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. >> 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? >> >> [...]
