On Tue, May 12, 2026 at 2:14 AM Stefan Hajnoczi <[email protected]> wrote:
>
> On Mon, May 11, 2026 at 10:04:04PM +0800, Zhang Chen wrote:
> > Currently, IOThreads do not maintain a record of which devices are
> > associated with them. This makes it difficult to monitor the
> > workload distribution of IOThreads, especially in complex
> > hotplug scenarios involving multiple virtio-blk or virtio-scsi devices.
> >
> > This patch introduces a reference counting and tracking mechanism
> > within the IOThread object:
> >
> > - iothread_ref(): Prepends the device's QOM path to a list.
> > - iothread_unref(): Searches for the device path using a custom
> > string comparison (g_strcmp0), releases the associated memory
> > upon a successful match.
> > - holders: A GList storing the QOM paths of attached devices
> > for runtime introspection.
> >
> > A later commit will add QMP commands to let management applications
> > query the attachment status of IOThreads.
> >
> > Signed-off-by: Zhang Chen <[email protected]>
> > ---
> > include/system/iothread.h | 5 +++
> > iothread.c | 67 +++++++++++++++++++++++++++++++++++++++
> > qapi/misc.json | 48 ++++++++++++++++++++++++++++
> > 3 files changed, 120 insertions(+)
> >
> > diff --git a/include/system/iothread.h b/include/system/iothread.h
> > index a1ef7696cb..2871b06edc 100644
> > --- a/include/system/iothread.h
> > +++ b/include/system/iothread.h
> > @@ -50,6 +50,11 @@ struct IOThread {
> > bool stopping; /* has iothread_stop() been called? */
> > bool running; /* should iothread_run() continue? */
> > int thread_id;
> > + /*
> > + * The list elements are of type IoThreadHolder, which can
> > + * represent either a QOM path or a block node name.
> > + */
> > + GList *holders;
> >
> > /* AioContext poll parameters */
> > int64_t poll_max_ns;
> > diff --git a/iothread.c b/iothread.c
> > index 3558535b40..b805e4f97d 100644
> > --- a/iothread.c
> > +++ b/iothread.c
> > @@ -25,6 +25,66 @@
> > #include "qemu/rcu.h"
> > #include "qemu/main-loop.h"
> >
> > +/*
> > + * Add the @holder path to the iothread's tracking list.
> > + * The @holder is a QOM path if it starts with '/', else a block node name.
> > + */
>
> It would be cleaner to use IoThreadHolder in the iothread_ref() and
> iothread_unref() APIs instead of a string:
>
> static void iothread_ref(IOThread *iothread, const IoThreadHolder *holder);
> static void iothread_unref(IOThread *iothread, const IoThreadHolder
> *holder);
>
> That way the caller already uses IO_THREAD_HOLDER_KIND_QOM_OBJECT or
> IO_THREAD_HOLDER_KIND_BLOCK_NODE to clearly distinguish QOM paths from
> block node names. No string parsing is necessary.
>
Good idea.
Will optimize it in the next version.
> > +static void iothread_ref(IOThread *iothread, const char *holder)
> > +{
> > + IoThreadHolder *h = g_new0(IoThreadHolder, 1);
>
> IoThreadHolder vs IOThread. Name it IOThreadHolder for consistency?
OK, will change the "IoThreadHolder" to "IOThreadHolder".
>
> > +
> > + assert(holder);
> > +
> > + if (holder[0] == '/') {
> > + h->type = IO_THREAD_HOLDER_KIND_QOM_OBJECT;
> > + h->u.qom_object.data = g_strdup(holder);
> > + } else {
> > + h->type = IO_THREAD_HOLDER_KIND_BLOCK_NODE;
> > + h->u.block_node.data = g_strdup(holder);
> > + }
> > +
> > + iothread->holders = g_list_prepend(iothread->holders, h);
> > +}
> > +
> > +static int iothread_holder_compare(gconstpointer a, gconstpointer b)
> > +{
> > + const IoThreadHolder *holder_node = a;
> > + const char *target_name = b;
> > + const char *current_name;
> > +
> > + if (holder_node->type == IO_THREAD_HOLDER_KIND_QOM_OBJECT) {
> > + current_name = holder_node->u.qom_object.data;
> > + } else if (holder_node->type == IO_THREAD_HOLDER_KIND_BLOCK_NODE) {
> > + current_name = holder_node->u.block_node.data;
> > + } else {
> > + /*
> > + * This should not happen. If it does, current_name remains
> > + * NULL and g_strcmp0 will handle it safely.
