drained_end(src);
+    bdrv_drained_end(mirror_top_bs);
+    s->in_drain = false;
+    bdrv_unref(mirror_top_bs);
+    bdrv_unref(src);
+
+    return ret;
+}
+
+static int mirror_prepare(Job *job)
+{
+    return mirror_exit_common(job);
+}
+
+static void mirror_abort(Job *job)
+{
+    int ret = mirror_exit_common(job);
+    assert(ret == 0);
+}
+
+static void coroutine_fn mirror_throttle(MirrorBlockJob *s)
+{
+    int64_t now = qemu_clock_get_ns(QEMU_CLOCK_REALTIME);
+
+    if (now - s->last_pause_ns > BLOCK_JOB_SLICE_TIME) {
+        s->last_pause_ns = now;
+        job_sleep_ns(&s->common.job, 0);
+    } else {
+        job_pause_point(&s->common.job);
+    }
+}
+
+static int coroutine_fn mirror_dirty_init(MirrorBlockJob *s)
+{
+    int64_t offset;
+    BlockDriverState *bs = s->mirror_top_bs->backing->bs;
+    BlockDriverState *target_bs = blk_bs(s->target);
+    int ret;
+    int64_t count;
+
+    if (s->zero_target) {
+        if (!bdrv_can_write_zeroes_with_unmap(target_bs)) {
+            bdrv_set_dirty_bitmap(s->dirty_bitmap, 0, s->bdev_length);
+            return 0;
+        }
+
+        s->initial_zeroing_ongoing = true;
+        for (offset = 0; offset < s->bdev_length; ) {
+            int bytes = MIN(s->bdev_length - offset,
+                            QEMU_ALIGN_DOWN(INT_MAX, s->granularity));
+
+            mirror_throttle(s);
+
+            if (job_is_cancelled(&s->common.job)) {
+                s->initial_zeroing_ongoing = false;
+                return 0;
+            }
+
+            if (s->in_flight >= MAX_IN_FLIGHT) {
+                trace_mirror_yield(s, UINT64_MAX, s->buf_free_count,
+                                   s->in_flight);
+                mirror_wait_for_free_in_flight_slot(s);
+                continue;
+            }
+
+            mirror_perform(s, offset, bytes, MIRROR_METHOD_ZERO);
+            offset += bytes;
+        }
+
+        mirror_wait_for_all_io(s);
+        s->initial_zeroing_ongoing = false;
+    }
+
+    /* First part, loop on the sectors and initialize the dirty bitmap.  */
+    for (offset = 0; offset < s->bdev_length; ) {
+        /* Just to make sure we are not exceeding int limit. */
+        int bytes = MIN(s->bdev_length - offset,
+                        QEMU_ALIGN_DOWN(INT_MAX, s->granularity));
+
+        mirror_throttle(s);
+
+        if (job_is_cancelled(&s->common.job)) {
+            return 0;
+        }
+
+        ret = bdrv_is_allocated_above(bs, s->base_overlay, true, offset, bytes,
+                                      &count);
+        if (ret < 0) {
+            return ret;
+        }
+
+        assert(count);
+        if (ret > 0) {
+            bdrv_set_dirty_bitmap(s->dirty_bitmap, offset, count);
+        }
+        offset += count;
+    }
+    return 0;
+}
+
+/* Called when going out of the streaming phase to flush the bulk of the
+ * data to the medium, or just before completing.
+ */
+static int mirror_flush(MirrorBlockJob *s)
+{
+    int ret = blk_flush(s->target);
+    if (ret < 0) {
+        if (mirror_error_action(s, false, -ret) == BLOCK_ERROR_ACTION_REPORT) {
+            s->ret = ret;
+        }
+    }
+    return ret;
+}
+
+static int coroutine_fn mirror_run(Job *job, Error **errp)
+{
+    MirrorBlockJob *s = container_of(job, MirrorBlockJob, common.job);
+    BlockDriverState *bs = s->mirror_top_bs->backing->bs;
+    BlockDriverState *target_bs = blk_bs(s->target);
+    bool need_drain = true;
+    int64_t length;
+    int64_t target_length;
+    BlockDriverInfo bdi;
+    char backing_filename[2]; /* we only need 2 characters because we are only
+                                 checking for a NULL string */
+    int ret = 0;
+
+    if (job_is_cancelled(&s->common.job)) {
+        goto immediate_exit;
+    }
+
+    s->bdev_length = bdrv_getlength(bs);
+    if (s->bdev_length < 0) {
+        ret = s->bdev_length;
+        goto immediate_exit;
+    }
+
+    target_length = blk_getlength(s->target);
+    if (target_length < 0) {
+        ret = target_length;
+        goto immediate_exit;
+    }
+
+    /* Active commit must resize the base image if its size differs from the
+     * active layer. */
+    if (s->base == blk_bs(s->target)) {
+        if (s->bdev_length > target_length) {
+            ret = blk_truncate(s->target, s->bdev_length, false,
+                               PREALLOC_MODE_OFF, 0, NULL);
+            if (ret < 0) {
+                goto immediate_exit;
+            }
+        }
+    } else if (s->bdev_length != target_length) {
+        error_setg(errp, "Source and target image have different sizes");
+        ret = -EINVAL;
+        goto immediate_exit;
+    }
+
+    if (s->bdev_length == 0) {
+        /* Transition to the READY state and wait for complete. */
+        job_transition_to_ready(&s->common.job);
+        s->actively_synced = true;
+        while (!job_cancel_requested(&s->common.job) && !s->should_complete) {
+            job_yield(&s->common.job);
+        }
+        goto immediate_exit;
+    }
+
+    length = DIV_ROUND_UP(s->bdev_length, s->granularity);
+    s->in_flight_bitmap = bitmap_new(length);
+
+    /* If we have no backing file yet in the destination, we cannot let
+     * the destination do COW.  Instead, we copy sectors around the
+     * dirty data if needed.  We need a bitmap to do that.
