[pve-devel] [PATCH qemu 3/3] blockdev: query file child QMP command

[Fabian Grünbichler]

On June 26, 2025 6:04 pm, Fiona Ebner wrote:
> There currently does not seem to be a good way to obtain information
> about the file child of a node, so add a custom command. The
> query-block and query-named-block-nodes commands lack the necessary
> info and while x-debug-query-block-graph exists, that is explicitly
> only for debugging and experimental.
> 
> This is requried for e.g. blockdev-mirror in PVE to determine the
> node below the top throttle node.
> 
> Signed-off-by: Fiona Ebner <f.ebner at proxmox.com>
> ---
> 
> Alternatively, we could think about adding the node name of the file
> child to the info returned by query-named-block-nodes. I can give that
> a shot and also ask upstream which approach is prefered.

that might be nice, or alternatively if we go with our custom solution
here for now - give it a prefixed name to avoid potential clashes with a
future upstream variant?

> 
>  blockdev.c           | 22 ++++++++++++++++++++++
>  qapi/block-core.json | 13 +++++++++++++
>  2 files changed, 35 insertions(+)
> 
> diff --git a/blockdev.c b/blockdev.c
> index 17de5d2ae4..3be209767a 100644
> --- a/blockdev.c
> +++ b/blockdev.c
> @@ -57,6 +57,7 @@
>  #include "system/system.h"
>  #include "system/iothread.h"
>  #include "block/block_int.h"
> +#include "block/qapi.h"
>  #include "block/trace.h"
>  #include "system/runstate.h"
>  #include "system/replay.h"
> @@ -2782,6 +2783,27 @@ XDbgBlockGraph *qmp_x_debug_query_block_graph(Error **errp)
>      return bdrv_get_xdbg_block_graph(errp);
>  }
>  
> +BlockDeviceInfo *qmp_block_node_query_file_child(const char *node_name,
> +                                                 Error **errp)
> +{
> +    GRAPH_RDLOCK_GUARD_MAINLOOP();
> +
> +    BlockDriverState *bs;
> +
> +    bs = bdrv_find_node(node_name);
> +    if (!bs) {
> +        error_setg(errp, "Failed to find node with node-name='%s'", node_name);
> +        return NULL;
> +    }
> +
> +    if (!bs->file || !bs->file->bs) {
> +        error_setg(errp, "Node '%s' has no 'file' child", node_name);
> +        return NULL;
> +    }
> +
> +    return bdrv_block_device_info(NULL, bs->file->bs, false, errp);
> +}
> +
>  void qmp_blockdev_backup(BlockdevBackup *backup, Error **errp)
>  {
>      TransactionAction action = {
> diff --git a/qapi/block-core.json b/qapi/block-core.json
> index f8ed564cf0..6c683b00ec 100644
> --- a/qapi/block-core.json
> +++ b/qapi/block-core.json
> @@ -2463,6 +2463,19 @@
>    'features': [ 'unstable' ],
>    'allow-preconfig': true }
>  
> +##
> +# @block-node-query-file-child:
> +#
> +# Get information about a child of the specified block node.
> +#
> +# @node-name: the block node name of the node to query.
> +#
> +# Returns: the BlockDeviceInfo of the file child.
> +##
> +{ 'command': 'block-node-query-file-child',
> +  'data': { 'node-name': 'str' },
> +  'returns': 'BlockDeviceInfo' }
> +
>  ##
>  # @drive-mirror:
>  #
> -- 
> 2.47.2
> 
> 
> 
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