Re: [PATCH v2 1/3] KVM: x86: implement KVM_{GET|SET}_TSC_STATE

[Marcelo Tosatti]

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? 

It seems to me using CLOCK_REALTIME as in the interface Maxim is
proposing is prone to difference in CLOCK_REALTIME itself.

Perhaps there is another way to measure that 0.1 sec which is
independent of the clock values of the source and destination hosts
(say by sending a packet once the clock stops counting).

Then on destination measure delta = clock_restart_time - packet_receival
and increase clock by that amount.



> Again, save-to-disk,
> reverse debugging and the like are a different story, which is why KVM
> should delegate policy to userspace (while documenting how to do it right).
> 
> Paolo
> 
> > CLOCK_REALTIME and CLOCK_TAI are off by the time the VM is paused and
> > this state persists up to the point where NTP corrects it with a time
> > jump.
> > 
> > So if migration takes 5 seconds then CLOCK_REALTIME is not off by 100ms
> > it's off by 5 seconds.
> > 
> > CLOCK_MONOTONIC/BOOTTIME might be off by 100ms between pause and resume.
> >