KVM: x86/pmu: Add hardware Topdown metrics support

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.

[1] https://lore.kernel.org/kvm/20231031090613.2872700-1-dape...
[2] https://lore.kernel.org/all/20230927033124.1226509-1-dape...
[3] https://lwn.net/ml/linux-kernel/20221212125844.41157-1-li...

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

Changes since V1:
- As suggested by Dapeng, implement a new selftest patch.
- Don't advertise fixed counter 3 if the host doesn't support it.
- Minor change in patch 1 to remove a magic number.

v1:
https://lore.kernel.org/kvm/20260226230606.146532-1-zide....

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 (2):
  KVM: x86/pmu: Do not map fixed counters >= 3 to generic perf events
  KVM: selftests: Add perf_metrics and fixed counter 3 tests

 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/cpuid.c                          |  8 ++-
 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 ++-
 tools/arch/x86/include/asm/msr-index.h        |  1 +
 tools/testing/selftests/kvm/include/x86/pmu.h |  3 +
 .../selftests/kvm/x86/pmu_counters_test.c     | 71 +++++++++++++++++--
 12 files changed, 148 insertions(+), 22 deletions(-)

-- 
2.53.0