NBDExport *nbd_export_new(BlockDriverState *bs, off_t dev_offset, off_t size,
                          uint16_t nbdflags, void (*close)(NBDExport *),
                          bool writethrough, BlockBackend *on_eject_blk,
                          Error **errp)
{
    BlockBackend *blk;
    NBDExport *exp = g_malloc0(sizeof(NBDExport));
    uint64_t perm;
    int ret;

    /* Don't allow resize while the NBD server is running, otherwise we don't
     * care what happens with the node. */
    perm = BLK_PERM_CONSISTENT_READ;
    if ((nbdflags & NBD_FLAG_READ_ONLY) == 0) {
        perm |= BLK_PERM_WRITE;
    }
    blk = blk_new(perm, BLK_PERM_CONSISTENT_READ | BLK_PERM_WRITE_UNCHANGED |
                        BLK_PERM_WRITE | BLK_PERM_GRAPH_MOD);
    ret = blk_insert_bs(blk, bs, errp);
    if (ret < 0) {
        goto fail;
    }
    blk_set_enable_write_cache(blk, !writethrough);

    exp->refcount = 1;
    QTAILQ_INIT(&exp->clients);
    exp->blk = blk;
    exp->dev_offset = dev_offset;
    exp->nbdflags = nbdflags;
    exp->size = size < 0 ? blk_getlength(blk) : size;
    if (exp->size < 0) {
        error_setg_errno(errp, -exp->size,
                         "Failed to determine the NBD export's length");
        goto fail;
    }
    exp->size -= exp->size % BDRV_SECTOR_SIZE;

    exp->close = close;
    exp->ctx = blk_get_aio_context(blk);
    blk_add_aio_context_notifier(blk, blk_aio_attached, blk_aio_detach, exp);

    if (on_eject_blk) {
        blk_ref(on_eject_blk);
        exp->eject_notifier_blk = on_eject_blk;
        exp->eject_notifier.notify = nbd_eject_notifier;
        blk_add_remove_bs_notifier(on_eject_blk, &exp->eject_notifier);
    }

    /*
     * NBD exports are used for non-shared storage migration.  Make sure
     * that BDRV_O_INACTIVE is cleared and the image is ready for write
     * access since the export could be available before migration handover.
     */
    aio_context_acquire(exp->ctx);
    blk_invalidate_cache(blk, NULL);
    aio_context_release(exp->ctx);
    return exp;

fail:
    blk_unref(blk);
    g_free(exp);
    return NULL;
}
NBDExport *nbd_export_new(BlockDriverState *bs, off_t dev_offset, off_t size,
                          uint16_t nbdflags, void (*close)(NBDExport *),
                          bool writethrough, BlockBackend *on_eject_blk,
                          Error **errp)
{
    BlockBackend *blk;
    NBDExport *exp = g_malloc0(sizeof(NBDExport));
    uint64_t perm;
    int ret;

     
    perm = BLK_PERM_CONSISTENT_READ;
    if ((nbdflags & NBD_FLAG_READ_ONLY) == 0) {
        perm |= BLK_PERM_WRITE;
    }
    blk = blk_new(perm, BLK_PERM_CONSISTENT_READ | BLK_PERM_WRITE_UNCHANGED |
                        BLK_PERM_WRITE | BLK_PERM_GRAPH_MOD);
    ret = blk_insert_bs(blk, bs, errp);
    if (ret < 0) {
        goto fail;
    }
    blk_set_enable_write_cache(blk, !writethrough);

    exp->refcount = 1;
    QTAILQ_INIT(&exp->clients);
    exp->blk = blk;
    exp->dev_offset = dev_offset;
    exp->nbdflags = nbdflags;
    exp->size = size < 0 ? blk_getlength(blk) : size;
    if (exp->size < 0) {
        error_setg_errno(errp, -exp->size,
                         "Failed to determine the NBD export's length");
        goto fail;
    }
    exp->size -= exp->size % BDRV_SECTOR_SIZE;

    exp->close = close;
    exp->ctx = blk_get_aio_context(blk);
    blk_add_aio_context_notifier(blk, blk_aio_attached, blk_aio_detach, exp);

    if (on_eject_blk) {
        blk_ref(on_eject_blk);
        exp->eject_notifier_blk = on_eject_blk;
        exp->eject_notifier.notify = nbd_eject_notifier;
        blk_add_remove_bs_notifier(on_eject_blk, &exp->eject_notifier);
    }

     
    aio_context_acquire(exp->ctx);
    blk_invalidate_cache(blk, NULL);
    aio_context_release(exp->ctx);
    return exp;

fail:
    blk_unref(blk);
    g_free(exp);
    return NULL;
}