Re: [PATCH 0/3] KVM: x86/pmu: Add hardware Topdown metrics support

[Mi, Dapeng]

Zide, it seems currently there is no KVM/selftests to cover the fixed
counter 3 and the topdown metrics support. We could enhance current PMU
selftest or add a new selftest to cover this case. Thanks.

On 2/27/2026 7:06 AM, 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 failed attempts have been made to virtualize this under the
> legacy vPMU model: [1], [2], [3].  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 overhead of trapping and emulating MSR accesses.
>
> [1] https://lore.kernel.org/kvm/20231031090613.2872700-1-dapeng1.mi@xxxxxxxxxxxxxxx/
> [2] https://lore.kernel.org/all/20230927033124.1226509-1-dapeng1.mi@xxxxxxxxxxxxxxx/T/
> [3] https://lwn.net/ml/linux-kernel/20221212125844.41157-1-likexu@xxxxxxxxxxx/
>
> Tested on an SPR.  Without this series, only raw topdown.*_slots events
> work in the guest, and metric events (e.g. cpu/topdown-bad-spec/) are
> not available.
>
> With this series, metric events are visible in the guest.  Run this
> command on both host and guest:
>
> $ perf stat --topdown --no-metric-only -- taskset -c 2 perf bench sched messaging
>
> Host results:
>
> # Running 'sched/messaging' benchmark:
> # 20 sender and receiver processes per group
> # 10 groups == 400 processes run
>
>      Total time: 1.500 [sec]
>
>  Performance counter stats for 'taskset -c 2 perf bench sched messaging':
>
>      4,266,060,558      TOPDOWN.SLOTS:u              #     32.0 %  tma_frontend_bound
>                                                      #      5.2 %  tma_bad_speculation
>        588,397,905      topdown-retiring:u           #     13.8 %  tma_retiring
>                                                      #     49.0 %  tma_backend_bound
>      1,376,283,990      topdown-fe-bound:u
>      2,096,827,304      topdown-be-bound:u
>        217,425,841      topdown-bad-spec:u
>          5,050,520      INT_MISC.UOP_DROPPING:u
>
>        1.755503765 seconds time elapsed
>
>        0.235965000 seconds user
>        1.500508000 seconds sys
>
> Guest results:
>
> # Running 'sched/messaging' benchmark:
> # 20 sender and receiver processes per group
> # 10 groups == 400 processes run
>
>      Total time: 1.558 [sec]
>
>  Performance counter stats for 'taskset -c 2 perf bench sched messaging':
>
>      5,148,818,712      TOPDOWN.SLOTS:u              #     34.0 %  tma_frontend_bound
>                                                      #      4.6 %  tma_bad_speculation
>        602,862,499      topdown-retiring:u           #     11.7 %  tma_retiring
>                                                      #     49.7 %  tma_backend_bound
>      1,759,698,259      topdown-fe-bound:u
>      2,565,571,672      topdown-be-bound:u
>        230,277,308      topdown-bad-spec:u
>          4,966,279      INT_MISC.UOP_DROPPING:u
>
>        1.783366587 seconds time elapsed
>
>        0.313692000 seconds user
>        1.446377000 seconds sys
>
> Dapeng Mi (2):
>   KVM: x86/pmu: Support Intel fixed counter 3 on mediated vPMU
>   KVM: x86/pmu: Support PERF_METRICS MSR in mediated vPMU
>
> Zide Chen (1):
>   KVM: x86/pmu: Do not map fixed counters >= 3 to generic perf events
>
>  arch/x86/include/asm/kvm_host.h   |  3 +-
>  arch/x86/include/asm/msr-index.h  |  1 +
>  arch/x86/include/asm/perf_event.h |  1 +
>  arch/x86/kvm/pmu.c                |  4 +++
>  arch/x86/kvm/vmx/pmu_intel.c      | 57 ++++++++++++++++++++++++-------
>  arch/x86/kvm/vmx/pmu_intel.h      |  5 +++
>  arch/x86/kvm/vmx/vmx.c            |  6 ++++
>  arch/x86/kvm/x86.c                | 10 ++++--
>  8 files changed, 71 insertions(+), 16 deletions(-)
>