+     */
+    bdrv_get_backing_filename(target_bs, backing_filename,
+                              sizeof(backing_filename));
+    if (!bdrv_get_info(target_bs, &bdi) && bdi.cluster_size) {
+        s->target_cluster_size = bdi.cluster_size;
+    } else {
+        s->target_cluster_size = BDRV_SECTOR_SIZE;
+    }
+    if (backing_filename[0] && !bdrv_backing_chain_next(target_bs) &&
+        s->granularity < s->target_cluster_size) {
+        s->buf_size = MAX(s->buf_size, s->target_cluster_size);
+        s->cow_bitmap = bitmap_new(length);
+    }
+    s->max_iov = MIN(bs->bl.max_iov, target_bs->bl.max_iov);
+
+    s->buf = qemu_try_blockalign(bs, s->buf_size);
+    if (s->buf == NULL) {
+        ret = -ENOMEM;
+        goto immediate_exit;
+    }
+
+    mirror_free_init(s);
+
+    s->last_pause_ns = qemu_clock_get_ns(QEMU_CLOCK_REALTIME);
+    if (!s->is_none_mode) {
+        ret = mirror_dirty_init(s);
+        if (ret < 0 || job_is_cancelled(&s->common.job)) {
+            goto immediate_exit;
+        }
+    }
+
+    assert(!s->dbi);
+    s->dbi = bdrv_dirty_iter_new(s->dirty_bitmap);
+    for (;;) {
+        uint64_t delay_ns = 0;
+        int64_t cnt, delta;
+        bool should_complete;
+
+        /* Do not start passive operations while there are active
+         * writes in progress */
+        while (s->in_active_write_counter) {
+            mirror_wait_for_any_operation(s, true);
+        }
+
+        if (s->ret < 0) {
+            ret = s->ret;
+            goto immediate_exit;
+        }
+
+        job_pause_point(&s->common.job);
+
+        if (job_is_cancelled(&s->common.job)) {
+            ret = 0;
+            goto immediate_exit;
+        }
+
+        cnt = bdrv_get_dirty_count(s->dirty_bitmap);
+        /* cnt is the number of dirty bytes remaining and s->bytes_in_flight is
+         * the number of bytes currently being processed; together those are
+         * the current remaining operation length */
+        job_progress_set_remaining(&s->common.job, s->bytes_in_flight + cnt);
+
+        /* Note that even when no rate limit is applied we need to yield
+         * periodically with no pending I/O so that bdrv_drain_all() returns.
+         * We do so every BLKOCK_JOB_SLICE_TIME nanoseconds, or when there is
+         * an error, or when the source is clean, whichever comes first. */
+        delta = qemu_clock_get_ns(QEMU_CLOCK_REALTIME) - s->last_pause_ns;
+        if (delta < BLOCK_JOB_SLICE_TIME &&
+            s->common.iostatus == BLOCK_DEVICE_IO_STATUS_OK) {
+            if (s->in_flight >= MAX_IN_FLIGHT || s->buf_free_count == 0 ||
+                (cnt == 0 && s->in_flight > 0)) {
+                trace_mirror_yield(s, cnt, s->buf_free_count, s->in_flight);
+                mirror_wait_for_free_in_flight_slot(s);
+                continue;
+            } else if (cnt != 0) {
+                delay_ns = mirror_iteration(s);
+            }
+        }
+
+        should_complete = false;
+        if (s->in_flight == 0 && cnt == 0) {
+            trace_mirror_before_flush(s);
+            if (!job_is_ready(&s->common.job)) {
+                if (mirror_flush(s) < 0) {
+                    /* Go check s->ret.  */
+                    continue;
+                }
+                /* We're out of the streaming phase.  From now on, if the job
+                 * is cancelled we will actually complete all pending I/O and
+                 * report completion.  This way, block-job-cancel will leave
+                 * the target in a consistent state.
+                 */
+                job_transition_to_ready(&s->common.job);
+                if (s->copy_mode != MIRROR_COPY_MODE_BACKGROUND) {
+                    s->actively_synced = true;
+                }
+            }
+
+            should_complete = s->should_complete ||
+                job_cancel_requested(&s->common.job);
+            cnt = bdrv_get_dirty_count(s->dirty_bitmap);
+        }
+
+        if (cnt == 0 && should_complete) {
+            /* The dirty bitmap is not updated while operations are pending.
+             * If we're about to exit, wait for pending operations before
+             * calling bdrv_get_dirty_count(bs), or we may exit while the
+             * source has dirty data to copy!
+             *
+             * Note that I/O can be submitted by the guest while
+             * mirror_populate runs, so pause it now.  Before deciding
+             * whether to switch to target check one last time if I/O has
+             * come in the meanwhile, and if not flush the data to disk.
