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, 2078 insertions, 0 deletions

diff --git a/block/nbd.c b/block/nbd.c
new file mode 100644
index 000000000..5ef462db1
--- /dev/null
+++ b/block/nbd.c
@@ -0,0 +1,2078 @@
+/*
+ * QEMU Block driver for  NBD
+ *
+ * Copyright (c) 2019 Virtuozzo International GmbH.
+ * Copyright (C) 2016 Red Hat, Inc.
+ * Copyright (C) 2008 Bull S.A.S.
+ *     Author: Laurent Vivier <Laurent.Vivier@bull.net>
+ *
+ * Some parts:
+ *    Copyright (C) 2007 Anthony Liguori <anthony@codemonkey.ws>
+ *
+ * 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 "trace.h"
+#include "qemu/uri.h"
+#include "qemu/option.h"
+#include "qemu/cutils.h"
+#include "qemu/main-loop.h"
+#include "qemu/atomic.h"
+
+#include "qapi/qapi-visit-sockets.h"
+#include "qapi/qmp/qstring.h"
+#include "qapi/clone-visitor.h"
+
+#include "block/qdict.h"
+#include "block/nbd.h"
+#include "block/block_int.h"
+#include "block/coroutines.h"
+
+#include "qemu/yank.h"
+
+#define EN_OPTSTR ":exportname="
+#define MAX_NBD_REQUESTS    16
+
+#define HANDLE_TO_INDEX(bs, handle) ((handle) ^ (uint64_t)(intptr_t)(bs))
+#define INDEX_TO_HANDLE(bs, index)  ((index)  ^ (uint64_t)(intptr_t)(bs))
+
+typedef struct {
+    Coroutine *coroutine;
+    uint64_t offset;        /* original offset of the request */
+    bool receiving;         /* sleeping in the yield in nbd_receive_replies */
+    bool reply_possible;    /* reply header not yet received */
+} NBDClientRequest;
+
+typedef enum NBDClientState {
+    NBD_CLIENT_CONNECTING_WAIT,
+    NBD_CLIENT_CONNECTING_NOWAIT,
+    NBD_CLIENT_CONNECTED,
+    NBD_CLIENT_QUIT
+} NBDClientState;
+
+typedef struct BDRVNBDState {
+    QIOChannel *ioc; /* The current I/O channel */
+    NBDExportInfo info;
+
+    CoMutex send_mutex;
+    CoQueue free_sema;
+
+    CoMutex receive_mutex;
+    int in_flight;
+    NBDClientState state;
+
+    QEMUTimer *reconnect_delay_timer;
+
+    NBDClientRequest requests[MAX_NBD_REQUESTS];
+    NBDReply reply;
+    BlockDriverState *bs;
+
+    /* Connection parameters */
+    uint32_t reconnect_delay;
+    SocketAddress *saddr;
+    char *export, *tlscredsid;
+    QCryptoTLSCreds *tlscreds;
+    const char *hostname;
+    char *x_dirty_bitmap;
+    bool alloc_depth;
+
+    NBDClientConnection *conn;
+} BDRVNBDState;
+
+static void nbd_yank(void *opaque);
+
+static void nbd_clear_bdrvstate(BlockDriverState *bs)
+{
+    BDRVNBDState *s = (BDRVNBDState *)bs->opaque;
+
+    nbd_client_connection_release(s->conn);
+    s->conn = NULL;
+
+    yank_unregister_instance(BLOCKDEV_YANK_INSTANCE(bs->node_name));
+
+    object_unref(OBJECT(s->tlscreds));
+    qapi_free_SocketAddress(s->saddr);
+    s->saddr = NULL;
+    g_free(s->export);
+    s->export = NULL;
+    g_free(s->tlscredsid);
+    s->tlscredsid = NULL;
+    g_free(s->x_dirty_bitmap);
+    s->x_dirty_bitmap = NULL;
+}
+
+static bool nbd_client_connected(BDRVNBDState *s)
+{
+    return qatomic_load_acquire(&s->state) == NBD_CLIENT_CONNECTED;
+}
+
+static bool nbd_recv_coroutine_wake_one(NBDClientRequest *req)
+{
+    if (req->receiving) {
+        req->receiving = false;
+        aio_co_wake(req->coroutine);
+        return true;
+    }
+
+    return false;
+}
+
+static void nbd_recv_coroutines_wake(BDRVNBDState *s, bool all)
+{
+    int i;
+
+    for (i = 0; i < MAX_NBD_REQUESTS; i++) {
+        if (nbd_recv_coroutine_wake_one(&s->requests[i]) && !all) {
+            return;
+        }
+    }
+}
+
+static void nbd_channel_error(BDRVNBDState *s, int ret)
+{
+    if (nbd_client_connected(s)) {
+        qio_channel_shutdown(s->ioc, QIO_CHANNEL_SHUTDOWN_BOTH, NULL);
+    }
+
+    if (ret == -EIO) {
+        if (nbd_client_connected(s)) {
+            s->state = s->reconnect_delay ? NBD_CLIENT_CONNECTING_WAIT :
+                                            NBD_CLIENT_CONNECTING_NOWAIT;
+        }
+    } else {
+        s->state = NBD_CLIENT_QUIT;
+    }
+
+    nbd_recv_coroutines_wake(s, true);
+}
+
+static void reconnect_delay_timer_del(BDRVNBDState *s)
+{
+    if (s->reconnect_delay_timer) {
+        timer_free(s->reconnect_delay_timer);
+        s->reconnect_delay_timer = NULL;
+    }
+}
+
+static void reconnect_delay_timer_cb(void *opaque)
+{
+    BDRVNBDState *s = opaque;
+
+    if (qatomic_load_acquire(&s->state) == NBD_CLIENT_CONNECTING_WAIT) {
+        s->state = NBD_CLIENT_CONNECTING_NOWAIT;
+        nbd_co_establish_connection_cancel(s->conn);
+        while (qemu_co_enter_next(&s->free_sema, NULL)) {
+            /* Resume all queued requests */
+        }
+    }
+
+    reconnect_delay_timer_del(s);
+}
+
+static void reconnect_delay_timer_init(BDRVNBDState *s, uint64_t expire_time_ns)
+{
+    if (qatomic_load_acquire(&s->state) != NBD_CLIENT_CONNECTING_WAIT) {
+        return;
+    }
+
+    assert(!s->reconnect_delay_timer);
+    s->reconnect_delay_timer = aio_timer_new(bdrv_get_aio_context(s->bs),
+                                             QEMU_CLOCK_REALTIME,
+                                             SCALE_NS,
+                                             reconnect_delay_timer_cb, s);
+    timer_mod(s->reconnect_delay_timer, expire_time_ns);
+}
+
+static void nbd_teardown_connection(BlockDriverState *bs)
+{
+    BDRVNBDState *s = (BDRVNBDState *)bs->opaque;
+
+    assert(!s->in_flight);
+
+    if (s->ioc) {
+        qio_channel_shutdown(s->ioc, QIO_CHANNEL_SHUTDOWN_BOTH, NULL);
+        yank_unregister_function(BLOCKDEV_YANK_INSTANCE(s->bs->node_name),
+                                 nbd_yank, s->bs);
+        object_unref(OBJECT(s->ioc));
+        s->ioc = NULL;
+    }
+
+    s->state = NBD_CLIENT_QUIT;
+}
+
+static bool nbd_client_connecting(BDRVNBDState *s)
+{
+    NBDClientState state = qatomic_load_acquire(&s->state);
+    return state == NBD_CLIENT_CONNECTING_WAIT ||
+        state == NBD_CLIENT_CONNECTING_NOWAIT;
+}
+
+static bool nbd_client_connecting_wait(BDRVNBDState *s)
+{
+    return qatomic_load_acquire(&s->state) == NBD_CLIENT_CONNECTING_WAIT;
+}
+
+/*
+ * Update @bs with information learned during a completed negotiation process.
+ * Return failure if the server's advertised options are incompatible with the
+ * client's needs.
+ */
+static int nbd_handle_updated_info(BlockDriverState *bs, Error **errp)
+{
+    BDRVNBDState *s = (BDRVNBDState *)bs->opaque;
+    int ret;
+
+    if (s->x_dirty_bitmap) {
+        if (!s->info.base_allocation) {
+            error_setg(errp, "requested x-dirty-bitmap %s not found",
+                       s->x_dirty_bitmap);
+            return -EINVAL;
+        }
+        if (strcmp(s->x_dirty_bitmap, "qemu:allocation-depth") == 0) {
+            s->alloc_depth = true;
+        }
+    }
+
+    if (s->info.flags & NBD_FLAG_READ_ONLY) {
+        ret = bdrv_apply_auto_read_only(bs, "NBD export is read-only", errp);
+        if (ret < 0) {
+            return ret;
+        }
+    }
+
+    if (s->info.flags & NBD_FLAG_SEND_FUA) {
+        bs->supported_write_flags = BDRV_REQ_FUA;
+        bs->supported_zero_flags |= BDRV_REQ_FUA;
+    }
+
+    if (s->info.flags & NBD_FLAG_SEND_WRITE_ZEROES) {
+        bs->supported_zero_flags |= BDRV_REQ_MAY_UNMAP;
+        if (s->info.flags & NBD_FLAG_SEND_FAST_ZERO) {
+            bs->supported_zero_flags |= BDRV_REQ_NO_FALLBACK;
+        }
+    }
+
+    trace_nbd_client_handshake_success(s->export);
+
+    return 0;
+}
+
+int coroutine_fn nbd_co_do_establish_connection(BlockDriverState *bs,
+                                                Error **errp)
+{
+    BDRVNBDState *s = (BDRVNBDState *)bs->opaque;
+    int ret;
+    bool blocking = nbd_client_connecting_wait(s);
+
+    assert(!s->ioc);
+
+    s->ioc = nbd_co_establish_connection(s->conn, &s->info, blocking, errp);
+    if (!s->ioc) {
+        return -ECONNREFUSED;
+    }
+
+    yank_register_function(BLOCKDEV_YANK_INSTANCE(s->bs->node_name), nbd_yank,
+                           bs);
+
+    ret = nbd_handle_updated_info(s->bs, NULL);
+    if (ret < 0) {
+        /*
+         * We have connected, but must fail for other reasons.
