Re: [PATCH RFC V6 22/24] monitor,qdev: Introduce 'device_set' to change admin state of existing devices

[Markus Armbruster <armbru@redhat.com>]

Igor Mammedov <imammedo@redhat.com> writes:

> On Mon, 20 Oct 2025 13:22:08 +0200
> Markus Armbruster <armbru@redhat.com> wrote:
>
>> Igor Mammedov <imammedo@redhat.com> writes:
>> 
>> > On Thu, 09 Oct 2025 16:55:54 +0200
>> > Markus Armbruster <armbru@redhat.com> wrote:

[...]

>> >> I feel it's best to start the design process with ensvisaged uses.  Can
>> >> you tell me a bit more about the uses you have in mind?  
>> >
>> > We have nic failover 'feature'
>> >    https://www.qemu.org/docs/master/system/virtio-net-failover.html
>> > to make it work we do abuse hotplug and that poses problem
>> > during migration, since:
>> >   - unplugging primary device releases resources (which might not be
>> >     possible to claim back in case migration failure)  
>> 
>> Serious reliability issue with no work-around.
>> 
>> >   - it's similar on destination side, where attempt to hotplug
>> >     primary might fail die to insufficient resources leaving guest
>> >     on 'degraded' virtio-net link.  
>> 
>> Obvious work-around is failing the migration.  Same as we do when we
>> can't create devices.
>> 
>> > Idea was that instead of hotplug we can power off primary device,
>> > (it will still exist and keep resources), initiate migration,
>> > and then on target do the same starting with primary fully realized
>> > but powered of (and failing migration early if it can't claim resources,
>> > safely resuming QEMU on source incl. primary link), and then guest
>> > failover driver on destination would power primary on as part of
>> > switching to primary link.  
>> 
>> I can see how power on / off makes more sense than hot plug / unplug.
>> 
>> > Above would require -device/device_add support for specifying device's
>> > power state as minimum.  
>> 
>> The obvious way to control a device's power state with -device /
>> device_add is a qdev property.  Easy enough.
>> 
>> Do we need to control a device's power state after it's created?  If I
>> understand your use case correctly, the answer is yes.  -device /
>> device_add can't do that.
>
> Could you elaborate why why -device/device_add can't do that?

-device / device_add create, configure, and realize a new device.

They can't reconfigure an existing device.  In particular, they can't be
used to control an existing device's power state.

>> qom-set could, but friends don't let friends use it in production.
>> 
>> Any other prior art for controlling device state at run time via QMP?
>> 
>> [...]