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

Marcelo Tosatti
2016-11-10T11:48:52Z

GOn Wed, Nov 09, 2016 at 05:23:50PM +0100, Paolo Bonzini 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.
>
> For now I'm more interested in having a version of the patch that is
> clean and doesn't need advance_clock.
>
> Paolo

Destination has to run the following logic:

If (source has KVM_CAP_ADVANCE_CLOCK)
use KVM_GET_CLOCK value
Else
read pvclock from guest

To support migration from older QEMU versions which do not have
KVM_CAP_ADVANCE_CLOCK (or new QEMU versions running on old
hosts without KVM_CAP_ADVANCE_CLOCK).

I don't see any clean way to give that information, except changing
the migration format to pass "host: kvm_cap_advance_clock=true/false"
information.

Ideas?