C++ qemu_blockalign函数代码示例

/**
 * Update header in-place (does not rewrite backing filename or other strings)
 *
 * This function only updates known header fields in-place and does not affect
 * extra data after the QED header.
 */
static void qed_write_header(BDRVQEDState *s, BlockDriverCompletionFunc cb,
                             void *opaque)
{
    /* We must write full sectors for O_DIRECT but cannot necessarily generate
     * the data following the header if an unrecognized compat feature is
     * active.  Therefore, first read the sectors containing the header, update
     * them, and write back.
     */

    int nsectors = (sizeof(QEDHeader) + BDRV_SECTOR_SIZE - 1) /
                   BDRV_SECTOR_SIZE;
    size_t len = nsectors * BDRV_SECTOR_SIZE;
    QEDWriteHeaderCB *write_header_cb = gencb_alloc(sizeof(*write_header_cb),
                                                    cb, opaque);

    write_header_cb->s = s;
    write_header_cb->nsectors = nsectors;
    write_header_cb->buf = qemu_blockalign(s->bs, len);
    write_header_cb->iov.iov_base = write_header_cb->buf;
    write_header_cb->iov.iov_len = len;
    qemu_iovec_init_external(&write_header_cb->qiov, &write_header_cb->iov, 1);

    bdrv_aio_readv(s->bs->file, 0, &write_header_cb->qiov, nsectors,
                   qed_write_header_read_cb, write_header_cb);
}

static void *qemu_io_alloc(size_t len, int pattern)
{
	void *buf;

	if (misalign)
		len += MISALIGN_OFFSET;
	buf = qemu_blockalign(bs, len);
	memset(buf, pattern, len);
	if (misalign)
		buf += MISALIGN_OFFSET;
	return buf;
}

/* Reads the log header, and subsequent descriptors (if any).  This
 * will allocate all the space for buffer, which must be NULL when
 * passed into this function. Each descriptor will also be validated,
 * and error returned if any are invalid. */
static int vhdx_log_read_desc(BlockDriverState *bs, BDRVVHDXState *s,
                              VHDXLogEntries *log, VHDXLogDescEntries **buffer)
{
    int ret = 0;
    uint32_t desc_sectors;
    uint32_t sectors_read;
    VHDXLogEntryHeader hdr;
    VHDXLogDescEntries *desc_entries = NULL;
    int i;

    assert(*buffer == NULL);

    ret = vhdx_log_peek_hdr(bs, log, &hdr);
    if (ret < 0) {
        goto exit;
    }
    vhdx_log_entry_hdr_le_import(&hdr);
    if (vhdx_log_hdr_is_valid(log, &hdr, s) == false) {
        ret = -EINVAL;
        goto exit;
    }

    desc_sectors = vhdx_compute_desc_sectors(hdr.descriptor_count);
    desc_entries = qemu_blockalign(bs, desc_sectors * VHDX_LOG_SECTOR_SIZE);

    ret = vhdx_log_read_sectors(bs, log, &sectors_read, desc_entries,
                                desc_sectors, false);
    if (ret < 0) {
        goto free_and_exit;
    }
    if (sectors_read != desc_sectors) {
        ret = -EINVAL;
        goto free_and_exit;
    }

    /* put in proper endianness, and validate each desc */
    for (i = 0; i < hdr.descriptor_count; i++) {
        vhdx_log_desc_le_import(&desc_entries->desc[i]);
        if (vhdx_log_desc_is_valid(&desc_entries->desc[i], &hdr) == false) {
            ret = -EINVAL;
            goto free_and_exit;
        }
    }

    *buffer = desc_entries;
    goto exit;

free_and_exit:
    qemu_vfree(desc_entries);
exit:
    return ret;
}

Qcow2Cache *qcow2_cache_create(BlockDriverState *bs, int num_tables)
{
    BDRVQcowState *s = bs->opaque;
    Qcow2Cache *c;
    int i;

    c = g_malloc0(sizeof(*c));
    c->size = num_tables;
    c->entries = g_malloc0(sizeof(*c->entries) * num_tables);

    for (i = 0; i < c->size; i++) {
        c->entries[i].table = qemu_blockalign(bs, s->cluster_size);
    }

    return c;
}

static int raw_open_common(BlockDriverState *bs, const char *filename,
                           int bdrv_flags, int open_flags)
{
    BDRVRawState *s = bs->opaque;
    int fd, ret;

    posix_aio_init();

    s->lseek_err_cnt = 0;

    s->open_flags = open_flags | O_BINARY;
    s->open_flags &= ~O_ACCMODE;
    if ((bdrv_flags & BDRV_O_ACCESS) == BDRV_O_RDWR) {
        s->open_flags |= O_RDWR;
    } else {
        s->open_flags |= O_RDONLY;
        bs->read_only = 1;
    }

