Re: [PATCH v6 5/8] KVM: x86/pmu: Support PERF_METRICS MSR in mediated vPMU

[Jim Mattson]

On Fri, Jul 24, 2026 at 1:47 PM Chen, Zide <zide.chen@xxxxxxxxx> wrote:
>
>
>
> On 7/23/2026 10:15 PM, Jim Mattson wrote:
> > On Thu, Jul 23, 2026 at 4:44 PM Chen, Zide <zide.chen@xxxxxxxxx> wrote:
> >> The SDM guidance that "fixed counter 3 must be restored before
> >> PERF_METRICS" appears to apply only to a running PMU, which is not the
> >> case here.
> >
> > That's not my reading of the SDM. There are three bullet points about
> > save/restore at the end of volume 3, section 22.3.9.3:
> >
> > * PERF_METRICS and fixed-function performance-monitoring counter 3
> > should be saved and restored together.
> > * To ensure that PERF_METRICS and fixed-function
> > performance-monitoring counter 3 remain synchronized,
> > both should be disabled during both save and restore. Software should
> > enable/disable them atomically, with a
> > single write to IA32_PERF_GLOBAL_CTRL to set/clear both
> > EN_PERF_METRICS[bit 48] and
> > EN_FIXED_CTR3[bit 35].
> > * On state restore, fixed-function performance-monitoring counter 3
> > must be restored before PERF_METRICS,
> > otherwise undefined results may be observed.
> >
> > The second bullet contradicts your claim above.
>
> Thanks for pointing this out. I followed up with an internal ucode
> expert, and my earlier explanation was incorrect.
>
> The ordering requirement is not dependent on whether the PMU is enabled.
> The relevant recalculation happens on the WRMSR itself, so the SDM
> guidance to restore fixed counter 3 before PERF_METRICS still applies
> even when counting is disabled.
>
> That said, for the code path being discussed, both PERF_METRICS and
> fixed counter 3 are written with 0. I was advised that this specific
> all-zero case still converges to the intended hardware state regardless
> of write order, so the write ordering here does not affect the final result.
>
> Sorry for the confusion, and thanks for the correction.

At the very least, the reliance on an undocumented exception to the
requirement stated in the SDM warrants an explicit comment.