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

[Andy Lutomirski]

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.