    /* Use O_DSYNC for write-through caching, no flags for write-back caching,
     * and O_DIRECT for no caching. */
    if ((bdrv_flags & BDRV_O_NOCACHE))
        s->open_flags |= O_DIRECT;
    else if (!(bdrv_flags & BDRV_O_CACHE_WB))
        s->open_flags |= O_DSYNC;

    s->fd = -1;
    fd = open(filename, s->open_flags, 0644);
    if (fd < 0) {
        ret = -errno;
        if (ret == -EROFS)
            ret = -EACCES;
        return ret;
    }
    s->fd = fd;
    s->aligned_buf = NULL;
    if ((bdrv_flags & BDRV_O_NOCACHE)) {
        s->aligned_buf = qemu_blockalign(bs, ALIGNED_BUFFER_SIZE);
        if (s->aligned_buf == NULL) {
            ret = -errno;
            close(fd);
            return ret;
        }
    }
    return 0;
}

/**
 * Write out an updated part or all of a table
 *
 * @s:          QED state
 * @offset:     Offset of table in image file, in bytes
 * @table:      Table
 * @index:      Index of first element
 * @n:          Number of elements
 * @flush:      Whether or not to sync to disk
 * @cb:         Completion function
 * @opaque:     Argument for completion function
 */
static void qed_write_table(BDRVQEDState *s, uint64_t offset, QEDTable *table,
                            unsigned int index, unsigned int n, bool flush,
                            BlockDriverCompletionFunc *cb, void *opaque)
{
    QEDWriteTableCB *write_table_cb;
    BlockDriverAIOCB *aiocb;
    unsigned int sector_mask = BDRV_SECTOR_SIZE / sizeof(uint64_t) - 1;
    unsigned int start, end, i;
    size_t len_bytes;

    trace_qed_write_table(s, offset, table, index, n);

    /* Calculate indices of the first and one after last elements */
    start = index & ~sector_mask;
    end = (index + n + sector_mask) & ~sector_mask;

    len_bytes = (end - start) * sizeof(uint64_t);

    write_table_cb = gencb_alloc(sizeof(*write_table_cb), cb, opaque);
    write_table_cb->s = s;
    write_table_cb->orig_table = table;
    write_table_cb->flush = flush;
    write_table_cb->table = qemu_blockalign(s->bs, len_bytes);
    write_table_cb->iov.iov_base = write_table_cb->table->offsets;
    write_table_cb->iov.iov_len = len_bytes;
    qemu_iovec_init_external(&write_table_cb->qiov, &write_table_cb->iov, 1);

    /* Byteswap table */
    for (i = start; i < end; i++) {
        uint64_t le_offset = cpu_to_le64(table->offsets[i]);
        write_table_cb->table->offsets[i - start] = le_offset;
    }

    /* Adjust for offset into table */
    offset += start * sizeof(uint64_t);

    aiocb = bdrv_aio_writev(s->bs->file, offset / BDRV_SECTOR_SIZE,
                            &write_table_cb->qiov,
                            write_table_cb->qiov.size / BDRV_SECTOR_SIZE,
                            qed_write_table_cb, write_table_cb);
    if (!aiocb) {
        qed_write_table_cb(write_table_cb, -EIO);
    }
}

static NBDRequest *nbd_request_get(NBDClient *client)
{
    NBDRequest *req;
    NBDExport *exp = client->exp;

    assert(client->nb_requests <= MAX_NBD_REQUESTS - 1);
    client->nb_requests++;

    if (QSIMPLEQ_EMPTY(&exp->requests)) {
        req = g_malloc0(sizeof(NBDRequest));
        req->data = qemu_blockalign(exp->bs, NBD_BUFFER_SIZE);
    } else {
        req = QSIMPLEQ_FIRST(&exp->requests);
        QSIMPLEQ_REMOVE_HEAD(&exp->requests, entry);
    }
    nbd_client_get(client);
    req->client = client;
    return req;
}

