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

[Paolo Bonzini]

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".  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.