On Tue, Dec 15, 2020 at 3:35 AM Marcelo Tosatti <mtosatti@xxxxxxxxxx> wrote: > > On Fri, Dec 11, 2020 at 10:59:59PM +0100, Paolo Bonzini wrote: > > On 11/12/20 22:04, Thomas Gleixner wrote: > > > > Its 100ms off with migration, and can be reduced further (customers > > > > complained about 5 seconds but seem happy with 0.1ms). > > > What is 100ms? Guaranteed maximum migration time? > > > > I suppose it's the length between the time from KVM_GET_CLOCK and > > KVM_GET_MSR(IA32_TSC) to KVM_SET_CLOCK and KVM_SET_MSR(IA32_TSC). But the > > VM is paused for much longer, the sequence for the non-live part of the > > migration (aka brownout) is as follows: > > > > pause > > finish sending RAM receive RAM ~1 sec > > send paused-VM state finish receiving RAM \ > > receive paused-VM state ) 0.1 sec > > restart / > > > > The nanosecond and TSC times are sent as part of the paused-VM state at the > > very end of the live migration process. > > > > So it's still true that the time advances during live migration brownout; > > 0.1 seconds is just the final part of the live migration process. But for > > _live_ migration there is no need to design things according to "people are > > happy if their clock is off by 0.1 seconds only". > > Agree. What would be a good way to fix this? > Could you implement the Hyper-V clock interface? It's much, much simpler than the kvmclock interface. It has the downside that CLOCK_BOOTTIME won't do what you want, but I'm not really convinced that's a problem, and you could come up with a minimal extension to fix that.