+             */
+            trace_mirror_before_drain(s, cnt);
+
+            s->in_drain = true;
+            bdrv_drained_begin(bs);
+            cnt = bdrv_get_dirty_count(s->dirty_bitmap);
+            if (cnt > 0 || mirror_flush(s) < 0) {
+                bdrv_drained_end(bs);
+                s->in_drain = false;
+                continue;
+            }
+
+            /* The two disks are in sync.  Exit and report successful
+             * completion.
+             */
+            assert(QLIST_EMPTY(&bs->tracked_requests));
+            need_drain = false;
+            break;
+        }
+
+        if (job_is_ready(&s->common.job) && !should_complete) {
+            delay_ns = (s->in_flight == 0 &&
+                        cnt == 0 ? BLOCK_JOB_SLICE_TIME : 0);
+        }
+        trace_mirror_before_sleep(s, cnt, job_is_ready(&s->common.job),
+                                  delay_ns);
+        job_sleep_ns(&s->common.job, delay_ns);
+        s->last_pause_ns = qemu_clock_get_ns(QEMU_CLOCK_REALTIME);
+    }
+
+immediate_exit:
+    if (s->in_flight > 0) {
+        /* We get here only if something went wrong.  Either the job failed,
+         * or it was cancelled prematurely so that we do not guarantee that
+         * the target is a copy of the source.
+         */
+        assert(ret < 0 || job_is_cancelled(&s->common.job));
+        assert(need_drain);
+        mirror_wait_for_all_io(s);
+    }
+
+    assert(s->in_flight == 0);
+    qemu_vfree(s->buf);
+    g_free(s->cow_bitmap);
+    g_free(s->in_flight_bitmap);
+    bdrv_dirty_iter_free(s->dbi);
+
+    if (need_drain) {
+        s->in_drain = true;
+        bdrv_drained_begin(bs);
+    }
+
+    return ret;
+}
+
+static void mirror_complete(Job *job, Error **errp)
+{
+    MirrorBlockJob *s = container_of(job, MirrorBlockJob, common.job);
+
+    if (!job_is_ready(job)) {
+        error_setg(errp, "The active block job '%s' cannot be completed",
+                   job->id);
+        return;
+    }
+
+    /* block all operations on to_replace bs */
+    if (s->replaces) {
+        AioContext *replace_aio_context;
+
+        s->to_replace = bdrv_find_node(s->replaces);
+        if (!s->to_replace) {
+            error_setg(errp, "Node name '%s' not found", s->replaces);
+            return;
+        }
+
+        replace_aio_context = bdrv_get_aio_context(s->to_replace);
+        aio_context_acquire(replace_aio_context);
+
+        /* TODO Translate this into permission system. Current definition of
+         * GRAPH_MOD would require to request it for the parents; they might
+         * not even be BlockDriverStates, however, so a BdrvChild can't address
+         * them. May need redefinition of GRAPH_MOD. */
+        error_setg(&s->replace_blocker,
+                   "block device is in use by block-job-complete");
+        bdrv_op_block_all(s->to_replace, s->replace_blocker);
+        bdrv_ref(s->to_replace);
+
+        aio_context_release(replace_aio_context);
+    }
+
+    s->should_complete = true;
+
+    /* If the job is paused, it will be re-entered when it is resumed */
+    if (!job->paused) {
+        job_enter(job);
+    }
+}
+
+static void coroutine_fn mirror_pause(Job *job)
+{
+    MirrorBlockJob *s = container_of(job, MirrorBlockJob, common.job);
+
+    mirror_wait_for_all_io(s);
+}
+
+static bool mirror_drained_poll(BlockJob *job)
+{
+    MirrorBlockJob *s = container_of(job, MirrorBlockJob, common);
+
+    /* If the job isn't paused nor cancelled, we can't be sure that it won't
+     * issue more requests. We make an exception if we've reached this point
+     * from one of our own drain sections, to avoid a deadlock waiting for
+     * ourselves.
+     */
+    if (!s->common.job.paused && !job_is_cancelled(&job->job) && !s->in_drain) {
+        return true;
+    }
+
+    return !!s->in_flight;
+}
+
+static bool mirror_cancel(Job *job, bool force)
+{
+    MirrorBlockJob *s = container_of(job, MirrorBlockJob, common.job);
+    BlockDriverState *target = blk_bs(s->target);
+
+    /*
+     * Before the job is READY, we treat any cancellation like a
+     * force-cancellation.
+     */
+    force = force || !job_is_ready(job);
+
+    if (force) {
+        bdrv_cancel_in_flight(target);
+    }
+    return force;
+}
+
+static bool commit_active_cancel(Job *job, bool force)
+{
+    /* Same as above in mirror_cancel() */
+    return force || !job_is_ready(job);
+}
+
+static const BlockJobDriver mirror_job_driver = {
+    .job_driver = {
+        .instance_size          = sizeof(MirrorBlockJob),
+        .job_type               = JOB_TYPE_MIRROR,
+        .free                   = block_job_free,
+        .user_resume            = block_job_user_resume,
+        .run                    = mirror_run,
+        .prepare                = mirror_prepare,
+        .abort                  = mirror_abort,
+        .pause                  = mirror_pause,
+        .complete               = mirror_complete,
+        .cancel                 = mirror_cancel,
+    },
+    .drained_poll           = mirror_drained_poll,
+};
+
+static const BlockJobDriver commit_active_job_driver = {
+    .job_driver = {
+        .instance_size          = sizeof(MirrorBlockJob),
+        .job_type               = JOB_TYPE_COMMIT,
+        .free                   = block_job_free,
+        .user_resume            = block_job_user_resume,
+        .run                    = mirror_run,
+        .prepare                = mirror_prepare,
+        .abort                  = mirror_abort,
+        .pause                  = mirror_pause,
+        .complete               = mirror_complete,
+        .cancel                 = commit_active_cancel,
+    },
+    .drained_poll           = mirror_drained_poll,
+};
+
+static void coroutine_fn
+do_sync_target_write(MirrorBlockJob *job, MirrorMethod method,
+                     uint64_t offset, uint64_t bytes,
+                     QEMUIOVector *qiov, int flags)
+{
+    int ret;
+    size_t qiov_offset = 0;
+    int64_t bitmap_offset, bitmap_end;
+
+    if (!QEMU_IS_ALIGNED(offset, job->granularity) &&
+        bdrv_dirty_bitmap_get(job->dirty_bitmap, offset))
+    {
+            /*
+             * Dirty unaligned padding: ignore it.
