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

On Tue, Feb 11, 2014 at 11:48:58AM +0100, Michal Privoznik wrote:
> On 11.02.2014 01:04, Marcelo Tosatti wrote:
> >On Mon, Feb 10, 2014 at 03:42:45PM -0700, Eric Blake wrote:
> >>On 02/10/2014 03:10 PM, Marcelo Tosatti wrote:
> >>>
> >>>Since QEMU commit "kvmclock: clock should count only if vm is running"
> >>>(http://lists.gnu.org/archive/html/qemu-devel/2013-06/msg01225.html),
> >>>guests have their realtime clock stopped during pause.
> >>>
> >>>To correct the situation, invoke guest agent to sync time from
> >>>host time.
> >>
> >>This needs to be conditional on new API (either a flag to existing API
> >>[except that virDomainResumeFlags is not quite existing yet], or new API
> >>altogether).
> >>
> >>We cannot require an API to depend on guest interaction (which an agent
> >>command does) without an explicit flag mentioning that it is okay.  We
> >>may even decide to have conditional ACL checks (allowing some users the
> >>ability to restart a domain, but not to interact with the guest agent).
> >>
> >>For that matter, when we proposed adding virDomainResumeFlags as the way
> >>to add in a flag for requesting a sync via this API, Dan suggested it
> >>would be better to add a new API altogether instead of piggybacking on
> >>top of the existing API:
> >>
> >>https://www.redhat.com/archives/libvir-list/2014-February/msg00104.html
> >
> >"So there's interesting, and I'm not sure I entirely agree about
> >adding flags here. It seems to me that if the QEMU agent has a
> >"set-time" capability we'd be better off having an explicit API
> >virDomainSetTime(...).  The action of resume + set time cannot
> >be atomic so I don't see any point in overloading the "set time"
> >functionality into the resume API call. Just let apps call the
> >set time method if they so desire."
> >
> >Apps desire the guest time to be synchronized to host time, which is the
> >standard 99% of usecases configuration.
> >
> >Ok, i was thinking of adding a new element GuestSyncTimeOnResume to
> >control this, which would default to on (as users expect their guests
> >time to be correct at all times, without additional commands, and
> >rightly so).
> >
> >It seems unreasonable to me that every API user has to keep track of
> >every location which a guest can possibly resume execution from a paused
> >state: this can be hidden inside libvirt.
> >
> >Daniel, all the logic behind the necessity to set-time after guest
> >resume (*) can be hidden inside libvirt.
> >
> >(*) all instances of guest resume: upon migration continuation, upon
> >ENOSPACE pause continuation, all of them.
> >
> >Don't see the need to force the knowledge and maintenance of this state
> >on libvirt users.
> >
> >Are you OK with the new knob, default on, to control sync time on
> >guest resume, then?
> >
> 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.

I agree that this should be a completely separate command, not
merely a flag to the Resume API.  The reason is that you cannot
do sensible error reporting if you overload this in one API
call.  ie consider that resuming the CPUs succeeds, but syncing
time fails. If you return "success" to the caller you are lieing
about the result of time sync. If you return "error" to the caller
you are lieing about the result of resuming the CPUs.

If there is a separate API to invoke then the caller can clearly
see which operation succeeded vs failed.

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