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

[Chen, Zide]

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.