Re: [PATCH v2 2/6] KVM-HV: KVM Steal time implementation

Avi Kivity
2011-02-01T17:09:23Z

On 02/01/2011 05:48 PM, Glauber Costa wrote:
> > > @@ -2106,6 +2120,25 @@ void kvm_arch_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
> > > kvm_migrate_timers(vcpu);
> > > vcpu->cpu = cpu;
> > > }
> > > +
> > > + if (vcpu->arch.this_time_out) {
> > > + u64 to = (get_kernel_ns() - vcpu->arch.this_time_out);
> > > + /*
> > > + * using nanoseconds introduces noise, which accumulates easily
> > > + * leading to big steal time values. We want, however, to keep the
> > > + * interface nanosecond-based for future-proofness.
> > > + */
> > > + to /= NSEC_PER_USEC;
> > > + to *= NSEC_PER_USEC;
> >
> > Seems there is a real problem and that this is just papering it over.
> > I'd like to understand the root cause.
> Okay, my self-explanation seemed reasonable to me, but since both you
> and Peter dislike it, I think it is important enough to get a more
> thorough investigation before a second round.

Yes please.

> But in this case,
> I keep that using nanoseconds may then not be the best approach here. We
> also have to keep in mind that the host and guest clocks may be running
> at different resolutions.

We need to choose a resolution for the clock (or negotiate one), an
nanoseconds seems as good as any from a range and precision
considerations, and is convenient for the host and Linux guests. So why
not pick it?

> > > + vcpu->arch.sversion += 2;
> >
> > Doesn't survive live migration. You need to use the version from the
> > guest area.
> Why not? Who said versions need to always increase? If current version
> is 102324, and we live migrate and it becomes 0, what is the problem?

Guest reads version (result: 2)
Guest starts reading data
Live migration; vcpu->arch.sversion is zeroed
Steal time update; vcpu->arch.sversion += 2; write to guest
Guest continues reading data
Guest reads version (result: 2)

So the guest is unaware that an update has occurred while it was reading
the data.

--
error compiling committee.c: too many arguments to function