+             *
+             * Reasoning:
+             * 1. If we copy it, we can't reset corresponding bit in
+             *    dirty_bitmap as there may be some "dirty" bytes still not
+             *    copied.
+             * 2. It's already dirty, so skipping it we don't diverge mirror
+             *    progress.
+             *
+             * Note, that because of this, guest write may have no contribution
+             * into mirror converge, but that's not bad, as we have background
+             * process of mirroring. If under some bad circumstances (high guest
+             * IO load) background process starve, we will not converge anyway,
+             * even if each write will contribute, as guest is not guaranteed to
+             * rewrite the whole disk.
+             */
+            qiov_offset = QEMU_ALIGN_UP(offset, job->granularity) - offset;
+            if (bytes <= qiov_offset) {
+                /* nothing to do after shrink */
+                return;
+            }
+            offset += qiov_offset;
+            bytes -= qiov_offset;
+    }
+
+    if (!QEMU_IS_ALIGNED(offset + bytes, job->granularity) &&
+        bdrv_dirty_bitmap_get(job->dirty_bitmap, offset + bytes - 1))
+    {
+        uint64_t tail = (offset + bytes) % job->granularity;
+
+        if (bytes <= tail) {
+            /* nothing to do after shrink */
+            return;
+        }
+        bytes -= tail;
+    }
+
+    /*
+     * Tails are either clean or shrunk, so for bitmap resetting
+     * we safely align the range down.
+     */
+    bitmap_offset = QEMU_ALIGN_UP(offset, job->granularity);
+    bitmap_end = QEMU_ALIGN_DOWN(offset + bytes, job->granularity);
+    if (bitmap_offset < bitmap_end) {
+        bdrv_reset_dirty_bitmap(job->dirty_bitmap, bitmap_offset,
+                                bitmap_end - bitmap_offset);
+    }
+
+    job_progress_increase_remaining(&job->common.job, bytes);
+
+    switch (method) {
+    case MIRROR_METHOD_COPY:
+        ret = blk_co_pwritev_part(job->target, offset, bytes,
+                                  qiov, qiov_offset, flags);
+        break;
+
+    case MIRROR_METHOD_ZERO:
+        assert(!qiov);
+        ret = blk_co_pwrite_zeroes(job->target, offset, bytes, flags);
+        break;
+
+    case MIRROR_METHOD_DISCARD:
+        assert(!qiov);
+        ret = blk_co_pdiscard(job->target, offset, bytes);
+        break;
+
+    default:
+        abort();
+    }
+
+    if (ret >= 0) {
+        job_progress_update(&job->common.job, bytes);
+    } else {
+        BlockErrorAction action;
+
+        /*
+         * We failed, so we should mark dirty the whole area, aligned up.
+         * Note that we don't care about shrunk tails if any: they were dirty
+         * at function start, and they must be still dirty, as we've locked
+         * the region for in-flight op.
+         */
+        bitmap_offset = QEMU_ALIGN_DOWN(offset, job->granularity);
+        bitmap_end = QEMU_ALIGN_UP(offset + bytes, job->granularity);
+        bdrv_set_dirty_bitmap(job->dirty_bitmap, bitmap_offset,
+                              bitmap_end - bitmap_offset);
+        job->actively_synced = false;
+
+        action = mirror_error_action(job, false, -ret);
+        if (action == BLOCK_ERROR_ACTION_REPORT) {
+            if (!job->ret) {
+                job->ret = ret;
+            }
+        }
+    }
+}
+
+static MirrorOp *coroutine_fn active_write_prepare(MirrorBlockJob *s,
+                                                   uint64_t offset,
+                                                   uint64_t bytes)
+{
+    MirrorOp *op;
+    uint64_t start_chunk = offset / s->granularity;
+    uint64_t end_chunk = DIV_ROUND_UP(offset + bytes, s->granularity);
+
+    op = g_new(MirrorOp, 1);
+    *op = (MirrorOp){
+        .s                  = s,
+        .offset             = offset,
+        .bytes              = bytes,
+        .is_active_write    = true,
+        .is_in_flight       = true,
+        .co                 = qemu_coroutine_self(),
+    };
+    qemu_co_queue_init(&op->waiting_requests);
+    QTAILQ_INSERT_TAIL(&s->ops_in_flight, op, next);
+
+    s->in_active_write_counter++;
+
+    mirror_wait_on_conflicts(op, s, offset, bytes);
+
+    bitmap_set(s->in_flight_bitmap, start_chunk, end_chunk - start_chunk);
+
+    return op;
+}
+
+static void coroutine_fn active_write_settle(MirrorOp *op)
+{
+    uint64_t start_chunk = op->offset / op->s->granularity;
+    uint64_t end_chunk = DIV_ROUND_UP(op->offset + op->bytes,
+                                      op->s->granularity);
+
+    if (!--op->s->in_active_write_counter && op->s->actively_synced) {
+        BdrvChild *source = op->s->mirror_top_bs->backing;
+
+        if (QLIST_FIRST(&source->bs->parents) == source &&
+            QLIST_NEXT(source, next_parent) == NULL)
+        {
+            /* Assert that we are back in sync once all active write
+             * operations are settled.