+         * Send NBD_CMD_DISC as a courtesy to the server.
+         */
+        NBDRequest request = { .type = NBD_CMD_DISC };
+
+        nbd_send_request(s->ioc, &request);
+
+        yank_unregister_function(BLOCKDEV_YANK_INSTANCE(s->bs->node_name),
+                                 nbd_yank, bs);
+        object_unref(OBJECT(s->ioc));
+        s->ioc = NULL;
+
+        return ret;
+    }
+
+    qio_channel_set_blocking(s->ioc, false, NULL);
+    qio_channel_attach_aio_context(s->ioc, bdrv_get_aio_context(bs));
+
+    /* successfully connected */
+    s->state = NBD_CLIENT_CONNECTED;
+    qemu_co_queue_restart_all(&s->free_sema);
+
+    return 0;
+}
+
+/* called under s->send_mutex */
+static coroutine_fn void nbd_reconnect_attempt(BDRVNBDState *s)
+{
+    assert(nbd_client_connecting(s));
+    assert(s->in_flight == 0);
+
+    if (nbd_client_connecting_wait(s) && s->reconnect_delay &&
+        !s->reconnect_delay_timer)
+    {
+        /*
+         * It's first reconnect attempt after switching to
+         * NBD_CLIENT_CONNECTING_WAIT
+         */
+        reconnect_delay_timer_init(s,
+            qemu_clock_get_ns(QEMU_CLOCK_REALTIME) +
+            s->reconnect_delay * NANOSECONDS_PER_SECOND);
+    }
+
+    /*
+     * Now we are sure that nobody is accessing the channel, and no one will
+     * try until we set the state to CONNECTED.
+     */
+
+    /* Finalize previous connection if any */
+    if (s->ioc) {
+        qio_channel_detach_aio_context(QIO_CHANNEL(s->ioc));
+        yank_unregister_function(BLOCKDEV_YANK_INSTANCE(s->bs->node_name),
+                                 nbd_yank, s->bs);
+        object_unref(OBJECT(s->ioc));
+        s->ioc = NULL;
+    }
+
+    nbd_co_do_establish_connection(s->bs, NULL);
+}
+
+static coroutine_fn int nbd_receive_replies(BDRVNBDState *s, uint64_t handle)
+{
+    int ret;
+    uint64_t ind = HANDLE_TO_INDEX(s, handle), ind2;
+    QEMU_LOCK_GUARD(&s->receive_mutex);
+
+    while (true) {
+        if (s->reply.handle == handle) {
+            /* We are done */
+            return 0;
+        }
+
+        if (!nbd_client_connected(s)) {
+            return -EIO;
+        }
+
+        if (s->reply.handle != 0) {
+            /*
+             * Some other request is being handled now. It should already be
+             * woken by whoever set s->reply.handle (or never wait in this
+             * yield). So, we should not wake it here.
+             */
+            ind2 = HANDLE_TO_INDEX(s, s->reply.handle);
+            assert(!s->requests[ind2].receiving);
+
+            s->requests[ind].receiving = true;
+            qemu_co_mutex_unlock(&s->receive_mutex);
+
+            qemu_coroutine_yield();
+            /*
+             * We may be woken for 3 reasons:
+             * 1. From this function, executing in parallel coroutine, when our
+             *    handle is received.
+             * 2. From nbd_channel_error(), when connection is lost.
+             * 3. From nbd_co_receive_one_chunk(), when previous request is
+             *    finished and s->reply.handle set to 0.
+             * Anyway, it's OK to lock the mutex and go to the next iteration.
+             */
+
+            qemu_co_mutex_lock(&s->receive_mutex);
+            assert(!s->requests[ind].receiving);
+            continue;
+        }
+
+        /* We are under mutex and handle is 0. We have to do the dirty work. */
+        assert(s->reply.handle == 0);
+        ret = nbd_receive_reply(s->bs, s->ioc, &s->reply, NULL);
+        if (ret <= 0) {
+            ret = ret ? ret : -EIO;
+            nbd_channel_error(s, ret);
+            return ret;
+        }
+        if (nbd_reply_is_structured(&s->reply) && !s->info.structured_reply) {
+            nbd_channel_error(s, -EINVAL);
+            return -EINVAL;
+        }
+        if (s->reply.handle == handle) {
+            /* We are done */
+            return 0;
+        }
+        ind2 = HANDLE_TO_INDEX(s, s->reply.handle);
+        if (ind2 >= MAX_NBD_REQUESTS || !s->requests[ind2].reply_possible) {
+            nbd_channel_error(s, -EINVAL);
+            return -EINVAL;
+        }
+        nbd_recv_coroutine_wake_one(&s->requests[ind2]);
+    }
+}
+
+static int nbd_co_send_request(BlockDriverState *bs,
+                               NBDRequest *request,
+                               QEMUIOVector *qiov)
+{
+    BDRVNBDState *s = (BDRVNBDState *)bs->opaque;
+    int rc, i = -1;
+
+    qemu_co_mutex_lock(&s->send_mutex);
+
+    while (s->in_flight == MAX_NBD_REQUESTS ||
+           (!nbd_client_connected(s) && s->in_flight > 0))
+    {
+        qemu_co_queue_wait(&s->free_sema, &s->send_mutex);
+    }
+
+    if (nbd_client_connecting(s)) {
+        nbd_reconnect_attempt(s);
+    }
+
+    if (!nbd_client_connected(s)) {
+        rc = -EIO;
+        goto err;
+    }
+
+    s->in_flight++;
+
+    for (i = 0; i < MAX_NBD_REQUESTS; i++) {
+        if (s->requests[i].coroutine == NULL) {
+            break;
+        }
+    }
+
+    g_assert(qemu_in_coroutine());
+    assert(i < MAX_NBD_REQUESTS);
+
+    s->requests[i].coroutine = qemu_coroutine_self();
+    s->requests[i].offset = request->from;
+    s->requests[i].receiving = false;
+    s->requests[i].reply_possible = true;
+
+    request->handle = INDEX_TO_HANDLE(s, i);
+
+    assert(s->ioc);
+
+    if (qiov) {
+        qio_channel_set_cork(s->ioc, true);
+        rc = nbd_send_request(s->ioc, request);
+        if (nbd_client_connected(s) && rc >= 0) {
+            if (qio_channel_writev_all(s->ioc, qiov->iov, qiov->niov,
+                                       NULL) < 0) {
+                rc = -EIO;
+            }
+        } else if (rc >= 0) {
+            rc = -EIO;
+        }
+        qio_channel_set_cork(s->ioc, false);
+    } else {
+        rc = nbd_send_request(s->ioc, request);
+    }
+
+err:
+    if (rc < 0) {
+        nbd_channel_error(s, rc);
+        if (i != -1) {
+            s->requests[i].coroutine = NULL;
+            s->in_flight--;
+            qemu_co_queue_next(&s->free_sema);
+        }
+    }
+    qemu_co_mutex_unlock(&s->send_mutex);
+    return rc;
+}
+
+static inline uint16_t payload_advance16(uint8_t **payload)
+{
+    *payload += 2;
+    return lduw_be_p(*payload - 2);
+}
+
+static inline uint32_t payload_advance32(uint8_t **payload)
+{
+    *payload += 4;
+    return ldl_be_p(*payload - 4);
+}
+
+static inline uint64_t payload_advance64(uint8_t **payload)
+{
+    *payload += 8;
+    return ldq_be_p(*payload - 8);
+}
+
+static int nbd_parse_offset_hole_payload(BDRVNBDState *s,
+                                         NBDStructuredReplyChunk *chunk,
+                                         uint8_t *payload, uint64_t orig_offset,
+                                         QEMUIOVector *qiov, Error **errp)
+{
+    uint64_t offset;
+    uint32_t hole_size;
+
+    if (chunk->length != sizeof(offset) + sizeof(hole_size)) {
+        error_setg(errp, "Protocol error: invalid payload for "
+                         "NBD_REPLY_TYPE_OFFSET_HOLE");
+        return -EINVAL;
+    }
+
+    offset = payload_advance64(&payload);
+    hole_size = payload_advance32(&payload);
+
+    if (!hole_size || offset < orig_offset || hole_size > qiov->size ||
+        offset > orig_offset + qiov->size - hole_size) {
+        error_setg(errp, "Protocol error: server sent chunk exceeding requested"
+                         " region");
+        return -EINVAL;
+    }
+    if (s->info.min_block &&
+        !QEMU_IS_ALIGNED(hole_size, s->info.min_block)) {
+        trace_nbd_structured_read_compliance("hole");
+    }
+
+    qemu_iovec_memset(qiov, offset - orig_offset, 0, hole_size);
+
+    return 0;
+}
+
+/*
+ * nbd_parse_blockstatus_payload
+ * Based on our request, we expect only one extent in reply, for the
+ * base:allocation context.
