Re: [PATCH 03/10] KVM: x86: Add KVM_[GS]ET_CLOCK_GUEST for accurate KVM clock migration

[David Woodhouse]

On Mon, 2024-04-22 at 16:11 +0200, Paolo Bonzini wrote:
> On Thu, Apr 18, 2024 at 9:46 PM David Woodhouse <dwmw2@xxxxxxxxxxxxx> wrote:
> > +       curr_tsc_hz = get_cpu_tsc_khz() * 1000LL;
> > +       if (unlikely(curr_tsc_hz == 0)) {
> > +               rc = -EINVAL;
> > +               goto out;
> > +       }
> > +
> > +       if (kvm_caps.has_tsc_control)
> > +               curr_tsc_hz = kvm_scale_tsc(curr_tsc_hz,
> > +                                           v->arch.l1_tsc_scaling_ratio);
> > +
> > +       /*
> > +        * The scaling factors in the hv_clock do not depend solely on the
> > +        * TSC frequency *requested* by userspace. They actually use the
> > +        * host TSC frequency that was measured/detected by the host kernel,
> > +        * scaled by kvm_scale_tsc() with the vCPU's l1_tsc_scaling_ratio.
> > +        * So a sanity check that they *precisely* match would have false
> > +        * negatives. Allow for a discrepancy of 1 kHz either way.
> 
> This is not very clear - if kvm_caps.has_tsc_control, cur_tsc_hz is
> exactly the "host TSC frequency [...] scaled by kvm_scale_tsc() with
> the vCPU's l1_tsc_scaling_ratio". But even in that case there is a
> double rounding issue, I guess.

That's exactly what I'm saying, isn't it?

Perhaps the issue is clearer if I say "that was measured/detected by
*each* host kernel"?

The point is that if I boot on a kernel which measured its TSC against
the PIT and came up with a value of 3002MHz, and then migrate to an
"identical" host which measured against *its* PIT and decided its TSC
frequency was 2999MHz.... then migrate a guest with an explicit TSC
frequency of 2500MHz from one host to the other... their effective
tsc_to_system_mul and tsc_shift in the pvclock are *different*
because...

"The scaling factors in the hv_clock do not depend solely on the
TSC frequency *requested* by userspace. They actually use the
host TSC frequency that was measured/detected by each host kernel,
scaled by kvm_scale_tsc() with the vCPU's l1_tsc_scaling_ratio."

Or did I misunderstand your objection?

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