[libvirt] [PATCH v3 3/4] qemu_capabilities: Introduce QEMU_CAPS_PM_WAKEUP_SUPPORT

[Peter Krempa]

On Thu, Apr 11, 2019 at 13:37:49 +0200, Michal Privoznik wrote:
> On 4/11/19 12:12 PM, Daniel Henrique Barboza wrote:
> > <snip/>
> >
> > All that said, I think a good solution would be (2). dompmsuspend isn't a
> > performance sensitive command, thus the extra time to execute the API
> > can be ignored. Also, we can still check for
> > QEMU_CAPS_QUERY_CURRENT_MACHINE
> > inside qemuDomainPMSuspendForDuration to avoid firing up an error in a
> > QEMU version that doesn't know the API.
> 
> That's the thing. We can't. The capability doesn't exist yet. So every
> domain that is currently running thinks that qemu doesn't have the
> capability. And when updating libvirt to say next release only freshly
> started domains might/will have the capability. For all already running
> domains libvirt thinks the capability is not there. Only freshly started
> domains will get the capability. IOW, upgrading libvirt won't help you
> unless you restart your domains (might not want to do that).

If you just go ahead with the PM suspend in case the capability bit for
the presence of query-current-machine is not present you just will keep
the existing behaviour. I think that is perfectly tolerable.

Error should be reported only if query-current-machine is supported and
it returns false for wakeup-suspend-support. (Obviously also if our caps
indicate that query-current-machine is supported but returns an error).
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