+ */
+static int nbd_parse_blockstatus_payload(BDRVNBDState *s,
+                                         NBDStructuredReplyChunk *chunk,
+                                         uint8_t *payload, uint64_t orig_length,
+                                         NBDExtent *extent, Error **errp)
+{
+    uint32_t context_id;
+
+    /* The server succeeded, so it must have sent [at least] one extent */
+    if (chunk->length < sizeof(context_id) + sizeof(*extent)) {
+        error_setg(errp, "Protocol error: invalid payload for "
+                         "NBD_REPLY_TYPE_BLOCK_STATUS");
+        return -EINVAL;
+    }
+
+    context_id = payload_advance32(&payload);
+    if (s->info.context_id != context_id) {
+        error_setg(errp, "Protocol error: unexpected context id %d for "
+                         "NBD_REPLY_TYPE_BLOCK_STATUS, when negotiated context "
+                         "id is %d", context_id,
+                         s->info.context_id);
+        return -EINVAL;
+    }
+
+    extent->length = payload_advance32(&payload);
+    extent->flags = payload_advance32(&payload);
+
+    if (extent->length == 0) {
+        error_setg(errp, "Protocol error: server sent status chunk with "
+                   "zero length");
+        return -EINVAL;
+    }
+
+    /*
+     * A server sending unaligned block status is in violation of the
+     * protocol, but as qemu-nbd 3.1 is such a server (at least for
+     * POSIX files that are not a multiple of 512 bytes, since qemu
+     * rounds files up to 512-byte multiples but lseek(SEEK_HOLE)
+     * still sees an implicit hole beyond the real EOF), it's nicer to
+     * work around the misbehaving server. If the request included
+     * more than the final unaligned block, truncate it back to an
+     * aligned result; if the request was only the final block, round
+     * up to the full block and change the status to fully-allocated
+     * (always a safe status, even if it loses information).
+     */
+    if (s->info.min_block && !QEMU_IS_ALIGNED(extent->length,
+                                                   s->info.min_block)) {
+        trace_nbd_parse_blockstatus_compliance("extent length is unaligned");
+        if (extent->length > s->info.min_block) {
+            extent->length = QEMU_ALIGN_DOWN(extent->length,
+                                             s->info.min_block);
+        } else {
+            extent->length = s->info.min_block;
+            extent->flags = 0;
+        }
+    }
+
+    /*
+     * We used NBD_CMD_FLAG_REQ_ONE, so the server should not have
+     * sent us any more than one extent, nor should it have included
+     * status beyond our request in that extent. However, it's easy
+     * enough to ignore the server's noncompliance without killing the
+     * connection; just ignore trailing extents, and clamp things to
+     * the length of our request.
+     */
+    if (chunk->length > sizeof(context_id) + sizeof(*extent)) {
+        trace_nbd_parse_blockstatus_compliance("more than one extent");
+    }
+    if (extent->length > orig_length) {
+        extent->length = orig_length;
+        trace_nbd_parse_blockstatus_compliance("extent length too large");
+    }
+
+    /*
+     * HACK: if we are using x-dirty-bitmaps to access
+     * qemu:allocation-depth, treat all depths > 2 the same as 2,
+     * since nbd_client_co_block_status is only expecting the low two
+     * bits to be set.
+     */
+    if (s->alloc_depth && extent->flags > 2) {
+        extent->flags = 2;
+    }
+
+    return 0;
+}
+
+/*
+ * nbd_parse_error_payload
+ * on success @errp contains message describing nbd error reply
+ */
+static int nbd_parse_error_payload(NBDStructuredReplyChunk *chunk,
+                                   uint8_t *payload, int *request_ret,
+                                   Error **errp)
+{
+    uint32_t error;
+    uint16_t message_size;
+
+    assert(chunk->type & (1 << 15));
+
+    if (chunk->length < sizeof(error) + sizeof(message_size)) {
+        error_setg(errp,
+                   "Protocol error: invalid payload for structured error");
+        return -EINVAL;
+    }
+
+    error = nbd_errno_to_system_errno(payload_advance32(&payload));
+    if (error == 0) {
+        error_setg(errp, "Protocol error: server sent structured error chunk "
+                         "with error = 0");
+        return -EINVAL;
+    }
+
+    *request_ret = -error;
+    message_size = payload_advance16(&payload);
+
+    if (message_size > chunk->length - sizeof(error) - sizeof(message_size)) {
+        error_setg(errp, "Protocol error: server sent structured error chunk "
+                         "with incorrect message size");
+        return -EINVAL;
+    }
+
+    /* TODO: Add a trace point to mention the server complaint */
+
+    /* TODO handle ERROR_OFFSET */
+
+    return 0;
+}
+
+static int nbd_co_receive_offset_data_payload(BDRVNBDState *s,
+                                              uint64_t orig_offset,
+                                              QEMUIOVector *qiov, Error **errp)
+{
+    QEMUIOVector sub_qiov;
+    uint64_t offset;
+    size_t data_size;
+    int ret;
+    NBDStructuredReplyChunk *chunk = &s->reply.structured;
+
+    assert(nbd_reply_is_structured(&s->reply));
+
+    /* The NBD spec requires at least one byte of payload */
+    if (chunk->length <= sizeof(offset)) {
+        error_setg(errp, "Protocol error: invalid payload for "
+                         "NBD_REPLY_TYPE_OFFSET_DATA");
+        return -EINVAL;
+    }
+
+    if (nbd_read64(s->ioc, &offset, "OFFSET_DATA offset", errp) < 0) {
+        return -EIO;
+    }
+
+    data_size = chunk->length - sizeof(offset);
+    assert(data_size);
+    if (offset < orig_offset || data_size > qiov->size ||
+        offset > orig_offset + qiov->size - data_size) {
+        error_setg(errp, "Protocol error: server sent chunk exceeding requested"
+                         " region");
+        return -EINVAL;
+    }
+    if (s->info.min_block && !QEMU_IS_ALIGNED(data_size, s->info.min_block)) {
+        trace_nbd_structured_read_compliance("data");
+    }
+
+    qemu_iovec_init(&sub_qiov, qiov->niov);
+    qemu_iovec_concat(&sub_qiov, qiov, offset - orig_offset, data_size);
+    ret = qio_channel_readv_all(s->ioc, sub_qiov.iov, sub_qiov.niov, errp);
+    qemu_iovec_destroy(&sub_qiov);
+
+    return ret < 0 ? -EIO : 0;
+}
+
+#define NBD_MAX_MALLOC_PAYLOAD 1000
+static coroutine_fn int nbd_co_receive_structured_payload(
+        BDRVNBDState *s, void **payload, Error **errp)
+{
+    int ret;
+    uint32_t len;
+
+    assert(nbd_reply_is_structured(&s->reply));
+
+    len = s->reply.structured.length;
+
+    if (len == 0) {
+        return 0;
+    }
+
+    if (payload == NULL) {
+        error_setg(errp, "Unexpected structured payload");
+        return -EINVAL;
+    }
+
+    if (len > NBD_MAX_MALLOC_PAYLOAD) {
+        error_setg(errp, "Payload too large");
+        return -EINVAL;
+    }
+
+    *payload = g_new(char, len);
+    ret = nbd_read(s->ioc, *payload, len, "structured payload", errp);
+    if (ret < 0) {
+        g_free(*payload);
+        *payload = NULL;
+        return ret;
+    }
+
+    return 0;
+}
+
+/*
+ * nbd_co_do_receive_one_chunk
+ * for simple reply:
+ *   set request_ret to received reply error
+ *   if qiov is not NULL: read payload to @qiov
+ * for structured reply chunk:
+ *   if error chunk: read payload, set @request_ret, do not set @payload
+ *   else if offset_data chunk: read payload data to @qiov, do not set @payload
+ *   else: read payload to @payload
+ *
+ * If function fails, @errp contains corresponding error message, and the
+ * connection with the server is suspect.  If it returns 0, then the
+ * transaction succeeded (although @request_ret may be a negative errno
+ * corresponding to the server's error reply), and errp is unchanged.
