Re: [QEMU PATCH v2] kvmclock: advance clock by time window between vm_stop and pre_save

Dr. David Alan Gilbert
2016-11-09T16:28:52Z

* Paolo Bonzini (pbonzini@redhat.com) wrote:
>
>
> On 08/11/2016 11:22, Dr. David Alan Gilbert wrote:
> > * Marcelo Tosatti (mtosatti@redhat.com) wrote:
> >> On Mon, Nov 07, 2016 at 08:03:50PM +0000, Dr. David Alan Gilbert wrote:
> >>> * Marcelo Tosatti (mtosatti@redhat.com) wrote:
> >>>> On Mon, Nov 07, 2016 at 03:46:11PM +0000, Dr. David Alan Gilbert wrote:
> >>>>> * Marcelo Tosatti (mtosatti@redhat.com) wrote:
> >>>>>> This patch, relative to pre-copy migration codepath,
> >>>>>> measures the time between vm_stop() and pre_save(),
> >>>>>> which includes copying the remaining RAM to destination,
> >>>>>> and advances the clock by that amount.
> >>>>>>
> >>>>>> In a VM with 5 seconds downtime, this reduces the guest
> >>>>>> clock difference on destination from 5s to 0.2s.
> >>>>>>
> >>>>>> Tested with Linux and Windows 2012 R2 guests with -cpu XXX,+hv-time.
> >>>>>
> >>>>> One thing that bothers me is that it's only this clock that's
> >>>>> getting corrected; doesn't it cause things to get upset when
> >>>>> one clock moves and the others dont?
> >>>>
> >>>> If you are correlating the clocks, then yes.
> >>>>
> >>>> Older Linux guests get upset (marking the TSC clocksource unstable
> >>>> because the watchdog checks TSC vs kvmclock), but there is a workaround for it
> >>>> in newer guests
> >>>> (kvmclock interface to notify watchdog to not complain).
> >>>>
> >>>> Note marking TSC clocksource unstable on older guests is harmless
> >>>> because kvmclock is the standard clocksource.
> >>>>
> >>>> For Windows guests, i don't know that Windows correlates between different
> >>>> clocks.
> >>>>
> >>>> That is, there is relative control as to which software reads kvmclock
> >>>> or Windows TIMER MSR, so i don't see the need to advance every clock
> >>>> exposed.
> >>>>
> >>>>> Shouldn't the pause delay be recorded somewhere architecturally
> >>>>> independent and then be a thing that kvm-clock happens to use and
> >>>>> other clocks might as well?
> >>>>
> >>>> In theory, yes. In practice, i don't see the need for this...
> >>>
> >>> It seems unlikely to me that x86 is the only one that will want
> >>> to do something similar.
> >>
> >> Can't they copy what kvmclock is doing today?
> >
> > We shouldn't have copies of code all over should we?
>
> Let's cross the bridge when we get there.

That will mean it has the migration data in the wrong place
and any other clocks that need to be incremented by the same offset
will need a hook or be inconsistent with this calculation.

Dave

> For now I'm more interested in having a version of the patch that is
> clean and doesn't need advance_clock.
>
> Paolo
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK