blockdev: Allow creation of BDS trees without BB

If the "id" field is missing from the options given to blockdev-add,
just omit the BlockBackend and create the BlockDriverState tree alone.

However, if "id" is missing, "node-name" must be specified; otherwise,
the BDS tree would no longer be accessible.

Many BDS options which are not parsed by bdrv_open() (like caching)
cannot be specified for these BB-less BDS trees yet. A future patch will
remove this limitation.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Alberto Garcia <berto@igalia.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

blockdev.c

@@ -3026,17 +3026,12 @@ out:
 void qmp_blockdev_add(BlockdevOptions *options, Error **errp)
 {
     QmpOutputVisitor *ov = qmp_output_visitor_new();
-    BlockBackend *blk;
+    BlockDriverState *bs;
+    BlockBackend *blk = NULL;
     QObject *obj;
     QDict *qdict;
     Error *local_err = NULL;
 
-    /* Require an ID in the top level */
-    if (!options->has_id) {
-        error_setg(errp, "Block device needs an ID");
-        goto fail;
-    }
-
     /* TODO Sort it out in raw-posix and drive_new(): Reject aio=native with
      * cache.direct=false instead of silently switching to aio=threads, except
      * when called from drive_new().
@@ -3064,14 +3059,37 @@ void qmp_blockdev_add(BlockdevOptions *options, Error **errp)
 
     qdict_flatten(qdict);
 
-    blk = blockdev_init(NULL, qdict, &local_err);
-    if (local_err) {
-        error_propagate(errp, local_err);
-        goto fail;
+    if (options->has_id) {
+        blk = blockdev_init(NULL, qdict, &local_err);
+        if (local_err) {
+            error_propagate(errp, local_err);
+            goto fail;
+        }
+
+        bs = blk_bs(blk);
+    } else {
+        int ret;
+
+        if (!qdict_get_try_str(qdict, "node-name")) {
+            error_setg(errp, "'id' and/or 'node-name' need to be specified for "
+                       "the root node");
+            goto fail;
+        }
+
+        bs = NULL;
+        ret = bdrv_open(&bs, NULL, NULL, qdict, BDRV_O_RDWR | BDRV_O_CACHE_WB,
+                        errp);
+        if (ret < 0) {
+            goto fail;
+        }
     }
 
-    if (bdrv_key_required(blk_bs(blk))) {
-        blk_unref(blk);
+    if (bs && bdrv_key_required(bs)) {
+        if (blk) {
+            blk_unref(blk);
+        } else {
+            bdrv_unref(bs);
+        }
         error_setg(errp, "blockdev-add doesn't support encrypted devices");
         goto fail;
     }

qapi/block-core.json

@@ -1393,9 +1393,12 @@
 #
 # @driver:        block driver name
 # @id:            #optional id by which the new block device can be referred to.
-#                 This is a required option on the top level of blockdev-add, and
-#                 currently not allowed on any other level.
-# @node-name:     #optional the name of a block driver state node (Since 2.0)
+#                 This option is only allowed on the top level of blockdev-add.
+#                 A BlockBackend will be created by blockdev-add if and only if
+#                 this option is given.
+# @node-name:     #optional the name of a block driver state node (Since 2.0).
+#                 This option is required on the top level of blockdev-add if
+#                 the @id option is not given there.
 # @discard:       #optional discard-related options (default: ignore)
 # @cache:         #optional cache-related options
 # @aio:           #optional AIO backend (default: threads)
@@ -1859,7 +1862,9 @@
 ##
 # @blockdev-add:
 #
-# Creates a new block device.
+# Creates a new block device. If the @id option is given at the top level, a
+# BlockBackend will be created; otherwise, @node-name is mandatory at the top
+# level and no BlockBackend will be created.
 #
 # This command is still a work in progress.  It doesn't support all
 # block drivers, it lacks a matching blockdev-del, and more.  Stay

tests/qemu-iotests/087

@@ -54,7 +54,7 @@ size=128M
 _make_test_img $size
 
 echo
-echo === Missing ID ===
+echo === Missing ID and node-name ===
 echo
 
 run_qemu <<EOF

tests/qemu-iotests/087.out

@@ -1,12 +1,12 @@
 QA output created by 087
 Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134217728
 
-=== Missing ID ===
+=== Missing ID and node-name ===
 
 Testing:
 QMP_VERSION
 {"return": {}}
-{"error": {"class": "GenericError", "desc": "Block device needs an ID"}}
+{"error": {"class": "GenericError", "desc": "'id' and/or 'node-name' need to be specified for the root node"}}
 {"return": {}}
 {"timestamp": {"seconds":  TIMESTAMP, "microseconds":  TIMESTAMP}, "event": "SHUTDOWN"}