Re: [PATCH V2 0/4] KVM: x86/pmu: Add hardware Topdown metrics support

[Sean Christopherson <seanjc@google.com>]

On Thu, Apr 23, 2026, Zide Chen wrote:
> The Top-Down Microarchitecture Analysis (TMA) method is a structured
> approach for identifying performance bottlenecks in out-of-order
> processors.
> 
> Currently, guests support the TMA method by collecting Topdown events
> using GP counters, which may trigger multiplexing.  To free up scarce
> GP counters, eliminate multiplexing-induced skew, and obtain coherent
> Topdown metric ratios, it is desirable to expose fixed counter 3 and
> the IA32_PERF_METRICS MSR to guests.
> 
> Several attempts have been made to virtualize this under the legacy
> vPMU model [1][2][3], but they were unsuccessful.  With the new mediated
> vPMU, enabling TMA support in guests becomes much simpler.  It avoids
> invasive changes to the perf core, eliminates CPU pinning and
> fixed-counter affinity issues, and reduces the latge overhead of
> trapping and emulating MSR accesses.

Please submit a new version of this to address Dapeng's feedback, rebase on the
latest kvm-x86/next, and perhaps most importantly, to let Sashiko take a crack
at it.

Thanks!