block/qapi: include child references in block device info

In combination with using a throttle filter to enforce IO limits for
a guest device, knowing the 'file' child of a block device can be
useful. If the throttle filter is only intended for guest IO, block
jobs should not also be limited by the throttle filter, so the
block operations need to be done with the 'file' child of the top
throttle node as the target. In combination with mirroring, the name
of that child is not fixed.

Another scenario is when unplugging a guest device after mirroring
below a top throttle node, where the mirror target is added explicitly
via blockdev-add. After mirroring, the target becomes the new 'file'
child of the throttle node. For unplugging, both the top throttle node
and the mirror target need to be deleted, because only implicitly
added child nodes are deleted automatically, and the current 'file'
child of the throttle node was explicitly added (as the mirror
target).

In other scenarios, it could be useful to follow the backing chain.

Note that iotests 191 and 273 use _filter_img_info, so the 'children'
information is filtered out there.

Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>
Message-ID: <20250702123204.325470-2-f.ebner@proxmox.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

3 files changed, 34 insertions, 0 deletions

diff --git a/block/qapi.c b/block/qapi.c
index 2c50a6b..e08a1e9 100644
--- a/block/qapi.c
+++ b/block/qapi.c
@@ -51,6 +51,8 @@ BlockDeviceInfo *bdrv_block_device_info(BlockBackend *blk,
     ImageInfo *backing_info;
     BlockDriverState *backing;
     BlockDeviceInfo *info;
+    BlockdevChildList **children_list_tail;
+    BdrvChild *child;
 
     if (!bs->drv) {
         error_setg(errp, "Block device %s is ejected", bs->node_name);
@@ -77,6 +79,14 @@ BlockDeviceInfo *bdrv_block_device_info(BlockBackend *blk,
         info->node_name = g_strdup(bs->node_name);
     }
 
+    children_list_tail = &info->children;
+    QLIST_FOREACH(child, &bs->children, next) {
+        BlockdevChild *child_ref = g_new0(BlockdevChild, 1);
+        child_ref->child = g_strdup(child->name);
+        child_ref->node_name = g_strdup(child->bs->node_name);
+        QAPI_LIST_APPEND(children_list_tail, child_ref);
+    }
+
     backing = bdrv_cow_bs(bs);
     if (backing) {
         info->backing_file = g_strdup(backing->filename);
diff --git a/qapi/block-core.json b/qapi/block-core.json
index 1df6644..3e720af 100644
--- a/qapi/block-core.json
+++ b/qapi/block-core.json
@@ -462,6 +462,19 @@
             'no-flush': 'bool' } }
 
 ##
+# @BlockdevChild:
+#
+# @child: The name of the child, for example 'file' or 'backing'.
+#
+# @node-name: The name of the child's block driver node.
+#
+# Since: 10.1
+##
+{ 'struct': 'BlockdevChild',
+  'data': { 'child': 'str',
+            'node-name': 'str' } }
+
+##
 # @BlockDeviceInfo:
 #
 # Information about the backing device for a block device.
@@ -486,6 +499,8 @@
 # @backing_file_depth: number of files in the backing file chain
 #     (since: 1.2)
 #
+# @children: Information about child block nodes. (since: 10.1)
+#
 # @active: true if the backend is active; typical cases for inactive backends
 #     are on the migration source instance after migration completes and on the
 #     destination before it completes.  (since: 10.0)
@@ -560,6 +575,7 @@
 { 'struct': 'BlockDeviceInfo',
   'data': { 'file': 'str', '*node-name': 'str', 'ro': 'bool', 'drv': 'str',
             '*backing_file': 'str', 'backing_file_depth': 'int',
+            'children': ['BlockdevChild'],
             'active': 'bool', 'encrypted': 'bool',
             'detect_zeroes': 'BlockdevDetectZeroesOptions',
             'bps': 'int', 'bps_rd': 'int', 'bps_wr': 'int',
diff --git a/tests/qemu-iotests/184.out b/tests/qemu-iotests/184.out
index 52692b6..ef99bb2 100644
--- a/tests/qemu-iotests/184.out
+++ b/tests/qemu-iotests/184.out
@@ -41,6 +41,12 @@ Testing:
             },
             "iops_wr": 0,
             "ro": false,
+            "children": [
+                {
+                    "node-name": "disk0",
+                    "child": "file"
+                }
+            ],
             "node-name": "throttle0",
             "backing_file_depth": 1,
             "drv": "throttle",
@@ -69,6 +75,8 @@ Testing:
             },
             "iops_wr": 0,
             "ro": false,
+            "children": [
+            ],
             "node-name": "disk0",
             "backing_file_depth": 0,
             "drv": "null-co",