static BlockDriverAIOCB *blkverify_aio_readv(BlockDriverState *bs,
        int64_t sector_num, QEMUIOVector *qiov, int nb_sectors,
        BlockDriverCompletionFunc *cb, void *opaque)
{
    BDRVBlkverifyState *s = bs->opaque;
    BlkverifyAIOCB *acb = blkverify_aio_get(bs, false, sector_num, qiov,
                                            nb_sectors, cb, opaque);

    acb->verify = blkverify_verify_readv;
    acb->buf = qemu_blockalign(bs->file, qiov->size);
    qemu_iovec_init(&acb->raw_qiov, acb->qiov->niov);
    blkverify_iovec_clone(&acb->raw_qiov, qiov, acb->buf);

    bdrv_aio_readv(s->test_file, sector_num, qiov, nb_sectors,
                   blkverify_aio_cb, acb);
    bdrv_aio_readv(bs->file, sector_num, &acb->raw_qiov, nb_sectors,
                   blkverify_aio_cb, acb);
    return &acb->common;
}

static int compare_full_images (void)
{
    CompareFullCB *cf;
    int old_copy_on_read = FALSE;

    printf ("Performing a full comparison of the truth image and "
            "the test image...\n");

    if (!strncmp (bs->drv->format_name, "fvd", 3)) {
        /* Disable copy-on-read when scanning through the entire image. */
        old_copy_on_read = fvd_get_copy_on_read (bs);
        fvd_set_copy_on_read (bs, FALSE);
    }

    cf = g_malloc(sizeof(CompareFullCB));
    cf->max_nb_sectors = 1048576L / 512;
    cf->nb_sectors = MIN (cf->max_nb_sectors, total_sectors);
    if (posix_memalign ((void **) &cf->truth_buf, 512,
                        cf->max_nb_sectors * 512) != 0) {
        die ("posix_memalign");
    }
    cf->iov.iov_base = qemu_blockalign (bs, cf->max_nb_sectors * 512);
    cf->iov.iov_len = cf->nb_sectors * 512;
    cf->sector_num = 0;
    qemu_iovec_init_external (&cf->qiov, &cf->iov, 1);
    if (!bdrv_aio_readv (bs, cf->sector_num, &cf->qiov,
                         cf->nb_sectors, compare_full_images_cb, cf)) {
        die ("bdrv_aio_readv\n");
    }

    sim_all_tasks ();

    if (!strncmp (bs->drv->format_name, "fvd", 3)) {
        fvd_set_copy_on_read (bs, old_copy_on_read);
    }

    return 0;
}

QEDTable *qed_alloc_table(BDRVQEDState *s)
{
    /* Honor O_DIRECT memory alignment requirements */
    return qemu_blockalign(s->bs,
                           s->header.cluster_size * s->header.table_size);
}

//.........这里部分代码省略.........
            ret = nbd_receive_negotiate(sock, NULL, &nbdflags,
					&size, &blocksize);
            if (ret == -1) {
                ret = 1;
                goto out;
            }

            ret = nbd_init(fd, sock, size, blocksize);
            if (ret == -1) {
                ret = 1;
                goto out;
            }

            printf("NBD device %s is now connected to file %s\n",
                    device, argv[optind]);

	    /* update partition table */

            show_parts(device);

            ret = nbd_client(fd);
            if (ret) {
                ret = 1;
            }
            close(fd);
 out:
            kill(pid, SIGTERM);
            unlink(socket);

            return ret;
        }
        /* children */
    }

    sharing_fds = qemu_malloc((shared + 1) * sizeof(int));

    if (socket) {
        sharing_fds[0] = unix_socket_incoming(socket);
    } else {
        sharing_fds[0] = tcp_socket_incoming(bindto, port);
    }

    if (sharing_fds[0] == -1)
        return 1;
    max_fd = sharing_fds[0];
    nb_fds++;

    data = qemu_blockalign(bs, NBD_BUFFER_SIZE);
    if (data == NULL)
        errx(EXIT_FAILURE, "Cannot allocate data buffer");

    do {

        FD_ZERO(&fds);
        for (i = 0; i < nb_fds; i++)
            FD_SET(sharing_fds[i], &fds);

        ret = select(max_fd + 1, &fds, NULL, NULL, NULL);
        if (ret == -1)
            break;

