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 * '-mon' is obsolete, deprecated and replaced by '-object',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.
The log on libvirt's side is not very interesting:
2026-06-23 11:00:11.229+0000: 529808: debug :
virSecuritySELinuxSetDaemonSocketLabel:3190 : Setting VM ha socket context
unconfined_u:unconfined_r:unconfined_t:s0:c677,c680
2026-06-23 11:00:11.229+0000: 529807: info : qemuMonitorOpenInternal:608 :
QEMU_MONITOR_NEW: mon=0x7fff94001b80 fd=35
2026-06-23 11:00:11.229+0000: 529807: debug :
qemuDomainObjEnterMonitorInternal:5165 : Entering monitor (mon=0x7fff94001b80
vm=0x7fffac185b60 name=cd)
2026-06-23 11:00:11.229+0000: 529807: debug : qemuMonitorSetCapabilities:1437 :
mon:0x7fff94001b80 vm:0x7fffac185b60 monfd:35
2026-06-23 11:00:11.229+0000: 529807: info : qemuMonitorSend:822 :
QEMU_MONITOR_SEND_MSG: mon=0x7fff94001b80
msg={"execute":"qmp_capabilities","id":"libvirt-1"}^M
as nothing else ever comes from monitor 0x7fff94001b80