+             * Note that we can only assert this if the mirror node
+             * is the source node's only parent. */
+            assert(!bdrv_get_dirty_count(op->s->dirty_bitmap));
+        }
+    }
+    bitmap_clear(op->s->in_flight_bitmap, start_chunk, end_chunk - start_chunk);
+    QTAILQ_REMOVE(&op->s->ops_in_flight, op, next);
+    qemu_co_queue_restart_all(&op->waiting_requests);
+    g_free(op);
+}
+
+static int coroutine_fn bdrv_mirror_top_preadv(BlockDriverState *bs,
+    int64_t offset, int64_t bytes, QEMUIOVector *qiov, BdrvRequestFlags flags)
+{
+    return bdrv_co_preadv(bs->backing, offset, bytes, qiov, flags);
+}
+
+static int coroutine_fn bdrv_mirror_top_do_write(BlockDriverState *bs,
+    MirrorMethod method, uint64_t offset, uint64_t bytes, QEMUIOVector *qiov,
+    int flags)
+{
+    MirrorOp *op = NULL;
+    MirrorBDSOpaque *s = bs->opaque;
+    int ret = 0;
+    bool copy_to_target;
+
+    copy_to_target = s->job->ret >= 0 &&
+                     !job_is_cancelled(&s->job->common.job) &&
+                     s->job->copy_mode == MIRROR_COPY_MODE_WRITE_BLOCKING;
+
+    if (copy_to_target) {
+        op = active_write_prepare(s->job, offset, bytes);
+    }
+
+    switch (method) {
+    case MIRROR_METHOD_COPY:
+        ret = bdrv_co_pwritev(bs->backing, offset, bytes, qiov, flags);
+        break;
+
+    case MIRROR_METHOD_ZERO:
+        ret = bdrv_co_pwrite_zeroes(bs->backing, offset, bytes, flags);
+        break;
+
+    case MIRROR_METHOD_DISCARD:
+        ret = bdrv_co_pdiscard(bs->backing, offset, bytes);
+        break;
+
+    default:
+        abort();
+    }
+
+    if (ret < 0) {
+        goto out;
+    }
+
+    if (copy_to_target) {
+        do_sync_target_write(s->job, method, offset, bytes, qiov, flags);
+    }
+
+out:
+    if (copy_to_target) {
+        active_write_settle(op);
+    }
+    return ret;
+}
+
+static int coroutine_fn bdrv_mirror_top_pwritev(BlockDriverState *bs,
+    int64_t offset, int64_t bytes, QEMUIOVector *qiov, BdrvRequestFlags flags)
+{
+    MirrorBDSOpaque *s = bs->opaque;
+    QEMUIOVector bounce_qiov;
+    void *bounce_buf;
+    int ret = 0;
+    bool copy_to_target;
+
+    copy_to_target = s->job->ret >= 0 &&
+                     !job_is_cancelled(&s->job->common.job) &&
+                     s->job->copy_mode == MIRROR_COPY_MODE_WRITE_BLOCKING;
+
+    if (copy_to_target) {
+        /* The guest might concurrently modify the data to write; but
+         * the data on source and destination must match, so we have
+         * to use a bounce buffer if we are going to write to the
+         * target now. */
+        bounce_buf = qemu_blockalign(bs, bytes);
+        iov_to_buf_full(qiov->iov, qiov->niov, 0, bounce_buf, bytes);
+
+        qemu_iovec_init(&bounce_qiov, 1);
+        qemu_iovec_add(&bounce_qiov, bounce_buf, bytes);
+        qiov = &bounce_qiov;
+    }
+
+    ret = bdrv_mirror_top_do_write(bs, MIRROR_METHOD_COPY, offset, bytes, qiov,
+                                   flags);
+
+    if (copy_to_target) {
+        qemu_iovec_destroy(&bounce_qiov);
+        qemu_vfree(bounce_buf);
+    }
+
+    return ret;
+}
+
+static int coroutine_fn bdrv_mirror_top_flush(BlockDriverState *bs)
+{
+    if (bs->backing == NULL) {
+        /* we can be here after failed bdrv_append in mirror_start_job */
+        return 0;
+    }
+    return bdrv_co_flush(bs->backing->bs);
+}
+
+static int coroutine_fn bdrv_mirror_top_pwrite_zeroes(BlockDriverState *bs,
+    int64_t offset, int64_t bytes, BdrvRequestFlags flags)
+{
+    return bdrv_mirror_top_do_write(bs, MIRROR_METHOD_ZERO, offset, bytes, NULL,
+                                    flags);
+}
+
+static int coroutine_fn bdrv_mirror_top_pdiscard(BlockDriverState *bs,
+    int64_t offset, int64_t bytes)
+{
+    return bdrv_mirror_top_do_write(bs, MIRROR_METHOD_DISCARD, offset, bytes,
+                                    NULL, 0);
+}
+
+static void bdrv_mirror_top_refresh_filename(BlockDriverState *bs)
+{
+    if (bs->backing == NULL) {
+        /* we can be here after failed bdrv_attach_child in
+         * bdrv_set_backing_hd */
+        return;
+    }
+    pstrcpy(bs->exact_filename, sizeof(bs->exact_filename),
+            bs->backing->bs->filename);
+}
+
+static void bdrv_mirror_top_child_perm(BlockDriverState *bs, BdrvChild *c,
+                                       BdrvChildRole role,
+                                       BlockReopenQueue *reopen_queue,
+                                       uint64_t perm, uint64_t shared,
+                                       uint64_t *nperm, uint64_t *nshared)
+{
+    MirrorBDSOpaque *s = bs->opaque;
+
+    if (s->stop) {
+        /*
+         * If the job is to be stopped, we do not need to forward
+         * anything to the real image.