        if (FD_ISSET(sharing_fds[0], &fds))
            ret--;
        for (i = 1; i < nb_fds && ret; i++) {
            if (FD_ISSET(sharing_fds[i], &fds)) {
                if (nbd_trip(bs, sharing_fds[i], fd_size, dev_offset,
                    &offset, readonly, data, NBD_BUFFER_SIZE) != 0) {
                    close(sharing_fds[i]);
                    nb_fds--;
                    sharing_fds[i] = sharing_fds[nb_fds];
                    i--;
                }
                ret--;
            }
        }
        /* new connection ? */
        if (FD_ISSET(sharing_fds[0], &fds)) {
            if (nb_fds < shared + 1) {
                sharing_fds[nb_fds] = accept(sharing_fds[0],
                                             (struct sockaddr *)&addr,
                                             &addr_len);
                if (sharing_fds[nb_fds] != -1 &&
                    nbd_negotiate(sharing_fds[nb_fds], fd_size) != -1) {
                        if (sharing_fds[nb_fds] > max_fd)
                            max_fd = sharing_fds[nb_fds];
                        nb_fds++;
                }
            }
        }
    } while (persistent || nb_fds > 1);
    qemu_vfree(data);

    close(sharing_fds[0]);
    bdrv_close(bs);
    qemu_free(sharing_fds);
    if (socket)
        unlink(socket);

    return 0;
}

static void coroutine_fn commit_run(void *opaque)
{
    CommitBlockJob *s = opaque;
    BlockDriverState *active = s->active;
    BlockDriverState *top = s->top;
    BlockDriverState *base = s->base;
    BlockDriverState *overlay_bs = NULL;
    int64_t sector_num, end;
    int ret = 0;
    int n = 0;
    void *buf;
    int bytes_written = 0;
    int64_t base_len;

    ret = s->common.len = bdrv_getlength(top);


    if (s->common.len < 0) {
        goto exit_restore_reopen;
    }

    ret = base_len = bdrv_getlength(base);
    if (base_len < 0) {
        goto exit_restore_reopen;
    }

    if (base_len < s->common.len) {
        ret = bdrv_truncate(base, s->common.len);
        if (ret) {
            goto exit_restore_reopen;
        }
    }

    overlay_bs = bdrv_find_overlay(active, top);

    end = s->common.len >> BDRV_SECTOR_BITS;
    buf = qemu_blockalign(top, COMMIT_BUFFER_SIZE);

    for (sector_num = 0; sector_num < end; sector_num += n) {
        uint64_t delay_ms = 0;
        bool copy;

wait:
        /* Note that even when no rate limit is applied we need to yield
         * with no pending I/O here so that qemu_aio_flush() returns.
         */
        block_job_sleep(&s->common, rt_clock, delay_ms);
        if (block_job_is_cancelled(&s->common)) {
            break;
        }
        /* Copy if allocated above the base */
        ret = bdrv_co_is_allocated_above(top, base, sector_num,
                                         COMMIT_BUFFER_SIZE / BDRV_SECTOR_SIZE,
                                         &n);
        copy = (ret == 1);
        trace_commit_one_iteration(s, sector_num, n, ret);
        if (copy) {
            if (s->common.speed) {
                delay_ms = ratelimit_calculate_delay(&s->limit, n);
                if (delay_ms > 0) {
                    goto wait;
                }
            }
            ret = commit_populate(top, base, sector_num, n, buf);
            bytes_written += n * BDRV_SECTOR_SIZE;
        }
        if (ret < 0) {
            if (s->on_error == BLOCK_ERR_STOP_ANY    ||
                s->on_error == BLOCK_ERR_REPORT      ||
                (s->on_error == BLOCK_ERR_STOP_ENOSPC && ret == -ENOSPC)) {
                goto exit_free_buf;
            } else {
                n = 0;
                continue;
            }
        }
        /* Publish progress */
        s->common.offset += n * BDRV_SECTOR_SIZE;
    }

    ret = 0;

    if (!block_job_is_cancelled(&s->common) && sector_num == end) {
        /* success */
        ret = bdrv_drop_intermediate(active, top, base);
    }

exit_free_buf:
    qemu_vfree(buf);

exit_restore_reopen:
    /* restore base open flags here if appropriate (e.g., change the base back
     * to r/o). These reopens do not need to be atomic, since we won't abort
     * even on failure here */
    if (s->base_flags != bdrv_get_flags(base)) {
        bdrv_reopen(base, s->base_flags, NULL);
    }
    if (s->orig_overlay_flags != bdrv_get_flags(overlay_bs)) {
        bdrv_reopen(overlay_bs, s->orig_overlay_flags, NULL);
    }
//.........这里部分代码省略.........

