On Tue, Jun 23, 2026 at 12:38:10PM +0100, Daniel P. Berrangé wrote:
> On Tue, Jun 23, 2026 at 01:03:46PM +0200, Peter Krempa wrote:
> > On Tue, Jun 23, 2026 at 10:55:16 +0100, Daniel P. Berrangé via Devel wrote:
> > > Conceptually -object and object_add/object_del should be sufficient
> > > for essentially all QEMU configuration....if only we ported all our
> > > internal custom backends/devices/etc to QOM. That is of course a big
> > > job which is why it hasn't happened.
> > >
> > > This series started with the premise that the monitor is one of the
> > > easier areas to convert since we have no more than three classes,
> > > a common base, and QMP and HMP subclasses[1]. So why not give it a
> > > go and thus unlock the ability to dynamically create/delete monitors
> > > in QMP/HMP.
> > >
> > > This series does the conversion in a great many small steps to better
> > > understand the implications at each stage.
> > >
> > > The high level outcome of this series is
> > >
> > > * HMP and QMP monitors are QOM objects, 'monitor-hmp' and
> > > 'monitor-qmp' respectively
> > >
> > > * Both can be cold plugged and hot plugged. QMP only, can
> > > also be hot unplugged.
> > >
> > > * '-mon' is obsolete, deprecated and replaced by '-object',
> > > but -monitor, -qmp and kept as high level syntax sugar
> >
> > I was giving this a spin by implementing libvirt support for it. The
> > basic usage seems to work well but I've encoutered a regression in
> > behaviour which happens both when the QMP monitor is instantiated using
> > the new syntax but also the old one
> > ( -mon chardev=charmonitor,id=monitor,mode=control)
> >
> > When libvirt wants to re-connect to a qemu running with these patches it
> > re-issues a 'qmp_capabilities' command after connecting.
> >
> > With these patches qemu does not respond to that command even when the
> > old syntax is used. This makes libvirt stuck when re-connecting to VMs.
>
> Oh that's odd, thanks for the warning. I'll investigate this.
/face-palm - a misplaced a line in the auto-delete code, and did not
have test coverage of re-opening a monitor. Try testing libvirt with
this additional change on top
diff --git a/monitor/qmp.c b/monitor/qmp.c
index f6b7fe65bb..10d651ca5c 100644
--- a/monitor/qmp.c
+++ b/monitor/qmp.c
@@ -613,7 +613,6 @@ static void monitor_qmp_event(void *opaque, QEMUChrEvent
event)
qobject_unref(data);
break;
case CHR_EVENT_CLOSED:
- mon->delete_pending = true;
/*
* Note: this is only useful when the output of the chardev
* backend is still open. For example, when the backend is
@@ -629,6 +628,7 @@ static void monitor_qmp_event(void *opaque, QEMUChrEvent
event)
case MONITOR_QMP_CLOSE_ACTION_NONE:
break; /* nada */
case MONITOR_QMP_CLOSE_ACTION_DELETE:
+ mon->delete_pending = true;
/*
* Do NOT run in the AIO context associated with the
* monitor. We need to run in the default AIO context
With regards,
Daniel
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