+ */
+static coroutine_fn int nbd_co_do_receive_one_chunk(
+        BDRVNBDState *s, uint64_t handle, bool only_structured,
+        int *request_ret, QEMUIOVector *qiov, void **payload, Error **errp)
+{
+    int ret;
+    int i = HANDLE_TO_INDEX(s, handle);
+    void *local_payload = NULL;
+    NBDStructuredReplyChunk *chunk;
+
+    if (payload) {
+        *payload = NULL;
+    }
+    *request_ret = 0;
+
+    nbd_receive_replies(s, handle);
+    if (!nbd_client_connected(s)) {
+        error_setg(errp, "Connection closed");
+        return -EIO;
+    }
+    assert(s->ioc);
+
+    assert(s->reply.handle == handle);
+
+    if (nbd_reply_is_simple(&s->reply)) {
+        if (only_structured) {
+            error_setg(errp, "Protocol error: simple reply when structured "
+                             "reply chunk was expected");
+            return -EINVAL;
+        }
+
+        *request_ret = -nbd_errno_to_system_errno(s->reply.simple.error);
+        if (*request_ret < 0 || !qiov) {
+            return 0;
+        }
+
+        return qio_channel_readv_all(s->ioc, qiov->iov, qiov->niov,
+                                     errp) < 0 ? -EIO : 0;
+    }
+
+    /* handle structured reply chunk */
+    assert(s->info.structured_reply);
+    chunk = &s->reply.structured;
+
+    if (chunk->type == NBD_REPLY_TYPE_NONE) {
+        if (!(chunk->flags & NBD_REPLY_FLAG_DONE)) {
+            error_setg(errp, "Protocol error: NBD_REPLY_TYPE_NONE chunk without"
+                       " NBD_REPLY_FLAG_DONE flag set");
+            return -EINVAL;
+        }
+        if (chunk->length) {
+            error_setg(errp, "Protocol error: NBD_REPLY_TYPE_NONE chunk with"
+                       " nonzero length");
+            return -EINVAL;
+        }
+        return 0;
+    }
+
+    if (chunk->type == NBD_REPLY_TYPE_OFFSET_DATA) {
+        if (!qiov) {
+            error_setg(errp, "Unexpected NBD_REPLY_TYPE_OFFSET_DATA chunk");
+            return -EINVAL;
+        }
+
+        return nbd_co_receive_offset_data_payload(s, s->requests[i].offset,
+                                                  qiov, errp);
+    }
+
+    if (nbd_reply_type_is_error(chunk->type)) {
+        payload = &local_payload;
+    }
+
+    ret = nbd_co_receive_structured_payload(s, payload, errp);
+    if (ret < 0) {
+        return ret;
+    }
+
+    if (nbd_reply_type_is_error(chunk->type)) {
+        ret = nbd_parse_error_payload(chunk, local_payload, request_ret, errp);
+        g_free(local_payload);
+        return ret;
+    }
+
+    return 0;
+}
+
+/*
+ * nbd_co_receive_one_chunk
+ * Read reply, wake up connection_co and set s->quit if needed.
+ * Return value is a fatal error code or normal nbd reply error code
+ */
+static coroutine_fn int nbd_co_receive_one_chunk(
+        BDRVNBDState *s, uint64_t handle, bool only_structured,
+        int *request_ret, QEMUIOVector *qiov, NBDReply *reply, void **payload,
+        Error **errp)
+{
+    int ret = nbd_co_do_receive_one_chunk(s, handle, only_structured,
+                                          request_ret, qiov, payload, errp);
+
+    if (ret < 0) {
+        memset(reply, 0, sizeof(*reply));
+        nbd_channel_error(s, ret);
+    } else {
+        /* For assert at loop start in nbd_connection_entry */
+        *reply = s->reply;
+    }
+    s->reply.handle = 0;
+
+    nbd_recv_coroutines_wake(s, false);
+
+    return ret;
+}
+
+typedef struct NBDReplyChunkIter {
+    int ret;
+    int request_ret;
+    Error *err;
+    bool done, only_structured;
+} NBDReplyChunkIter;
+
+static void nbd_iter_channel_error(NBDReplyChunkIter *iter,
+                                   int ret, Error **local_err)
+{
+    assert(local_err && *local_err);
+    assert(ret < 0);
+
+    if (!iter->ret) {
+        iter->ret = ret;
+        error_propagate(&iter->err, *local_err);
+    } else {
+        error_free(*local_err);
+    }
+
+    *local_err = NULL;
+}
+
+static void nbd_iter_request_error(NBDReplyChunkIter *iter, int ret)
+{
+    assert(ret < 0);
+
+    if (!iter->request_ret) {
+        iter->request_ret = ret;
+    }
+}
+
+/*
+ * NBD_FOREACH_REPLY_CHUNK
+ * The pointer stored in @payload requires g_free() to free it.
+ */
+#define NBD_FOREACH_REPLY_CHUNK(s, iter, handle, structured, \
+                                qiov, reply, payload) \
+    for (iter = (NBDReplyChunkIter) { .only_structured = structured }; \
+         nbd_reply_chunk_iter_receive(s, &iter, handle, qiov, reply, payload);)
+
+/*
+ * nbd_reply_chunk_iter_receive
+ * The pointer stored in @payload requires g_free() to free it.
+ */
+static bool nbd_reply_chunk_iter_receive(BDRVNBDState *s,
+                                         NBDReplyChunkIter *iter,
+                                         uint64_t handle,
+                                         QEMUIOVector *qiov, NBDReply *reply,
+                                         void **payload)
+{
+    int ret, request_ret;
+    NBDReply local_reply;
+    NBDStructuredReplyChunk *chunk;
+    Error *local_err = NULL;
+    if (!nbd_client_connected(s)) {
+        error_setg(&local_err, "Connection closed");
+        nbd_iter_channel_error(iter, -EIO, &local_err);
+        goto break_loop;
+    }
+
+    if (iter->done) {
+        /* Previous iteration was last. */
+        goto break_loop;
+    }
+
+    if (reply == NULL) {
+        reply = &local_reply;
+    }
+
+    ret = nbd_co_receive_one_chunk(s, handle, iter->only_structured,
+                                   &request_ret, qiov, reply, payload,
+                                   &local_err);
+    if (ret < 0) {
+        nbd_iter_channel_error(iter, ret, &local_err);
+    } else if (request_ret < 0) {
+        nbd_iter_request_error(iter, request_ret);
+    }
+
+    /* Do not execute the body of NBD_FOREACH_REPLY_CHUNK for simple reply. */
+    if (nbd_reply_is_simple(reply) || !nbd_client_connected(s)) {
+        goto break_loop;
+    }
+
+    chunk = &reply->structured;
+    iter->only_structured = true;
+
+    if (chunk->type == NBD_REPLY_TYPE_NONE) {
+        /* NBD_REPLY_FLAG_DONE is already checked in nbd_co_receive_one_chunk */
+        assert(chunk->flags & NBD_REPLY_FLAG_DONE);
+        goto break_loop;
+    }
+
+    if (chunk->flags & NBD_REPLY_FLAG_DONE) {
+        /* This iteration is last. */
+        iter->done = true;
+    }
+
+    /* Execute the loop body */
+    return true;
+
+break_loop:
+    s->requests[HANDLE_TO_INDEX(s, handle)].coroutine = NULL;
+
+    qemu_co_mutex_lock(&s->send_mutex);
+    s->in_flight--;
+    qemu_co_queue_next(&s->free_sema);
+    qemu_co_mutex_unlock(&s->send_mutex);
+
+    return false;
+}
+
+static int nbd_co_receive_return_code(BDRVNBDState *s, uint64_t handle,
+                                      int *request_ret, Error **errp)
+{
+    NBDReplyChunkIter iter;
+
+    NBD_FOREACH_REPLY_CHUNK(s, iter, handle, false, NULL, NULL, NULL) {
+        /* nbd_reply_chunk_iter_receive does all the work */
+    }
+
+    error_propagate(errp, iter.err);
+    *request_ret = iter.request_ret;
+    return iter.ret;
+}
+
+static int nbd_co_receive_cmdread_reply(BDRVNBDState *s, uint64_t handle,
+                                        uint64_t offset, QEMUIOVector *qiov,
+                                        int *request_ret, Error **errp)
+{
+    NBDReplyChunkIter iter;
+    NBDReply reply;
+    void *payload = NULL;
+    Error *local_err = NULL;
+
+    NBD_FOREACH_REPLY_CHUNK(s, iter, handle, s->info.structured_reply,
+                            qiov, &reply, &payload)
+    {
+        int ret;
+        NBDStructuredReplyChunk *chunk = &reply.structured;
+
+        assert(nbd_reply_is_structured(&reply));
+
+        switch (chunk->type) {
+        case NBD_REPLY_TYPE_OFFSET_DATA:
+            /*
+             * special cased in nbd_co_receive_one_chunk, data is already
+             * in qiov
+             */
+            break;
+        case NBD_REPLY_TYPE_OFFSET_HOLE:
+            ret = nbd_parse_offset_hole_payload(s, &reply.structured, payload,
+                                                offset, qiov, &local_err);
+            if (ret < 0) {
+                nbd_channel_error(s, ret);
+                nbd_iter_channel_error(&iter, ret, &local_err);
+            }
+            break;
+        default:
+            if (!nbd_reply_type_is_error(chunk->type)) {
+                /* not allowed reply type */
+                nbd_channel_error(s, -EINVAL);
+                error_setg(&local_err,
+                           "Unexpected reply type: %d (%s) for CMD_READ",
+                           chunk->type, nbd_reply_type_lookup(chunk->type));
+                nbd_iter_channel_error(&iter, -EINVAL, &local_err);
+            }
+        }
+
+        g_free(payload);
+        payload = NULL;
+    }
+
+    error_propagate(errp, iter.err);
+    *request_ret = iter.request_ret;
+    return iter.ret;
+}
+
+static int nbd_co_receive_blockstatus_reply(BDRVNBDState *s,
+                                            uint64_t handle, uint64_t length,
+                                            NBDExtent *extent,
