On Wed, 29 Oct 2025 12:38:02 +0100 Markus Armbruster <[email protected]> wrote:
> Igor Mammedov <[email protected]> writes: > > > On Mon, 20 Oct 2025 13:22:08 +0200 > > Markus Armbruster <[email protected]> wrote: > > > >> Igor Mammedov <[email protected]> writes: > >> > >> > On Thu, 09 Oct 2025 16:55:54 +0200 > >> > Markus Armbruster <[email protected]> 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. Sorry, I've misread as we can't use both for creating device in powered off state. Perhaps we should consider a new specialized QMP command to manipulate runtime power state. (Like it was suggested by Daniel) > > >> 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? > >> > >> [...] >