+         */
+        *nperm = 0;
+        *nshared = BLK_PERM_ALL;
+        return;
+    }
+
+    bdrv_default_perms(bs, c, role, reopen_queue,
+                       perm, shared, nperm, nshared);
+
+    if (s->is_commit) {
+        /*
+         * For commit jobs, we cannot take CONSISTENT_READ, because
+         * that permission is unshared for everything above the base
+         * node (except for filters on the base node).
+         * We also have to force-share the WRITE permission, or
+         * otherwise we would block ourselves at the base node (if
+         * writes are blocked for a node, they are also blocked for
+         * its backing file).
+         * (We could also share RESIZE, because it may be needed for
+         * the target if its size is less than the top node's; but
+         * bdrv_default_perms_for_cow() automatically shares RESIZE
+         * for backing nodes if WRITE is shared, so there is no need
+         * to do it here.)
+         */
+        *nperm &= ~BLK_PERM_CONSISTENT_READ;
+        *nshared |= BLK_PERM_WRITE;
+    }
+}
+
+/* Dummy node that provides consistent read to its users without requiring it
+ * from its backing file and that allows writes on the backing file chain. */
+static BlockDriver bdrv_mirror_top = {
+    .format_name                = "mirror_top",
+    .bdrv_co_preadv             = bdrv_mirror_top_preadv,
+    .bdrv_co_pwritev            = bdrv_mirror_top_pwritev,
+    .bdrv_co_pwrite_zeroes      = bdrv_mirror_top_pwrite_zeroes,
+    .bdrv_co_pdiscard           = bdrv_mirror_top_pdiscard,
+    .bdrv_co_flush              = bdrv_mirror_top_flush,
+    .bdrv_refresh_filename      = bdrv_mirror_top_refresh_filename,
+    .bdrv_child_perm            = bdrv_mirror_top_child_perm,
+
+    .is_filter                  = true,
+};
+
+static BlockJob *mirror_start_job(
+                             const char *job_id, BlockDriverState *bs,
+                             int creation_flags, BlockDriverState *target,
+                             const char *replaces, int64_t speed,
+                             uint32_t granularity, int64_t buf_size,
+                             BlockMirrorBackingMode backing_mode,
+                             bool zero_target,
+                             BlockdevOnError on_source_error,
+                             BlockdevOnError on_target_error,
+                             bool unmap,
+                             BlockCompletionFunc *cb,
+                             void *opaque,
+                             const BlockJobDriver *driver,
+                             bool is_none_mode, BlockDriverState *base,
+                             bool auto_complete, const char *filter_node_name,
+                             bool is_mirror, MirrorCopyMode copy_mode,
+                             Error **errp)
+{
+    MirrorBlockJob *s;
+    MirrorBDSOpaque *bs_opaque;
+    BlockDriverState *mirror_top_bs;
+    bool target_is_backing;
+    uint64_t target_perms, target_shared_perms;
+    int ret;
+
+    if (granularity == 0) {
+        granularity = bdrv_get_default_bitmap_granularity(target);
+    }
+
+    assert(is_power_of_2(granularity));
+
+    if (buf_size < 0) {
+        error_setg(errp, "Invalid parameter 'buf-size'");
+        return NULL;
+    }
+
+    if (buf_size == 0) {
+        buf_size = DEFAULT_MIRROR_BUF_SIZE;
+    }
+
+    if (bdrv_skip_filters(bs) == bdrv_skip_filters(target)) {
+        error_setg(errp, "Can't mirror node into itself");
+        return NULL;
+    }
+
+    target_is_backing = bdrv_chain_contains(bs, target);
+
+    /* In the case of active commit, add dummy driver to provide consistent
+     * reads on the top, while disabling it in the intermediate nodes, and make
+     * the backing chain writable. */
+    mirror_top_bs = bdrv_new_open_driver(&bdrv_mirror_top, filter_node_name,
+                                         BDRV_O_RDWR, errp);
+    if (mirror_top_bs == NULL) {
+        return NULL;
+    }
+    if (!filter_node_name) {
+        mirror_top_bs->implicit = true;
+    }
+
+    /* So that we can always drop this node */
+    mirror_top_bs->never_freeze = true;
+
+    mirror_top_bs->total_sectors = bs->total_sectors;
+    mirror_top_bs->supported_write_flags = BDRV_REQ_WRITE_UNCHANGED;
+    mirror_top_bs->supported_zero_flags = BDRV_REQ_WRITE_UNCHANGED |
+                                          BDRV_REQ_NO_FALLBACK;
+    bs_opaque = g_new0(MirrorBDSOpaque, 1);
+    mirror_top_bs->opaque = bs_opaque;
+
+    bs_opaque->is_commit = target_is_backing;
+
+    bdrv_drained_begin(bs);
+    ret = bdrv_append(mirror_top_bs, bs, errp);
+    bdrv_drained_end(bs);
+
+    if (ret < 0) {
+        bdrv_unref(mirror_top_bs);
+        return NULL;
+    }
+
+    /* Make sure that the source is not resized while the job is running */
+    s = block_job_create(job_id, driver, NULL, mirror_top_bs,
+                         BLK_PERM_CONSISTENT_READ,
+                         BLK_PERM_CONSISTENT_READ | BLK_PERM_WRITE_UNCHANGED |
+                         BLK_PERM_WRITE | BLK_PERM_GRAPH_MOD, speed,
+                         creation_flags, cb, opaque, errp);
+    if (!s) {
+        goto fail;
+    }
+    bs_opaque->job = s;
+
+    /* The block job now has a reference to this node */
+    bdrv_unref(mirror_top_bs);
+
+    s->mirror_top_bs = mirror_top_bs;
+
+    /* No resize for the target either; while the mirror is still running, a
+     * consistent read isn't necessarily possible. We could possibly allow
+     * writes and graph modifications, though it would likely defeat the
+     * purpose of a mirror, so leave them blocked for now.
