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

[Michal Privoznik]

On 4/17/19 8:27 PM, Daniel Henrique Barboza wrote:
> 
> 
> On 4/11/19 8:43 AM, Peter Krempa wrote:
>> 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).
> 
> I agree with this approach.
> 
> Michal, do you agree agree with this? If so, let me know if you plan
> to re-spin this series again. Otherwise I can do it.

My only concern is that we don't recreate capabilities for running 
domains. That means that domains which are already running won't benefit 
from this feature. On the other hand, I guess this is almost a corner 
case and thus probably not worth spending too much time on it.

So I say go for it. I don't plan to post patches anytime soon (burried 
under tonns of other stuff) so if you want to post the patches, please do.

Michal