Re: KVM: x86: fix tsc catchup issue with tsc scaling

[Paolo Bonzini <pbonzini@redhat.com>]

Il 15/01/2014 17:37, Marcelo Tosatti ha scritto:
> > Right.  Another question, what about this comment:
> > 
> >         /* Reset of TSC must disable overshoot protection below */
> >         vcpu->arch.hv_clock.tsc_timestamp = 0;
> >         vcpu->arch.last_guest_tsc = data;
> > 
> > Should it be instead like this:
> > 
> > 	vcpu->arch.hv_clock.tsc_timestamp += data - vcpu->arch.last_guest_tsc;
> > 	vcpu->arch.last_guest_tsc = data;
> > 
> > ?
> 
> Don't see why it should?

Setting tsc_timestamp to 0 makes no sense with the current code, I'm
trying to understand if the right fix is to delete that line or
something else.  What I proposed (plus a version increase) would make a
pvclock read continuous before and after the change.

But looking more at the surrounding code, the calls to kvm_write_tsc
ultimately result in a master clock update as soon as all TSCs agree and
the master clock is re-enabled.  This master clock update rewrites
tsc_timestamp.  Then I think that the line that sets tsc_timestamp to 0
can be deleted.

Paolo