On Thu, Mar 12, 2026 at 5:16 PM Markus Armbruster <[email protected]> wrote: > > Context... we're talking about this command: > > ## > # @x-blockdev-set-iothread: > # > # Move @node and its children into the @iothread. If @iothread is > # null then move @node and its children into the main loop. > # > # The node must not be attached to a BlockBackend. > # > # @node-name: the name of the block driver node > # > # @iothread: the name of the IOThread object or null for the main loop > # > # @force: true if the node and its children should be moved when a > # BlockBackend is already attached > # > # Features: > # > # @unstable: This command is experimental and intended for test cases > # that need control over IOThreads only. > # > # Since: 2.12 > # > # .. qmp-example:: > # :title: Move a node into an IOThread > # > # -> { "execute": "x-blockdev-set-iothread", > # "arguments": { "node-name": "disk1", > # "iothread": "iothread0" } } > # <- { "return": {} } > # > # .. qmp-example:: > # :title: Move a node into the main loop > # > # -> { "execute": "x-blockdev-set-iothread", > # "arguments": { "node-name": "disk1", > # "iothread": null } } > # <- { "return": {} } > ## > { 'command': 'x-blockdev-set-iothread', > 'data' : { 'node-name': 'str', > 'iothread': 'StrOrNull', > '*force': 'bool' }, > 'features': [ 'unstable' ], > 'allow-preconfig': true } > > > Stefan Hajnoczi <[email protected]> writes: > > > On Tue, Mar 10, 2026 at 06:02:54PM +0800, Zhang Chen wrote: > >> On Mon, Mar 9, 2026 at 4:15 PM Stefan Hajnoczi <[email protected]> wrote: > >> > > >> > On Thu, Mar 05, 2026 at 10:24:50PM +0800, Zhang Chen wrote: > >> > > Update the usage of "iothread_get_aio_context()". > >> > > > >> > > Signed-off-by: Zhang Chen <[email protected]> > >> > > --- > >> > > blockdev.c | 9 ++++++++- > >> > > 1 file changed, 8 insertions(+), 1 deletion(-) > >> > > > >> > > diff --git a/blockdev.c b/blockdev.c > >> > > index 6e86c6262f..01ccf64b3f 100644 > >> > > --- a/blockdev.c > >> > > +++ b/blockdev.c > >> > > @@ -3683,7 +3683,14 @@ void qmp_x_blockdev_set_iothread(const char > >> > > *node_name, StrOrNull *iothread, > >> > > goto out; > >> > > } > >> > > > >> > > - new_context = iothread_get_aio_context(obj); > >> > > + char *path = object_get_canonical_path(OBJECT(bs)); > >> > > >> > CCing Kevin and Markus in case they have an opinion on this. > >> > > >> > BlockDriverState is not a QOM Object so using OBJECT(bs) is undefined > >> > behavior and may crash. > > Yes. > > >> > node_name is unique across block driver graph nodes and could be used. > >> > Unfortunately it's not connected to the QOM Object hierarchy. > > Correct. > > >> > Maybe it's > >> > best to build a holder name that is an invalid QOM path so there can be > >> > no collisions between QOM paths and block driver graph nodes. > > I guess you're talking about the values that go into IOThreadInfo member > holders. From PATCH 13: > > # @holders: The parameter is an array of QOM paths indicating how many > # active devices are currently associated with this iothread > # (e.g. virtio-blk). In hotplug scenarios, users can > # pre-allocate multiple iothread objects to serve as a persistent > # thread pool. When a device is hot-unplugged, the corresponding > # IOThread is released but remains available, allowing subsequent > # hot-plugged devices to attach to and reuse the existing thread. > # Returns empty if no devices are attached. (since 11.0) > # > > I further guess you need it to refer to both QOM objects and block > nodes, and you worry about ambiguity. > > Ambiguity indeed exists: a block node name can be a valid QOM path. > > We could restrict QOM paths to absolute paths. These start with '/'. > If I remember correctly, node names cannot contain '/'. > > Note that canonical paths (returned object_get_canonical_path()) are > absolute. > > We ran into a similar design issue in review of Vladimir's "[PATCH v10 > 4/8] qapi: add blockdev-replace command" not too long ago: > > Subject: Re: [PATCH v10 4/8] qapi: add blockdev-replace command > Date: Wed, 04 Feb 2026 13:26:35 +0100 > Message-ID: <[email protected]> > > There, the new command needs to refer to QOM object, block node, or > block export. > > >> > g_autofree char *holder = g_strdup_printf("BlockDriverState %s", > >> > node_name); > >> > > >> > (A cleaner long-term solution would be making BlockDriverStates QOM > >> > Objects so they have a proper path.) > > Yes, but that's a beefy project, isn't it? > > >> If no other comments, it's OK for me. This issue like I mentioned in > >> patch 7 and 9. > > > > A thought about the QAPI interface: > > > > QAPI expresses as much information in the schema as possible, so I think > > the right approach would be a {'union': 'IOThreadHolder', > > 'discriminator': 'type', ...} that supports at least "qom" and > > "block-node". That way there are proper types to encode QOM Object paths > > vs block node-names. Let's avoid having a single string value that takes > > on different meaning depending on the type of holder. > > This shifts the complexity from semantics to syntax. > > Semantics: the member can have multiple meanings, and you have to > examine its value to decide which one applies. The member's > documentation should specify how to decide. Say something like "if the > value starts with '/', it's an absolute QOM path, else it's a block node > name". > > Syntax: meaning is syntactically obvious. For instance, union of QOM > path and block node name. > > Complex semantics tend to require more complex documentation. > > Which choice is better depends on the specific case. I generally lean > towards syntax. >
I agree Markus's suggestion. Compare with standard QOM path: /machine/peripheral/blk0/virtio-backend I will try to implement block nodes path like this: /machine/blockdriverstate/node-name And Markus, could you please take a look at patch 13? I will prepare the V6 after your comments. Thanks Chen