+     *
+     * In the case of active commit, things look a bit different, though,
+     * because the target is an already populated backing file in active use.
+     * We can allow anything except resize there.*/
+
+    target_perms = BLK_PERM_WRITE;
+    target_shared_perms = BLK_PERM_WRITE_UNCHANGED;
+
+    if (target_is_backing) {
+        int64_t bs_size, target_size;
+        bs_size = bdrv_getlength(bs);
+        if (bs_size < 0) {
+            error_setg_errno(errp, -bs_size,
+                             "Could not inquire top image size");
+            goto fail;
+        }
+
+        target_size = bdrv_getlength(target);
+        if (target_size < 0) {
+            error_setg_errno(errp, -target_size,
+                             "Could not inquire base image size");
+            goto fail;
+        }
+
+        if (target_size < bs_size) {
+            target_perms |= BLK_PERM_RESIZE;
+        }
+
+        target_shared_perms |= BLK_PERM_CONSISTENT_READ
+                            |  BLK_PERM_WRITE
+                            |  BLK_PERM_GRAPH_MOD;
+    } else if (bdrv_chain_contains(bs, bdrv_skip_filters(target))) {
+        /*
+         * We may want to allow this in the future, but it would
+         * require taking some extra care.
+         */
+        error_setg(errp, "Cannot mirror to a filter on top of a node in the "
+                   "source's backing chain");
+        goto fail;
+    }
+
+    if (backing_mode != MIRROR_LEAVE_BACKING_CHAIN) {
+        target_perms |= BLK_PERM_GRAPH_MOD;
+    }
+
+    s->target = blk_new(s->common.job.aio_context,
+                        target_perms, target_shared_perms);
+    ret = blk_insert_bs(s->target, target, errp);
+    if (ret < 0) {
+        goto fail;
+    }
+    if (is_mirror) {
+        /* XXX: Mirror target could be a NBD server of target QEMU in the case
+         * of non-shared block migration. To allow migration completion, we
+         * have to allow "inactivate" of the target BB.  When that happens, we
+         * know the job is drained, and the vcpus are stopped, so no write
+         * operation will be performed. Block layer already has assertions to
+         * ensure that. */
+        blk_set_force_allow_inactivate(s->target);
+    }
+    blk_set_allow_aio_context_change(s->target, true);
+    blk_set_disable_request_queuing(s->target, true);
+
+    s->replaces = g_strdup(replaces);
+    s->on_source_error = on_source_error;
+    s->on_target_error = on_target_error;
+    s->is_none_mode = is_none_mode;
+    s->backing_mode = backing_mode;
+    s->zero_target = zero_target;
+    s->copy_mode = copy_mode;
+    s->base = base;
+    s->base_overlay = bdrv_find_overlay(bs, base);
+    s->granularity = granularity;
+    s->buf_size = ROUND_UP(buf_size, granularity);
+    s->unmap = unmap;
+    if (auto_complete) {
+        s->should_complete = true;
+    }
+
+    s->dirty_bitmap = bdrv_create_dirty_bitmap(bs, granularity, NULL, errp);
+    if (!s->dirty_bitmap) {
+        goto fail;
+    }
+    if (s->copy_mode == MIRROR_COPY_MODE_WRITE_BLOCKING) {
+        bdrv_disable_dirty_bitmap(s->dirty_bitmap);
+    }
+
+    ret = block_job_add_bdrv(&s->common, "source", bs, 0,
+                             BLK_PERM_WRITE_UNCHANGED | BLK_PERM_WRITE |
+                             BLK_PERM_CONSISTENT_READ,
+                             errp);
+    if (ret < 0) {
+        goto fail;
+    }
+
+    /* Required permissions are already taken with blk_new() */
+    block_job_add_bdrv(&s->common, "target", target, 0, BLK_PERM_ALL,
+                       &error_abort);
+
+    /* In commit_active_start() all intermediate nodes disappear, so
+     * any jobs in them must be blocked */
+    if (target_is_backing) {
+        BlockDriverState *iter, *filtered_target;
+        uint64_t iter_shared_perms;
+
+        /*
+         * The topmost node with
+         * bdrv_skip_filters(filtered_target) == bdrv_skip_filters(target)
+         */
+        filtered_target = bdrv_cow_bs(bdrv_find_overlay(bs, target));
+
+        assert(bdrv_skip_filters(filtered_target) ==
+               bdrv_skip_filters(target));
+
+        /*
+         * XXX BLK_PERM_WRITE needs to be allowed so we don't block
+         * ourselves at s->base (if writes are blocked for a node, they are
+         * also blocked for its backing file). The other options would be a
+         * second filter driver above s->base (== target).