> > + */
> > + current_name = NULL;
> > + }
> > +
> > + return g_strcmp0(current_name, target_name);
> > +}
> > +
> > +/*
> > + * This function removes the @holder from the @iothread's tracking list.
> > + * The @holder string must match the one used previously in iothread_ref().
> > + * It is a programming error to call this with a @holder that is not
> > + * currently associated with the @iothread.
> > + */
> > +static void iothread_unref(IOThread *iothread, const char *holder)
> > +{
> > + GList *link = g_list_find_custom(iothread->holders, holder,
> > +
> > (GCompareFunc)iothread_holder_compare);
> > +
> > + assert(link);
> > +
> > + IoThreadHolder *h = (IoThreadHolder *)link->data;
> > + qapi_free_IoThreadHolder(h);
> > + iothread->holders = g_list_delete_link(iothread->holders, link);
> > +}
> > +
> > static void *iothread_run(void *opaque)
> > {
> > IOThread *iothread = opaque;
> > @@ -108,6 +168,9 @@ static void iothread_instance_finalize(Object *obj)
> >
> > iothread_stop(iothread);
> >
> > + /* We don't support finalize without holders */
>
> The double-negative is confusing and the logic seems incorrect: we don't
> support finalize while there are holders. A clearer comment would be:
>
> /* All holders must have called iothread_unref() */
>
Sure.
Thanks
Chen
> > + assert(iothread->holders == NULL);
> > +
> > /*
> > * Before glib2 2.33.10, there is a glib2 bug that GSource context
> > * pointer may not be cleared even if the context has already been
> > @@ -356,6 +419,10 @@ char *iothread_get_id(IOThread *iothread)
> >
> > AioContext *iothread_get_aio_context(IOThread *iothread)
> > {
> > + /* Remove in next patch for build */
> > + iothread_ref(iothread, "tmp");
> > + iothread_unref(iothread, "tmp");
> > +
> > return iothread->ctx;
> > }
> >
> > diff --git a/qapi/misc.json b/qapi/misc.json
> > index c71a5fe657..5fb7dcfcad 100644
> > --- a/qapi/misc.json
> > +++ b/qapi/misc.json
> > @@ -67,6 +67,54 @@
> > ##
> > { 'command': 'query-name', 'returns': 'NameInfo', 'allow-preconfig': true }
> >
> > +
> > +##
> > +# @IoThreadHolderBlockNode:
> > +#
> > +# @data: Block node name.
> > +#
> > +# Since: 11.1
> > +#
> > +##
> > +{ 'struct': 'IoThreadHolderBlockNode',
> > + 'data': { 'data': 'str' } }
> > +
> > +##
> > +# @IoThreadHolderQomObject:
> > +#
> > +# @data: Absolute @qom-path.
> > +#
> > +# Since: 11.1
> > +#
> > +##
> > +{ 'struct': 'IoThreadHolderQomObject',
> > + 'data': { 'data': 'str' } }
> > +
> > +##
> > +# @IoThreadHolderKind:
> > +#
> > +# @block-node: Block node name.
> > +# @qom-object: Absolute @qom-path.
> > +#
> > +# Since: 11.1
> > +##
> > +{ 'enum': 'IoThreadHolderKind',
> > + 'data': [ 'block-node', 'qom-object' ] }
> > +
> > +##
> > +# @IoThreadHolder:
> > +#
> > +# @type: the kind of I/O thread holder.
> > +#
> > +# Since: 11.1
> > +##
> > +{ 'union': 'IoThreadHolder',
> > + 'base': { 'type': 'IoThreadHolderKind' },
> > + 'discriminator': 'type',
> > + 'data': {
> > + 'block-node': 'IoThreadHolderBlockNode',
> > + 'qom-object': 'IoThreadHolderQomObject' } }
> > +
> > ##
> > # @IOThreadInfo:
> > #
> > --
> > 2.49.0
> >