+                                            int *request_ret, Error **errp)
+{
+    NBDReplyChunkIter iter;
+    NBDReply reply;
+    void *payload = NULL;
+    Error *local_err = NULL;
+    bool received = false;
+
+    assert(!extent->length);
+    NBD_FOREACH_REPLY_CHUNK(s, iter, handle, false, NULL, &reply, &payload) {
+        int ret;
+        NBDStructuredReplyChunk *chunk = &reply.structured;
+
+        assert(nbd_reply_is_structured(&reply));
+
+        switch (chunk->type) {
+        case NBD_REPLY_TYPE_BLOCK_STATUS:
+            if (received) {
+                nbd_channel_error(s, -EINVAL);
+                error_setg(&local_err, "Several BLOCK_STATUS chunks in reply");
+                nbd_iter_channel_error(&iter, -EINVAL, &local_err);
+            }
+            received = true;
+
+            ret = nbd_parse_blockstatus_payload(s, &reply.structured,
+                                                payload, length, extent,
+                                                &local_err);
+            if (ret < 0) {
+                nbd_channel_error(s, ret);
+                nbd_iter_channel_error(&iter, ret, &local_err);
+            }
+            break;
+        default:
+            if (!nbd_reply_type_is_error(chunk->type)) {
+                nbd_channel_error(s, -EINVAL);
+                error_setg(&local_err,
+                           "Unexpected reply type: %d (%s) "
+                           "for CMD_BLOCK_STATUS",
+                           chunk->type, nbd_reply_type_lookup(chunk->type));
+                nbd_iter_channel_error(&iter, -EINVAL, &local_err);
+            }
+        }
+
+        g_free(payload);
+        payload = NULL;
+    }
+
+    if (!extent->length && !iter.request_ret) {
+        error_setg(&local_err, "Server did not reply with any status extents");
+        nbd_iter_channel_error(&iter, -EIO, &local_err);
+    }
+
+    error_propagate(errp, iter.err);
+    *request_ret = iter.request_ret;
+    return iter.ret;
+}
+
+static int nbd_co_request(BlockDriverState *bs, NBDRequest *request,
+                          QEMUIOVector *write_qiov)
+{
+    int ret, request_ret;
+    Error *local_err = NULL;
+    BDRVNBDState *s = (BDRVNBDState *)bs->opaque;
+
+    assert(request->type != NBD_CMD_READ);
+    if (write_qiov) {
+        assert(request->type == NBD_CMD_WRITE);
+        assert(request->len == iov_size(write_qiov->iov, write_qiov->niov));
+    } else {
+        assert(request->type != NBD_CMD_WRITE);
+    }
+
+    do {
+        ret = nbd_co_send_request(bs, request, write_qiov);
+        if (ret < 0) {
+            continue;
+        }
+
+        ret = nbd_co_receive_return_code(s, request->handle,
+                                         &request_ret, &local_err);
+        if (local_err) {
+            trace_nbd_co_request_fail(request->from, request->len,
+                                      request->handle, request->flags,
+                                      request->type,
+                                      nbd_cmd_lookup(request->type),
+                                      ret, error_get_pretty(local_err));
+            error_free(local_err);
+            local_err = NULL;
+        }
+    } while (ret < 0 && nbd_client_connecting_wait(s));
+
+    return ret ? ret : request_ret;
+}
+
+static int nbd_client_co_preadv(BlockDriverState *bs, int64_t offset,
+                                int64_t bytes, QEMUIOVector *qiov,
+                                BdrvRequestFlags flags)
+{
+    int ret, request_ret;
+    Error *local_err = NULL;
+    BDRVNBDState *s = (BDRVNBDState *)bs->opaque;
+    NBDRequest request = {
+        .type = NBD_CMD_READ,
+        .from = offset,
+        .len = bytes,
+    };
+
+    assert(bytes <= NBD_MAX_BUFFER_SIZE);
+    assert(!flags);
+
+    if (!bytes) {
+        return 0;
+    }
+    /*
+     * Work around the fact that the block layer doesn't do
+     * byte-accurate sizing yet - if the read exceeds the server's
+     * advertised size because the block layer rounded size up, then
+     * truncate the request to the server and tail-pad with zero.
+     */
+    if (offset >= s->info.size) {
+        assert(bytes < BDRV_SECTOR_SIZE);
+        qemu_iovec_memset(qiov, 0, 0, bytes);
+        return 0;
+    }
+    if (offset + bytes > s->info.size) {
+        uint64_t slop = offset + bytes - s->info.size;
+
+        assert(slop < BDRV_SECTOR_SIZE);
+        qemu_iovec_memset(qiov, bytes - slop, 0, slop);
+        request.len -= slop;
+    }
+
+    do {
+        ret = nbd_co_send_request(bs, &request, NULL);
+        if (ret < 0) {
+            continue;
+        }
+
+        ret = nbd_co_receive_cmdread_reply(s, request.handle, offset, qiov,
+                                           &request_ret, &local_err);
+        if (local_err) {
+            trace_nbd_co_request_fail(request.from, request.len, request.handle,
+                                      request.flags, request.type,
+                                      nbd_cmd_lookup(request.type),
+                                      ret, error_get_pretty(local_err));
+            error_free(local_err);
+            local_err = NULL;
+        }
+    } while (ret < 0 && nbd_client_connecting_wait(s));
+
+    return ret ? ret : request_ret;
+}
+
+static int nbd_client_co_pwritev(BlockDriverState *bs, int64_t offset,
+                                 int64_t bytes, QEMUIOVector *qiov,
+                                 BdrvRequestFlags flags)
+{
+    BDRVNBDState *s = (BDRVNBDState *)bs->opaque;
+    NBDRequest request = {
+        .type = NBD_CMD_WRITE,
+        .from = offset,
+        .len = bytes,
+    };
+
+    assert(!(s->info.flags & NBD_FLAG_READ_ONLY));
+    if (flags & BDRV_REQ_FUA) {
+        assert(s->info.flags & NBD_FLAG_SEND_FUA);
+        request.flags |= NBD_CMD_FLAG_FUA;
+    }
+
+    assert(bytes <= NBD_MAX_BUFFER_SIZE);
+
+    if (!bytes) {
+        return 0;
+    }
+    return nbd_co_request(bs, &request, qiov);
+}
+
+static int nbd_client_co_pwrite_zeroes(BlockDriverState *bs, int64_t offset,
+                                       int64_t bytes, BdrvRequestFlags flags)
+{
+    BDRVNBDState *s = (BDRVNBDState *)bs->opaque;
+    NBDRequest request = {
+        .type = NBD_CMD_WRITE_ZEROES,
+        .from = offset,
+        .len = bytes,  /* .len is uint32_t actually */
+    };
+
+    assert(bytes <= UINT32_MAX); /* rely on max_pwrite_zeroes */
+
+    assert(!(s->info.flags & NBD_FLAG_READ_ONLY));
+    if (!(s->info.flags & NBD_FLAG_SEND_WRITE_ZEROES)) {
+        return -ENOTSUP;
+    }
+
+    if (flags & BDRV_REQ_FUA) {
+        assert(s->info.flags & NBD_FLAG_SEND_FUA);
+        request.flags |= NBD_CMD_FLAG_FUA;
+    }
+    if (!(flags & BDRV_REQ_MAY_UNMAP)) {
+        request.flags |= NBD_CMD_FLAG_NO_HOLE;
+    }
+    if (flags & BDRV_REQ_NO_FALLBACK) {
+        assert(s->info.flags & NBD_FLAG_SEND_FAST_ZERO);
+        request.flags |= NBD_CMD_FLAG_FAST_ZERO;
+    }
+
+    if (!bytes) {
+        return 0;
+    }
+    return nbd_co_request(bs, &request, NULL);
+}
+
+static int nbd_client_co_flush(BlockDriverState *bs)
+{
+    BDRVNBDState *s = (BDRVNBDState *)bs->opaque;
+    NBDRequest request = { .type = NBD_CMD_FLUSH };
+
+    if (!(s->info.flags & NBD_FLAG_SEND_FLUSH)) {
+        return 0;
+    }
+
+    request.from = 0;
+    request.len = 0;
+
+    return nbd_co_request(bs, &request, NULL);
+}
+
+static int nbd_client_co_pdiscard(BlockDriverState *bs, int64_t offset,
+                                  int64_t bytes)
+{
+    BDRVNBDState *s = (BDRVNBDState *)bs->opaque;
+    NBDRequest request = {
+        .type = NBD_CMD_TRIM,
+        .from = offset,
+        .len = bytes, /* len is uint32_t */
+    };
+
+    assert(bytes <= UINT32_MAX); /* rely on max_pdiscard */
+
+    assert(!(s->info.flags & NBD_FLAG_READ_ONLY));
+    if (!(s->info.flags & NBD_FLAG_SEND_TRIM) || !bytes) {
+        return 0;
+    }
+
+    return nbd_co_request(bs, &request, NULL);
+}
+
+static int coroutine_fn nbd_client_co_block_status(
+        BlockDriverState *bs, bool want_zero, int64_t offset, int64_t bytes,
+        int64_t *pnum, int64_t *map, BlockDriverState **file)
+{
+    int ret, request_ret;
+    NBDExtent extent = { 0 };
+    BDRVNBDState *s = (BDRVNBDState *)bs->opaque;
+    Error *local_err = NULL;
+
+    NBDRequest request = {
+        .type = NBD_CMD_BLOCK_STATUS,
+        .from = offset,
+        .len = MIN(QEMU_ALIGN_DOWN(INT_MAX, bs->bl.request_alignment),
+                   MIN(bytes, s->info.size - offset)),
+        .flags = NBD_CMD_FLAG_REQ_ONE,
+    };
+
+    if (!s->info.base_allocation) {
+        *pnum = bytes;
+        *map = offset;
+        *file = bs;
+        return BDRV_BLOCK_DATA | BDRV_BLOCK_OFFSET_VALID;
+    }
+
+    /*
+     * Work around the fact that the block layer doesn't do
+     * byte-accurate sizing yet - if the status request exceeds the
+     * server's advertised size because the block layer rounded size
+     * up, we truncated the request to the server (above), or are
+     * called on just the hole.
