[PATCH v2 0/2] block: Wire up 'flat' mode also for 'query-block'

[PATCH v2 0/2] block: Wire up 'flat' mode also for 'query-block'

[Peter Krempa]

v2:
 - More descriptive docs text for the new QAPI option (Markus; new
   wording not approved)
 - use new options in hmp_nbd_server_start to avoid fetching unused
   backing image info, impossible in 'info block' worker (Markus)

Peter Krempa (2):
  block: Wire up 'flat' mode also for 'query-block'
  hmp_nbd_server_start: Don't ask for backing image data

 block/monitor/block-hmp-cmds.c | 4 ++--
 block/qapi.c                   | 9 +++++----
 qapi/block-core.json           | 7 +++++++
 ui/cocoa.m                     | 2 +-
 4 files changed, 15 insertions(+), 7 deletions(-)

-- 
2.52.0

[PATCH v2 1/2] block: Wire up 'flat' mode also for 'query-block'

[Peter Krempa]

From: Peter Krempa <pkrempa@redhat.com>

Some time ago (commit facda5443f5a8) I've added 'flat' mode (which
omits 'backing-image' key in reply) to 'query-named-block-nodes' to
minimize the size of the returned JSON for deeper backing chains.

While 'query-block' behaved slightly better it turns out that in libvirt
we do call 'query-block' to figure out some information about the
block device (e.g. throttling info) but we don't look at the backing
chain itself.

Wire up 'flat' for 'query-block' so that libvirt can ask for an
abbreviated output. The implementation is much simpler as the internals
are shared with 'query-named-block-nodes'.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Acked-by: Markus Armbruster <armbru@redhat.com>
---
 block/monitor/block-hmp-cmds.c | 4 ++--
 block/qapi.c                   | 9 +++++----
 qapi/block-core.json           | 7 +++++++
 ui/cocoa.m                     | 2 +-
 4 files changed, 15 insertions(+), 7 deletions(-)

diff --git a/block/monitor/block-hmp-cmds.c b/block/monitor/block-hmp-cmds.c
index 3391cee4d2..bde25bb588 100644
--- a/block/monitor/block-hmp-cmds.c
+++ b/block/monitor/block-hmp-cmds.c
@@ -422,7 +422,7 @@ void hmp_nbd_server_start(Monitor *mon, const QDict *qdict)
     /* Then try adding all block devices.  If one fails, close all and
      * exit.
      */
-    block_list = qmp_query_block(NULL);
+    block_list = qmp_query_block(false, false, NULL);

     for (info = block_list; info; info = info->next) {
         if (!info->value->inserted) {
@@ -741,7 +741,7 @@ void hmp_info_block(Monitor *mon, const QDict *qdict)

     /* Print BlockBackend information */
     if (!nodes) {
-        block_list = qmp_query_block(NULL);
+        block_list = qmp_query_block(false, false, NULL);
     } else {
         block_list = NULL;
     }
diff --git a/block/qapi.c b/block/qapi.c
index 27e0ac6a32..3688d0e713 100644
--- a/block/qapi.c
+++ b/block/qapi.c
@@ -456,7 +456,7 @@ fail:

 /* @p_info will be set only on success. */
 static void GRAPH_RDLOCK
-bdrv_query_info(BlockBackend *blk, BlockInfo **p_info, Error **errp)
+bdrv_query_info(BlockBackend *blk, bool flat, BlockInfo **p_info, Error **errp)
 {
     BlockInfo *info = g_malloc0(sizeof(*info));
     BlockDriverState *bs = blk_bs(blk);
@@ -488,7 +488,7 @@ bdrv_query_info(BlockBackend *blk, BlockInfo **p_info, Error **errp)
     }

     if (bs && bs->drv) {
-        info->inserted = bdrv_block_device_info(blk, bs, false, errp);
+        info->inserted = bdrv_block_device_info(blk, bs, flat, errp);
         if (info->inserted == NULL) {
             goto err;
         }
@@ -698,11 +698,12 @@ bdrv_query_bds_stats(BlockDriverState *bs, bool blk_level)
     return s;
 }

-BlockInfoList *qmp_query_block(Error **errp)
+BlockInfoList *qmp_query_block(bool has_flat, bool flat, Error **errp)
 {
     BlockInfoList *head = NULL, **p_next = &head;
     BlockBackend *blk;
     Error *local_err = NULL;
+    bool return_flat = has_flat && flat;

     GRAPH_RDLOCK_GUARD_MAINLOOP();