static int coroutine_fn stream_run(Job *job, Error **errp)
{
    StreamBlockJob *s = container_of(job, StreamBlockJob, common.job);
    BlockBackend *blk = s->common.blk;
    BlockDriverState *bs = blk_bs(blk);
    BlockDriverState *base = s->base;
    int64_t len;
    int64_t offset = 0;
    uint64_t delay_ns = 0;
    int error = 0;
    int ret = 0;
    int64_t n = 0; /* bytes */
    void *buf;

    if (!bs->backing) {
        goto out;
    }

    len = bdrv_getlength(bs);
    if (len < 0) {
        ret = len;
        goto out;
    }
    job_progress_set_remaining(&s->common.job, len);

    buf = qemu_blockalign(bs, STREAM_BUFFER_SIZE);

    /* Turn on copy-on-read for the whole block device so that guest read
     * requests help us make progress.  Only do this when copying the entire
     * backing chain since the copy-on-read operation does not take base into
     * account.
     */
    if (!base) {
        bdrv_enable_copy_on_read(bs);
    }

    for ( ; offset < len; offset += n) {
        bool copy;

        /* Note that even when no rate limit is applied we need to yield
         * with no pending I/O here so that bdrv_drain_all() returns.
         */
        job_sleep_ns(&s->common.job, delay_ns);
        if (job_is_cancelled(&s->common.job)) {
            break;
        }

        copy = false;

        ret = bdrv_is_allocated(bs, offset, STREAM_BUFFER_SIZE, &n);
        if (ret == 1) {
            /* Allocated in the top, no need to copy.  */
        } else if (ret >= 0) {
            /* Copy if allocated in the intermediate images.  Limit to the
             * known-unallocated area [offset, offset+n*BDRV_SECTOR_SIZE).  */
            ret = bdrv_is_allocated_above(backing_bs(bs), base,
                                          offset, n, &n);

            /* Finish early if end of backing file has been reached */
            if (ret == 0 && n == 0) {
                n = len - offset;
            }

            copy = (ret == 1);
        }
        trace_stream_one_iteration(s, offset, n, ret);
        if (copy) {
            ret = stream_populate(blk, offset, n, buf);
        }
        if (ret < 0) {
            BlockErrorAction action =
                block_job_error_action(&s->common, s->on_error, true, -ret);
            if (action == BLOCK_ERROR_ACTION_STOP) {
                n = 0;
                continue;
            }
            if (error == 0) {
                error = ret;
            }
            if (action == BLOCK_ERROR_ACTION_REPORT) {
                break;
            }
        }
        ret = 0;

        /* Publish progress */
        job_progress_update(&s->common.job, n);
        if (copy) {
            delay_ns = block_job_ratelimit_get_delay(&s->common, n);
        } else {
            delay_ns = 0;
        }
    }

    if (!base) {
        bdrv_disable_copy_on_read(bs);
    }

    /* Do not remove the backing file if an error was there but ignored.  */
    ret = error;
//.........这里部分代码省略.........

static void coroutine_fn mirror_run(void *opaque)
{
    MirrorBlockJob *s = opaque;
    BlockDriverState *bs = s->common.bs;
    int64_t sector_num, end, sectors_per_chunk, length;
    uint64_t last_pause_ns;
    BlockDriverInfo bdi;
    char backing_filename[1024];
    int ret = 0;
    int n;

    if (block_job_is_cancelled(&s->common)) {
        goto immediate_exit;
    }

    s->common.len = bdrv_getlength(bs);
    if (s->common.len <= 0) {
        block_job_completed(&s->common, s->common.len);
        return;
    }

    length = (bdrv_getlength(bs) + s->granularity - 1) / 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(s->target, backing_filename,
                              sizeof(backing_filename));
    if (backing_filename[0] && !s->target->backing_hd) {
        bdrv_get_info(s->target, &bdi);
        if (s->granularity < bdi.cluster_size) {
            s->buf_size = MAX(s->buf_size, bdi.cluster_size);
            s->cow_bitmap = bitmap_new(length);
        }
    }

    end = s->common.len >> BDRV_SECTOR_BITS;
    s->buf = qemu_blockalign(bs, s->buf_size);
    sectors_per_chunk = s->granularity >> BDRV_SECTOR_BITS;
    mirror_free_init(s);

