Igor Mammedov <[email protected]> writes: > On Thu, 09 Oct 2025 16:55:54 +0200 > Markus Armbruster <[email protected]> wrote: > >> Igor Mammedov <[email protected]> writes:
[...] >> > It's likely /me who to blame for asking to invent generic >> > device-set QMP command. >> > I see another application (beside ARM CPU power-on/off) for it, >> > PCI devices to simulate powering on/off them at runtime without >> > actually removing device. >> >> I prefer generic commands over collecting ad hoc single-purpose >> commands, too. Getting the design right can be difficult. >> >> > wrt command, >> > I'd use only 'id' with it to identify target device >> > (i.e. no template matching nor QMP path either). >> > To enforce rule, what user hasn't named explicitly by providing 'id' >> > isn't meant to be accessed/manged by user later on. >> >> Works well, except when we need to access / manage onboard devices. >> That's still an unsolved problem. >> >> > potentially we can invent specialized power_set/get command as >> > an alternative if it makes design easier. >> > But then we would be spawning similar commands for other things, >> > where as device-set would cover it all. But then I might be >> > over-complicating things by suggesting a generic approach. >> >> Unclear. >> >> 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. 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? [...]