@@ -714,7 +715,7 @@ BlockInfoList *qmp_query_block(Error **errp)
         }

         info = g_malloc0(sizeof(*info));
-        bdrv_query_info(blk, &info->value, &local_err);
+        bdrv_query_info(blk, return_flat, &info->value, &local_err);
         if (local_err) {
             error_propagate(errp, local_err);
             g_free(info);
diff --git a/qapi/block-core.json b/qapi/block-core.json
index b82af74256..f55cf01dca 100644
--- a/qapi/block-core.json
+++ b/qapi/block-core.json
@@ -855,6 +855,12 @@
 #
 # Get a list of `BlockInfo` for all virtual block devices.
 #
+# @flat: Omit the nested data about backing image (Omitted are
+#        contents of 'backing-image' key of the 'ImageInfo' struct
+#        which is returned as 'image' key of 'BlockDeviceInfo' struct
+#        returned as 'inserted' key in  'BlockInfo' struct returned
+#        by this command).  Default is false (Since 11.0)
+#
 # Returns: a list describing each virtual block device.  Filter nodes
 #     that were created implicitly are skipped over.
 #
@@ -945,6 +951,7 @@
 #        }
 ##
 { 'command': 'query-block', 'returns': ['BlockInfo'],
+  'data': { '*flat': 'bool' },
   'allow-preconfig': true }

 ##
diff --git a/ui/cocoa.m b/ui/cocoa.m
index 23b7a736d7..5b21fe3aea 100644
--- a/ui/cocoa.m
+++ b/ui/cocoa.m
@@ -1849,7 +1849,7 @@ static void addRemovableDevicesMenuItems(void)
     BlockInfoList *currentDevice, *pointerToFree;
     NSString *deviceName;

-    currentDevice = qmp_query_block(NULL);
+    currentDevice = qmp_query_block(false, false, NULL);
     pointerToFree = currentDevice;

     menu = [[[NSApp mainMenu] itemWithTitle:@"Machine"] submenu];
-- 
2.52.0

Re: [PATCH v2 1/2] block: Wire up 'flat' mode also for 'query-block'

[Eric Blake]

On Tue, Jan 20, 2026 at 03:45:56PM +0100, Peter Krempa wrote:
> From: Peter Krempa <pkrempa@redhat.com>
> 
> Some time ago (commit facda5443f5a8) I've added 'flat' mode (which
> omits 'backing-image' key in reply) to 'query-named-block-nodes' to
> minimize the size of the returned JSON for deeper backing chains.
> 
> While 'query-block' behaved slightly better it turns out that in libvirt
> we do call 'query-block' to figure out some information about the
> block device (e.g. throttling info) but we don't look at the backing
> chain itself.
> 
> Wire up 'flat' for 'query-block' so that libvirt can ask for an
> abbreviated output. The implementation is much simpler as the internals
> are shared with 'query-named-block-nodes'.
> 
> Signed-off-by: Peter Krempa <pkrempa@redhat.com>
> Acked-by: Markus Armbruster <armbru@redhat.com>
> ---
>  block/monitor/block-hmp-cmds.c | 4 ++--
>  block/qapi.c                   | 9 +++++----
>  qapi/block-core.json           | 7 +++++++
>  ui/cocoa.m                     | 2 +-
>  4 files changed, 15 insertions(+), 7 deletions(-)
> +++ b/qapi/block-core.json
> @@ -855,6 +855,12 @@
>  #
>  # Get a list of `BlockInfo` for all virtual block devices.
>  #
> +# @flat: Omit the nested data about backing image (Omitted are
> +#        contents of 'backing-image' key of the 'ImageInfo' struct
> +#        which is returned as 'image' key of 'BlockDeviceInfo' struct
> +#        returned as 'inserted' key in  'BlockInfo' struct returned

Extra space.

> +#        by this command).  Default is false (Since 11.0)

Still hard to parse.  Maybe:

@flat: Omit nested data about the backing image (ie. the contents at
       'inserted.image.backing-image' are trimmed to avoid duplicate
       data about that backing image).  Default is false (Since 11.0)

The idea makes sense to me, but I will defer to Markus on whether the
wording still needs a tweak.