+     */
+    if (offset >= s->info.size) {
+        *pnum = bytes;
+        assert(bytes < BDRV_SECTOR_SIZE);
+        /* Intentionally don't report offset_valid for the hole */
+        return BDRV_BLOCK_ZERO;
+    }
+
+    if (s->info.min_block) {
+        assert(QEMU_IS_ALIGNED(request.len, s->info.min_block));
+    }
+    do {
+        ret = nbd_co_send_request(bs, &request, NULL);
+        if (ret < 0) {
+            continue;
+        }
+
+        ret = nbd_co_receive_blockstatus_reply(s, request.handle, bytes,
+                                               &extent, &request_ret,
+                                               &local_err);
+        if (local_err) {
+            trace_nbd_co_request_fail(request.from, request.len, request.handle,
+                                      request.flags, request.type,
+                                      nbd_cmd_lookup(request.type),
+                                      ret, error_get_pretty(local_err));
+            error_free(local_err);
+            local_err = NULL;
+        }
+    } while (ret < 0 && nbd_client_connecting_wait(s));
+
+    if (ret < 0 || request_ret < 0) {
+        return ret ? ret : request_ret;
+    }
+
+    assert(extent.length);
+    *pnum = extent.length;
+    *map = offset;
+    *file = bs;
+    return (extent.flags & NBD_STATE_HOLE ? 0 : BDRV_BLOCK_DATA) |
+        (extent.flags & NBD_STATE_ZERO ? BDRV_BLOCK_ZERO : 0) |
+        BDRV_BLOCK_OFFSET_VALID;
+}
+
+static int nbd_client_reopen_prepare(BDRVReopenState *state,
+                                     BlockReopenQueue *queue, Error **errp)
+{
+    BDRVNBDState *s = (BDRVNBDState *)state->bs->opaque;
+
+    if ((state->flags & BDRV_O_RDWR) && (s->info.flags & NBD_FLAG_READ_ONLY)) {
+        error_setg(errp, "Can't reopen read-only NBD mount as read/write");
+        return -EACCES;
+    }
+    return 0;
+}
+
+static void nbd_yank(void *opaque)
+{
+    BlockDriverState *bs = opaque;
+    BDRVNBDState *s = (BDRVNBDState *)bs->opaque;
+
+    qatomic_store_release(&s->state, NBD_CLIENT_QUIT);
+    qio_channel_shutdown(QIO_CHANNEL(s->ioc), QIO_CHANNEL_SHUTDOWN_BOTH, NULL);
+}
+
+static void nbd_client_close(BlockDriverState *bs)
+{
+    BDRVNBDState *s = (BDRVNBDState *)bs->opaque;
+    NBDRequest request = { .type = NBD_CMD_DISC };
+
+    if (s->ioc) {
+        nbd_send_request(s->ioc, &request);
+    }
+
+    nbd_teardown_connection(bs);
+}
+
+
+/*
+ * Parse nbd_open options
+ */
+
+static int nbd_parse_uri(const char *filename, QDict *options)
+{
+    URI *uri;
+    const char *p;
+    QueryParams *qp = NULL;
+    int ret = 0;
+    bool is_unix;
+
+    uri = uri_parse(filename);
+    if (!uri) {
+        return -EINVAL;
+    }
+
+    /* transport */
+    if (!g_strcmp0(uri->scheme, "nbd")) {
+        is_unix = false;
+    } else if (!g_strcmp0(uri->scheme, "nbd+tcp")) {
+        is_unix = false;
+    } else if (!g_strcmp0(uri->scheme, "nbd+unix")) {
+        is_unix = true;
+    } else {
+        ret = -EINVAL;
+        goto out;
+    }
+
+    p = uri->path ? uri->path : "";
+    if (p[0] == '/') {
+        p++;
+    }
+    if (p[0]) {
+        qdict_put_str(options, "export", p);
+    }
+
+    qp = query_params_parse(uri->query);
+    if (qp->n > 1 || (is_unix && !qp->n) || (!is_unix && qp->n)) {
+        ret = -EINVAL;
+        goto out;
+    }
+
+    if (is_unix) {
+        /* nbd+unix:///export?socket=path */
+        if (uri->server || uri->port || strcmp(qp->p[0].name, "socket")) {
+            ret = -EINVAL;
+            goto out;
+        }
+        qdict_put_str(options, "server.type", "unix");
+        qdict_put_str(options, "server.path", qp->p[0].value);
+    } else {
+        QString *host;
+        char *port_str;
+
+        /* nbd[+tcp]://host[:port]/export */
+        if (!uri->server) {
+            ret = -EINVAL;
+            goto out;
+        }
+
+        /* strip braces from literal IPv6 address */
+        if (uri->server[0] == '[') {
+            host = qstring_from_substr(uri->server, 1,
+                                       strlen(uri->server) - 1);
+        } else {
+            host = qstring_from_str(uri->server);
+        }
+
+        qdict_put_str(options, "server.type", "inet");
+        qdict_put(options, "server.host", host);
+
+        port_str = g_strdup_printf("%d", uri->port ?: NBD_DEFAULT_PORT);
+        qdict_put_str(options, "server.port", port_str);
+        g_free(port_str);
+    }
+
+out:
+    if (qp) {
+        query_params_free(qp);
+    }
+    uri_free(uri);
+    return ret;
+}
+
+static bool nbd_has_filename_options_conflict(QDict *options, Error **errp)
+{
+    const QDictEntry *e;
+
+    for (e = qdict_first(options); e; e = qdict_next(options, e)) {
+        if (!strcmp(e->key, "host") ||
+            !strcmp(e->key, "port") ||
+            !strcmp(e->key, "path") ||
+            !strcmp(e->key, "export") ||
+            strstart(e->key, "server.", NULL))
+        {
+            error_setg(errp, "Option '%s' cannot be used with a file name",
+                       e->key);
+            return true;
+        }
+    }
+
+    return false;
+}
+
+static void nbd_parse_filename(const char *filename, QDict *options,
+                               Error **errp)
+{
+    g_autofree char *file = NULL;
+    char *export_name;
+    const char *host_spec;
+    const char *unixpath;
+
+    if (nbd_has_filename_options_conflict(options, errp)) {
+        return;
+    }
+
+    if (strstr(filename, "://")) {
+        int ret = nbd_parse_uri(filename, options);
+        if (ret < 0) {
+            error_setg(errp, "No valid URL specified");
+        }
+        return;
+    }
+
+    file = g_strdup(filename);
+
+    export_name = strstr(file, EN_OPTSTR);
+    if (export_name) {
+        if (export_name[strlen(EN_OPTSTR)] == 0) {
+            return;
+        }
+        export_name[0] = 0; /* truncate 'file' */
+        export_name += strlen(EN_OPTSTR);
+
+        qdict_put_str(options, "export", export_name);
+    }
+
+    /* extract the host_spec - fail if it's not nbd:... */
+    if (!strstart(file, "nbd:", &host_spec)) {
+        error_setg(errp, "File name string for NBD must start with 'nbd:'");
+        return;
+    }
+
+    if (!*host_spec) {
+        return;
+    }
+
+    /* are we a UNIX or TCP socket? */
+    if (strstart(host_spec, "unix:", &unixpath)) {
+        qdict_put_str(options, "server.type", "unix");
+        qdict_put_str(options, "server.path", unixpath);
+    } else {
+        InetSocketAddress *addr = g_new(InetSocketAddress, 1);
+
+        if (inet_parse(addr, host_spec, errp)) {
+            goto out_inet;
+        }
+
+        qdict_put_str(options, "server.type", "inet");
+        qdict_put_str(options, "server.host", addr->host);
+        qdict_put_str(options, "server.port", addr->port);
+    out_inet:
+        qapi_free_InetSocketAddress(addr);
+    }
+}
+
+static bool nbd_process_legacy_socket_options(QDict *output_options,
+                                              QemuOpts *legacy_opts,
+                                              Error **errp)
+{
+    const char *path = qemu_opt_get(legacy_opts, "path");
+    const char *host = qemu_opt_get(legacy_opts, "host");
+    const char *port = qemu_opt_get(legacy_opts, "port");
+    const QDictEntry *e;
+
+    if (!path && !host && !port) {
+        return true;
+    }
+
+    for (e = qdict_first(output_options); e; e = qdict_next(output_options, e))
+    {
+        if (strstart(e->key, "server.", NULL)) {
+            error_setg(errp, "Cannot use 'server' and path/host/port at the "
+                       "same time");
+            return false;
+        }
+    }
+
+    if (path && host) {
+        error_setg(errp, "path and host may not be used at the same time");
+        return false;
+    } else if (path) {
+        if (port) {
+            error_setg(errp, "port may not be used without host");
+            return false;
+        }
+
+        qdict_put_str(output_options, "server.type", "unix");
+        qdict_put_str(output_options, "server.path", path);
+    } else if (host) {
+        qdict_put_str(output_options, "server.type", "inet");