    if (s->mode != MIRROR_SYNC_MODE_NONE) {
        /* First part, loop on the sectors and initialize the dirty bitmap.  */
        BlockDriverState *base;
        base = s->mode == MIRROR_SYNC_MODE_FULL ? NULL : bs->backing_hd;
        for (sector_num = 0; sector_num < end; ) {
            int64_t next = (sector_num | (sectors_per_chunk - 1)) + 1;
            ret = bdrv_is_allocated_above(bs, base,
                                          sector_num, next - sector_num, &n);

            if (ret < 0) {
                goto immediate_exit;
            }

            assert(n > 0);
            if (ret == 1) {
                bdrv_set_dirty(bs, sector_num, n);
                sector_num = next;
            } else {
                sector_num += n;
            }
        }
    }

    bdrv_dirty_iter_init(bs, &s->hbi);
    last_pause_ns = qemu_clock_get_ns(QEMU_CLOCK_REALTIME);
    for (;;) {
        uint64_t delay_ns;
        int64_t cnt;
        bool should_complete;

        if (s->ret < 0) {
            ret = s->ret;
            goto immediate_exit;
        }

        cnt = bdrv_get_dirty_count(bs);

        /* Note that even when no rate limit is applied we need to yield
         * periodically with no pending I/O so that qemu_aio_flush() returns.
         * We do so every SLICE_TIME nanoseconds, or when there is an error,
         * or when the source is clean, whichever comes first.
         */
        if (qemu_clock_get_ns(QEMU_CLOCK_REALTIME) - last_pause_ns < 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, s->in_flight, s->buf_free_count, cnt);
                qemu_coroutine_yield();
                continue;
            } else if (cnt != 0) {
                mirror_iteration(s);
                continue;
            }
        }

        should_complete = false;
        if (s->in_flight == 0 && cnt == 0) {
//.........这里部分代码省略.........

/* Flushes the descriptor described by desc to the VHDX image file.
 * If the descriptor is a data descriptor, than 'data' must be non-NULL,
 * and >= 4096 bytes (VHDX_LOG_SECTOR_SIZE), containing the data to be
 * written.
 *
 * Verification is performed to make sure the sequence numbers of a data
 * descriptor match the sequence number in the desc.
 *
 * For a zero descriptor, it may describe multiple sectors to fill with zeroes.
 * In this case, it should be noted that zeroes are written to disk, and the
 * image file is not extended as a sparse file.  */
static int vhdx_log_flush_desc(BlockDriverState *bs, VHDXLogDescriptor *desc,
                               VHDXLogDataSector *data)
{
    int ret = 0;
    uint64_t seq, file_offset;
    uint32_t offset = 0;
    void *buffer = NULL;
    uint64_t count = 1;
    int i;

    buffer = qemu_blockalign(bs, VHDX_LOG_SECTOR_SIZE);

    if (!memcmp(&desc->signature, "desc", 4)) {
        /* data sector */
        if (data == NULL) {
            ret = -EFAULT;
            goto exit;
        }

        /* The sequence number of the data sector must match that
         * in the descriptor */
        seq = data->sequence_high;
        seq <<= 32;
        seq |= data->sequence_low & 0xffffffff;

        if (seq != desc->sequence_number) {
            ret = -EINVAL;
            goto exit;
        }

        /* Each data sector is in total 4096 bytes, however the first
         * 8 bytes, and last 4 bytes, are located in the descriptor */
        memcpy(buffer, &desc->leading_bytes, 8);
        offset += 8;

        memcpy(buffer+offset, data->data, 4084);
        offset += 4084;

        memcpy(buffer+offset, &desc->trailing_bytes, 4);

    } else if (!memcmp(&desc->signature, "zero", 4)) {
        /* write 'count' sectors of sector */
        memset(buffer, 0, VHDX_LOG_SECTOR_SIZE);
        count = desc->zero_length / VHDX_LOG_SECTOR_SIZE;
    }

    file_offset = desc->file_offset;

    /* count is only > 1 if we are writing zeroes */
    for (i = 0; i < count; i++) {
        ret = bdrv_pwrite_sync(bs->file, file_offset, buffer,
                               VHDX_LOG_SECTOR_SIZE);
        if (ret < 0) {
            goto exit;
        }
        file_offset += VHDX_LOG_SECTOR_SIZE;
    }

exit:
    qemu_vfree(buffer);
    return ret;
}