-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.
Virtualization:  qemu.org | libguestfs.org

Re: [PATCH v2 1/2] block: Wire up 'flat' mode also for 'query-block'

[Peter Krempa]

On Mon, Jan 26, 2026 at 13:27:33 -0600, Eric Blake wrote:
> On Tue, Jan 20, 2026 at 03:45:56PM +0100, Peter Krempa wrote:
> > From: Peter Krempa <pkrempa@redhat.com>
> > 
> > Some time ago (commit facda5443f5a8) I've added 'flat' mode (which
> > omits 'backing-image' key in reply) to 'query-named-block-nodes' to
> > minimize the size of the returned JSON for deeper backing chains.
> > 
> > While 'query-block' behaved slightly better it turns out that in libvirt
> > we do call 'query-block' to figure out some information about the
> > block device (e.g. throttling info) but we don't look at the backing
> > chain itself.
> > 
> > Wire up 'flat' for 'query-block' so that libvirt can ask for an
> > abbreviated output. The implementation is much simpler as the internals
> > are shared with 'query-named-block-nodes'.
> > 
> > Signed-off-by: Peter Krempa <pkrempa@redhat.com>
> > Acked-by: Markus Armbruster <armbru@redhat.com>
> > ---
> >  block/monitor/block-hmp-cmds.c | 4 ++--
> >  block/qapi.c                   | 9 +++++----
> >  qapi/block-core.json           | 7 +++++++
> >  ui/cocoa.m                     | 2 +-
> >  4 files changed, 15 insertions(+), 7 deletions(-)
> > +++ b/qapi/block-core.json
> > @@ -855,6 +855,12 @@
> >  #
> >  # Get a list of `BlockInfo` for all virtual block devices.
> >  #
> > +# @flat: Omit the nested data about backing image (Omitted are
> > +#        contents of 'backing-image' key of the 'ImageInfo' struct
> > +#        which is returned as 'image' key of 'BlockDeviceInfo' struct
> > +#        returned as 'inserted' key in  'BlockInfo' struct returned
> 
> Extra space.
> 
> > +#        by this command).  Default is false (Since 11.0)
> 
> Still hard to parse.  Maybe:
> 
> @flat: Omit nested data about the backing image (ie. the contents at
>        'inserted.image.backing-image' are trimmed to avoid duplicate
>        data about that backing image).  Default is false (Since 11.0)

In this case the 'backing-image' data is not duplicate as it was the
case with 'query-named-block-nodes', as it is not repeated on top level.

Here using 'flat' decreases the amount of returned information, but in
case of libvirt we don't use anything from the 'backing-image', just
stuff that is related to the frontend device which is not queryable via
'query-named-block-nodes'.

So then perhaps just shorten it to:

 @flat: Omit nested data about the backing image (ie. the contents at
        'inserted.image.backing-image' are trimmed).
        Default is false (Since 11.0)

> The idea makes sense to me, but I will defer to Markus on whether the
> wording still needs a tweak.

Should I post a v3?

[PATCH v2 2/2] hmp_nbd_server_start: Don't ask for backing image data

[Peter Krempa]

From: Peter Krempa <pkrempa@redhat.com>

'hmp_nbd_server_start' uses only the device name from the data returned
from 'qmp_query_block', thus no backing file information. Use the new
options to suppress asking for the unused parts to save on resources.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
---
 block/monitor/block-hmp-cmds.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/block/monitor/block-hmp-cmds.c b/block/monitor/block-hmp-cmds.c
index bde25bb588..1fd28d59eb 100644
--- a/block/monitor/block-hmp-cmds.c
+++ b/block/monitor/block-hmp-cmds.c
@@ -422,7 +422,7 @@ void hmp_nbd_server_start(Monitor *mon, const QDict *qdict)
     /* Then try adding all block devices.  If one fails, close all and
      * exit.
      */
-    block_list = qmp_query_block(false, false, NULL);
+    block_list = qmp_query_block(true, true, NULL);

     for (info = block_list; info; info = info->next) {
         if (!info->value->inserted) {
-- 
2.52.0

Re: [PATCH v2 2/2] hmp_nbd_server_start: Don't ask for backing image data

[Eric Blake]

On Tue, Jan 20, 2026 at 03:45:57PM +0100, Peter Krempa wrote:
> From: Peter Krempa <pkrempa@redhat.com>
> 
> 'hmp_nbd_server_start' uses only the device name from the data returned
> from 'qmp_query_block', thus no backing file information. Use the new
> options to suppress asking for the unused parts to save on resources.
> 
> Signed-off-by: Peter Krempa <pkrempa@redhat.com>
> ---
>  block/monitor/block-hmp-cmds.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)

Reviewed-by: Eric Blake <eblake@redhat.com>

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.
Virtualization:  qemu.org | libguestfs.org