+        qdict_put_str(output_options, "server.host", host);
+        qdict_put_str(output_options, "server.port",
+                      port ?: stringify(NBD_DEFAULT_PORT));
+    }
+
+    return true;
+}
+
+static SocketAddress *nbd_config(BDRVNBDState *s, QDict *options,
+                                 Error **errp)
+{
+    SocketAddress *saddr = NULL;
+    QDict *addr = NULL;
+    Visitor *iv = NULL;
+
+    qdict_extract_subqdict(options, &addr, "server.");
+    if (!qdict_size(addr)) {
+        error_setg(errp, "NBD server address missing");
+        goto done;
+    }
+
+    iv = qobject_input_visitor_new_flat_confused(addr, errp);
+    if (!iv) {
+        goto done;
+    }
+
+    if (!visit_type_SocketAddress(iv, NULL, &saddr, errp)) {
+        goto done;
+    }
+
+    if (socket_address_parse_named_fd(saddr, errp) < 0) {
+        qapi_free_SocketAddress(saddr);
+        saddr = NULL;
+        goto done;
+    }
+
+done:
+    qobject_unref(addr);
+    visit_free(iv);
+    return saddr;
+}
+
+static QCryptoTLSCreds *nbd_get_tls_creds(const char *id, Error **errp)
+{
+    Object *obj;
+    QCryptoTLSCreds *creds;
+
+    obj = object_resolve_path_component(
+        object_get_objects_root(), id);
+    if (!obj) {
+        error_setg(errp, "No TLS credentials with id '%s'",
+                   id);
+        return NULL;
+    }
+    creds = (QCryptoTLSCreds *)
+        object_dynamic_cast(obj, TYPE_QCRYPTO_TLS_CREDS);
+    if (!creds) {
+        error_setg(errp, "Object with id '%s' is not TLS credentials",
+                   id);
+        return NULL;
+    }
+
+    if (!qcrypto_tls_creds_check_endpoint(creds,
+                                          QCRYPTO_TLS_CREDS_ENDPOINT_CLIENT,
+                                          errp)) {
+        return NULL;
+    }
+    object_ref(obj);
+    return creds;
+}
+
+
+static QemuOptsList nbd_runtime_opts = {
+    .name = "nbd",
+    .head = QTAILQ_HEAD_INITIALIZER(nbd_runtime_opts.head),
+    .desc = {
+        {
+            .name = "host",
+            .type = QEMU_OPT_STRING,
+            .help = "TCP host to connect to",
+        },
+        {
+            .name = "port",
+            .type = QEMU_OPT_STRING,
+            .help = "TCP port to connect to",
+        },
+        {
+            .name = "path",
+            .type = QEMU_OPT_STRING,
+            .help = "Unix socket path to connect to",
+        },
+        {
+            .name = "export",
+            .type = QEMU_OPT_STRING,
+            .help = "Name of the NBD export to open",
+        },
+        {
+            .name = "tls-creds",
+            .type = QEMU_OPT_STRING,
+            .help = "ID of the TLS credentials to use",
+        },
+        {
+            .name = "x-dirty-bitmap",
+            .type = QEMU_OPT_STRING,
+            .help = "experimental: expose named dirty bitmap in place of "
+                    "block status",
+        },
+        {
+            .name = "reconnect-delay",
+            .type = QEMU_OPT_NUMBER,
+            .help = "On an unexpected disconnect, the nbd client tries to "
+                    "connect again until succeeding or encountering a serious "
+                    "error.  During the first @reconnect-delay seconds, all "
+                    "requests are paused and will be rerun on a successful "
+                    "reconnect. After that time, any delayed requests and all "
+                    "future requests before a successful reconnect will "
+                    "immediately fail. Default 0",
+        },
+        { /* end of list */ }
+    },
+};
+
+static int nbd_process_options(BlockDriverState *bs, QDict *options,
+                               Error **errp)
+{
+    BDRVNBDState *s = bs->opaque;
+    QemuOpts *opts;
+    int ret = -EINVAL;
+
+    opts = qemu_opts_create(&nbd_runtime_opts, NULL, 0, &error_abort);
+    if (!qemu_opts_absorb_qdict(opts, options, errp)) {
+        goto error;
+    }
+
+    /* Translate @host, @port, and @path to a SocketAddress */
+    if (!nbd_process_legacy_socket_options(options, opts, errp)) {
+        goto error;
+    }
+
+    /* Pop the config into our state object. Exit if invalid. */
+    s->saddr = nbd_config(s, options, errp);
+    if (!s->saddr) {
+        goto error;
+    }
+
+    s->export = g_strdup(qemu_opt_get(opts, "export"));
+    if (s->export && strlen(s->export) > NBD_MAX_STRING_SIZE) {
+        error_setg(errp, "export name too long to send to server");
+        goto error;
+    }
+
+    s->tlscredsid = g_strdup(qemu_opt_get(opts, "tls-creds"));
+    if (s->tlscredsid) {
+        s->tlscreds = nbd_get_tls_creds(s->tlscredsid, errp);
+        if (!s->tlscreds) {
+            goto error;
+        }
+
+        /* TODO SOCKET_ADDRESS_KIND_FD where fd has AF_INET or AF_INET6 */
+        if (s->saddr->type != SOCKET_ADDRESS_TYPE_INET) {
+            error_setg(errp, "TLS only supported over IP sockets");
+            goto error;
+        }
+        s->hostname = s->saddr->u.inet.host;
+    }
+
+    s->x_dirty_bitmap = g_strdup(qemu_opt_get(opts, "x-dirty-bitmap"));
+    if (s->x_dirty_bitmap && strlen(s->x_dirty_bitmap) > NBD_MAX_STRING_SIZE) {
+        error_setg(errp, "x-dirty-bitmap query too long to send to server");
+        goto error;
+    }
+
+    s->reconnect_delay = qemu_opt_get_number(opts, "reconnect-delay", 0);
+
+    ret = 0;
+
+ error:
+    qemu_opts_del(opts);
+    return ret;
+}
+
+static int nbd_open(BlockDriverState *bs, QDict *options, int flags,
+                    Error **errp)
+{
+    int ret;
+    BDRVNBDState *s = (BDRVNBDState *)bs->opaque;
+
+    s->bs = bs;
+    qemu_co_mutex_init(&s->send_mutex);
+    qemu_co_queue_init(&s->free_sema);
+    qemu_co_mutex_init(&s->receive_mutex);
+
+    if (!yank_register_instance(BLOCKDEV_YANK_INSTANCE(bs->node_name), errp)) {
+        return -EEXIST;
+    }
+
+    ret = nbd_process_options(bs, options, errp);
+    if (ret < 0) {
+        goto fail;
+    }
+
+    s->conn = nbd_client_connection_new(s->saddr, true, s->export,
+                                        s->x_dirty_bitmap, s->tlscreds);
+
+    /* TODO: Configurable retry-until-timeout behaviour. */
+    s->state = NBD_CLIENT_CONNECTING_WAIT;
+    ret = nbd_do_establish_connection(bs, errp);
+    if (ret < 0) {
+        goto fail;
+    }
+
+    nbd_client_connection_enable_retry(s->conn);
+
+    return 0;
+
+fail:
+    nbd_clear_bdrvstate(bs);
+    return ret;
+}
+
+static int nbd_co_flush(BlockDriverState *bs)
+{
+    return nbd_client_co_flush(bs);
+}
+
+static void nbd_refresh_limits(BlockDriverState *bs, Error **errp)
+{
+    BDRVNBDState *s = (BDRVNBDState *)bs->opaque;
+    uint32_t min = s->info.min_block;
+    uint32_t max = MIN_NON_ZERO(NBD_MAX_BUFFER_SIZE, s->info.max_block);
+
+    /*
+     * If the server did not advertise an alignment:
+     * - a size that is not sector-aligned implies that an alignment
+     *   of 1 can be used to access those tail bytes
+     * - advertisement of block status requires an alignment of 1, so
+     *   that we don't violate block layer constraints that block
+     *   status is always aligned (as we can't control whether the
+     *   server will report sub-sector extents, such as a hole at EOF
+     *   on an unaligned POSIX file)
+     * - otherwise, assume the server is so old that we are safer avoiding
+     *   sub-sector requests
+     */
+    if (!min) {
+        min = (!QEMU_IS_ALIGNED(s->info.size, BDRV_SECTOR_SIZE) ||
+               s->info.base_allocation) ? 1 : BDRV_SECTOR_SIZE;
+    }
+
+    bs->bl.request_alignment = min;
+    bs->bl.max_pdiscard = QEMU_ALIGN_DOWN(INT_MAX, min);
+    bs->bl.max_pwrite_zeroes = max;
+    bs->bl.max_transfer = max;
+
+    if (s->info.opt_block &&
+        s->info.opt_block > bs->bl.opt_transfer) {
+        bs->bl.opt_transfer = s->info.opt_block;
+    }
+}
+
+static void nbd_close(BlockDriverState *bs)
+{
+    nbd_client_close(bs);
+    nbd_clear_bdrvstate(bs);
+}
+
+/*
+ * NBD cannot truncate, but if the caller asks to truncate to the same size, or
+ * to a smaller size with exact=false, there is no reason to fail the
+ * operation.
