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

[Marcelo Tosatti <mtosatti@redhat.com>]

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?