[libvirt] [PATCH] qemu driver: sync guest time via qemu-guest-agent when domain is resumed

[Michal Privoznik]

On 11.02.2014 22:45, Eric Blake wrote:
> On 02/11/2014 02:37 PM, Marcelo Tosatti wrote:
>
>>> I think we may need both approaches. I too think that resume with
>>> syncing guest time is so common use case that is deserves to be
>>> exposed as a single operation outside the libvirt.
>>>
>>> On the other hand, we certainly want to expose this as a new
>>> virDomain{Get,Set}Time() API.
>>
>> Why a new API which requires modifications to every libvirt user (read:
>> current libvirt users are broken until patched) ?
>
> ALL existing libvirt users are "broken", in that NO ONE has been able to
> automatically resync guest time yet (at least, using supported libvirt
> API).  This is a new feature; but as a new feature that involves guest
> cooperation, it must be explicitly requested by any client that wants to
> take advantage of it.  We cannot change the default and automatically
> turn this feature on, because it has back-compat implications to
> existing libvirt clients that are not expecting libvirt to automatically
> try and munge guest time.
>
>>
>> Note it requires applications to keep track of every pause/resume
>> instance (see item 1 above).
>
> Whether we make it a new API call that all clients must call every time
> they want, or a single knob that says that for the given VM, libvirt
> should automatically attempt to resync time after any resume action,
> isn't as important to me as the fact that it IS something new that gets
> explicitly requested.  We already have <on_poweroff> and friends in the
> guest XML that control what the guest does on a state transition to
> poweroff, maybe we could add an <on_resume> element that has
> instructions on what actions libvirt should take when a guest
> transitions from paused back to running.  But the point is that such an
> XML element is an explicit request, and only appropriate for guests
> where the management stack trusts interacting with the guest agent.
>
> Maybe even both additions would be appropriate (a knob for automatic
> libvirt syncing as well as a new API for explicit syncing outside the
> events where libvirt would normally do it automatically).

I don't think we should add the <on_resume/> element. After all, it 
would be the same as:

virDomainResumeFlags(dom, VIR_SYNC_TIME)

which as Dan correctly argues, makes sensible error reporting 
impossible. If the domain is resumed, libvirt should execute the 
<on_resume/> action, which may fail.

Michal