+ *
+ * Preallocation mode is ignored since it does not seems useful to fail when
+ * we never change anything.
+ */
+static int coroutine_fn nbd_co_truncate(BlockDriverState *bs, int64_t offset,
+                                        bool exact, PreallocMode prealloc,
+                                        BdrvRequestFlags flags, Error **errp)
+{
+    BDRVNBDState *s = bs->opaque;
+
+    if (offset != s->info.size && exact) {
+        error_setg(errp, "Cannot resize NBD nodes");
+        return -ENOTSUP;
+    }
+
+    if (offset > s->info.size) {
+        error_setg(errp, "Cannot grow NBD nodes");
+        return -EINVAL;
+    }
+
+    return 0;
+}
+
+static int64_t nbd_getlength(BlockDriverState *bs)
+{
+    BDRVNBDState *s = bs->opaque;
+
+    return s->info.size;
+}
+
+static void nbd_refresh_filename(BlockDriverState *bs)
+{
+    BDRVNBDState *s = bs->opaque;
+    const char *host = NULL, *port = NULL, *path = NULL;
+    size_t len = 0;
+
+    if (s->saddr->type == SOCKET_ADDRESS_TYPE_INET) {
+        const InetSocketAddress *inet = &s->saddr->u.inet;
+        if (!inet->has_ipv4 && !inet->has_ipv6 && !inet->has_to) {
+            host = inet->host;
+            port = inet->port;
+        }
+    } else if (s->saddr->type == SOCKET_ADDRESS_TYPE_UNIX) {
+        path = s->saddr->u.q_unix.path;
+    } /* else can't represent as pseudo-filename */
+
+    if (path && s->export) {
+        len = snprintf(bs->exact_filename, sizeof(bs->exact_filename),
+                       "nbd+unix:///%s?socket=%s", s->export, path);
+    } else if (path && !s->export) {
+        len = snprintf(bs->exact_filename, sizeof(bs->exact_filename),
+                       "nbd+unix://?socket=%s", path);
+    } else if (host && s->export) {
+        len = snprintf(bs->exact_filename, sizeof(bs->exact_filename),
+                       "nbd://%s:%s/%s", host, port, s->export);
+    } else if (host && !s->export) {
+        len = snprintf(bs->exact_filename, sizeof(bs->exact_filename),
+                       "nbd://%s:%s", host, port);
+    }
+    if (len >= sizeof(bs->exact_filename)) {
+        /* Name is too long to represent exactly, so leave it empty. */
+        bs->exact_filename[0] = '\0';
+    }
+}
+
+static char *nbd_dirname(BlockDriverState *bs, Error **errp)
+{
+    /* The generic bdrv_dirname() implementation is able to work out some
+     * directory name for NBD nodes, but that would be wrong. So far there is no
+     * specification for how "export paths" would work, so NBD does not have
+     * directory names. */
+    error_setg(errp, "Cannot generate a base directory for NBD nodes");
+    return NULL;
+}
+
+static const char *const nbd_strong_runtime_opts[] = {
+    "path",
+    "host",
+    "port",
+    "export",
+    "tls-creds",
+    "server.",
+
+    NULL
+};
+
+static void nbd_cancel_in_flight(BlockDriverState *bs)
+{
+    BDRVNBDState *s = (BDRVNBDState *)bs->opaque;
+
+    reconnect_delay_timer_del(s);
+
+    if (s->state == NBD_CLIENT_CONNECTING_WAIT) {
+        s->state = NBD_CLIENT_CONNECTING_NOWAIT;
+        qemu_co_queue_restart_all(&s->free_sema);
+    }
+
+    nbd_co_establish_connection_cancel(s->conn);
+}
+
+static BlockDriver bdrv_nbd = {
+    .format_name                = "nbd",
+    .protocol_name              = "nbd",
+    .instance_size              = sizeof(BDRVNBDState),
+    .bdrv_parse_filename        = nbd_parse_filename,
+    .bdrv_co_create_opts        = bdrv_co_create_opts_simple,
+    .create_opts                = &bdrv_create_opts_simple,
+    .bdrv_file_open             = nbd_open,
+    .bdrv_reopen_prepare        = nbd_client_reopen_prepare,
+    .bdrv_co_preadv             = nbd_client_co_preadv,
+    .bdrv_co_pwritev            = nbd_client_co_pwritev,
+    .bdrv_co_pwrite_zeroes      = nbd_client_co_pwrite_zeroes,
+    .bdrv_close                 = nbd_close,
+    .bdrv_co_flush_to_os        = nbd_co_flush,
+    .bdrv_co_pdiscard           = nbd_client_co_pdiscard,
+    .bdrv_refresh_limits        = nbd_refresh_limits,
+    .bdrv_co_truncate           = nbd_co_truncate,
+    .bdrv_getlength             = nbd_getlength,
+    .bdrv_refresh_filename      = nbd_refresh_filename,
+    .bdrv_co_block_status       = nbd_client_co_block_status,
+    .bdrv_dirname               = nbd_dirname,
+    .strong_runtime_opts        = nbd_strong_runtime_opts,
+    .bdrv_cancel_in_flight      = nbd_cancel_in_flight,
+};
+
+static BlockDriver bdrv_nbd_tcp = {
+    .format_name                = "nbd",
+    .protocol_name              = "nbd+tcp",
+    .instance_size              = sizeof(BDRVNBDState),
+    .bdrv_parse_filename        = nbd_parse_filename,
+    .bdrv_co_create_opts        = bdrv_co_create_opts_simple,
+    .create_opts                = &bdrv_create_opts_simple,
+    .bdrv_file_open             = nbd_open,
+    .bdrv_reopen_prepare        = nbd_client_reopen_prepare,
+    .bdrv_co_preadv             = nbd_client_co_preadv,
+    .bdrv_co_pwritev            = nbd_client_co_pwritev,
+    .bdrv_co_pwrite_zeroes      = nbd_client_co_pwrite_zeroes,
+    .bdrv_close                 = nbd_close,
+    .bdrv_co_flush_to_os        = nbd_co_flush,
+    .bdrv_co_pdiscard           = nbd_client_co_pdiscard,
+    .bdrv_refresh_limits        = nbd_refresh_limits,
+    .bdrv_co_truncate           = nbd_co_truncate,
+    .bdrv_getlength             = nbd_getlength,
+    .bdrv_refresh_filename      = nbd_refresh_filename,
+    .bdrv_co_block_status       = nbd_client_co_block_status,
+    .bdrv_dirname               = nbd_dirname,
+    .strong_runtime_opts        = nbd_strong_runtime_opts,
+    .bdrv_cancel_in_flight      = nbd_cancel_in_flight,
+};
+
+static BlockDriver bdrv_nbd_unix = {
+    .format_name                = "nbd",
+    .protocol_name              = "nbd+unix",
+    .instance_size              = sizeof(BDRVNBDState),
+    .bdrv_parse_filename        = nbd_parse_filename,
+    .bdrv_co_create_opts        = bdrv_co_create_opts_simple,
+    .create_opts                = &bdrv_create_opts_simple,
+    .bdrv_file_open             = nbd_open,
+    .bdrv_reopen_prepare        = nbd_client_reopen_prepare,
+    .bdrv_co_preadv             = nbd_client_co_preadv,
+    .bdrv_co_pwritev            = nbd_client_co_pwritev,
+    .bdrv_co_pwrite_zeroes      = nbd_client_co_pwrite_zeroes,
+    .bdrv_close                 = nbd_close,
+    .bdrv_co_flush_to_os        = nbd_co_flush,
+    .bdrv_co_pdiscard           = nbd_client_co_pdiscard,
+    .bdrv_refresh_limits        = nbd_refresh_limits,
+    .bdrv_co_truncate           = nbd_co_truncate,
+    .bdrv_getlength             = nbd_getlength,
+    .bdrv_refresh_filename      = nbd_refresh_filename,
+    .bdrv_co_block_status       = nbd_client_co_block_status,
+    .bdrv_dirname               = nbd_dirname,
+    .strong_runtime_opts        = nbd_strong_runtime_opts,
+    .bdrv_cancel_in_flight      = nbd_cancel_in_flight,
+};
+
+static void bdrv_nbd_init(void)
+{
+    bdrv_register(&bdrv_nbd);
+    bdrv_register(&bdrv_nbd_tcp);
+    bdrv_register(&bdrv_nbd_unix);
+}
+
+block_init(bdrv_nbd_init);