+         */
+        iter_shared_perms = BLK_PERM_WRITE_UNCHANGED | BLK_PERM_WRITE;
+
+        for (iter = bdrv_filter_or_cow_bs(bs); iter != target;
+             iter = bdrv_filter_or_cow_bs(iter))
+        {
+            if (iter == filtered_target) {
+                /*
+                 * From here on, all nodes are filters on the base.
+                 * This allows us to share BLK_PERM_CONSISTENT_READ.
+                 */
+                iter_shared_perms |= BLK_PERM_CONSISTENT_READ;
+            }
+
+            ret = block_job_add_bdrv(&s->common, "intermediate node", iter, 0,
+                                     iter_shared_perms, errp);
+            if (ret < 0) {
+                goto fail;
+            }
+        }
+
+        if (bdrv_freeze_backing_chain(mirror_top_bs, target, errp) < 0) {
+            goto fail;
+        }
+    }
+
+    QTAILQ_INIT(&s->ops_in_flight);
+
+    trace_mirror_start(bs, s, opaque);
+    job_start(&s->common.job);
+
+    return &s->common;
+
+fail:
+    if (s) {
+        /* Make sure this BDS does not go away until we have completed the graph
+         * changes below */
+        bdrv_ref(mirror_top_bs);
+
+        g_free(s->replaces);
+        blk_unref(s->target);
+        bs_opaque->job = NULL;
+        if (s->dirty_bitmap) {
+            bdrv_release_dirty_bitmap(s->dirty_bitmap);
+        }
+        job_early_fail(&s->common.job);
+    }
+
+    bs_opaque->stop = true;
+    bdrv_child_refresh_perms(mirror_top_bs, mirror_top_bs->backing,
+                             &error_abort);
+    bdrv_replace_node(mirror_top_bs, mirror_top_bs->backing->bs, &error_abort);
+
+    bdrv_unref(mirror_top_bs);
+
+    return NULL;
+}
+
+void mirror_start(const char *job_id, BlockDriverState *bs,
+                  BlockDriverState *target, const char *replaces,
+                  int creation_flags, int64_t speed,
+                  uint32_t granularity, int64_t buf_size,
+                  MirrorSyncMode mode, BlockMirrorBackingMode backing_mode,
+                  bool zero_target,
+                  BlockdevOnError on_source_error,
+                  BlockdevOnError on_target_error,
+                  bool unmap, const char *filter_node_name,
+                  MirrorCopyMode copy_mode, Error **errp)
+{
+    bool is_none_mode;
+    BlockDriverState *base;
+
+    if ((mode == MIRROR_SYNC_MODE_INCREMENTAL) ||
+        (mode == MIRROR_SYNC_MODE_BITMAP)) {
+        error_setg(errp, "Sync mode '%s' not supported",
+                   MirrorSyncMode_str(mode));
+        return;
+    }
+    is_none_mode = mode == MIRROR_SYNC_MODE_NONE;
+    base = mode == MIRROR_SYNC_MODE_TOP ? bdrv_backing_chain_next(bs) : NULL;
+    mirror_start_job(job_id, bs, creation_flags, target, replaces,
+                     speed, granularity, buf_size, backing_mode, zero_target,
+                     on_source_error, on_target_error, unmap, NULL, NULL,
+                     &mirror_job_driver, is_none_mode, base, false,
+                     filter_node_name, true, copy_mode, errp);
+}
+
+BlockJob *commit_active_start(const char *job_id, BlockDriverState *bs,
+                              BlockDriverState *base, int creation_flags,
+                              int64_t speed, BlockdevOnError on_error,
+                              const char *filter_node_name,
+                              BlockCompletionFunc *cb, void *opaque,
+                              bool auto_complete, Error **errp)
+{
+    bool base_read_only;
+    BlockJob *job;
+
+    base_read_only = bdrv_is_read_only(base);
+
+    if (base_read_only) {
+        if (bdrv_reopen_set_read_only(base, false, errp) < 0) {
+            return NULL;
+        }
+    }
+
+    job = mirror_start_job(
+                     job_id, bs, creation_flags, base, NULL, speed, 0, 0,
+                     MIRROR_LEAVE_BACKING_CHAIN, false,
+                     on_error, on_error, true, cb, opaque,
+                     &commit_active_job_driver, false, base, auto_complete,
+                     filter_node_name, false, MIRROR_COPY_MODE_BACKGROUND,
+                     errp);
+    if (!job) {
+        goto error_restore_flags;
+    }
+
+    return job;
+
+error_restore_flags:
+    /* ignore error and errp for bdrv_reopen, because we want to propagate
+     * the original error */
+    if (base_read_only) {
+        bdrv_reopen_set_read_only(base, true, NULL);
+    }
+    return NULL;
+}