Today when the perf tool runs in a guest on cores with PMUv3p4, it fails
to parse the default metrics with "Failure to read '#slots'", since perf
can only read 0 from sysfs caps/slots, which is backed by PMMIR_EL1.SLOTS
that KVM traps as RAZ/WI.

Taking into account backward compatibility and heterogeneous systems, the
exposure of PMMIR_EL1.SLOTS is gated behind a new vCPU feature flag:

- Patch 1 exposes PMMIR_EL1.SLOTS of the selected PMU under the (as yet
  unsettable) KVM_ARM_VCPU_PMU_V3_STRICT flag, and adds userspace get/set
  for PMMIR_EL1 so that SLOTS can be reset to 0 for backward
  compatibility.
- Patch 2 stops masking STALL_SLOT* in PMCEID1 under the flag.
- Patch 3 makes strict-mode userspace configure the number of event
  counters via the KVM_ARM_VCPU_PMU_V3_SET_NR_COUNTERS attribute rather
  than PMCR_EL0.N, whose meaning is context-dependent under NV.
- Patch 4 adds the KVM_ARM_VCPU_PMU_V3_STRICT flag itself: when set, KVM
  does not create a default PMU during vCPU init, and the VMM must select
  one explicitly via KVM_ARM_VCPU_PMU_V3_SET_PMU before the first
  KVM_RUN. This flips the gate on and can serve as an umbrella flag for
  future PMUv3 UAPI changes.

When the flag is not set, behaviors are unchanged.

v2: https://lore.kernel.org/r/20260702190421.420992-1-congkai@amazon.com
v1: https://lore.kernel.org/r/20260601193954.2103455-1-congkai@amazon.com

v2 -> v3 changes:
 - Reordered so the KVM_ARM_VCPU_PMU_V3_STRICT flag / KVM_CAP exposure
   comes last (now patch 4), after the flag-guarded behavior patches.
 - Reworked on top of Oliver Upton's refinements (thanks!):
   * Move the NULL-arm_pmu check into kvm_arm_pmu_v3_init() (returns
     -ENXIO) instead of failing at the first KVM_RUN, and report zero
     counters from kvm_arm_pmu_get_max_counters() before a PMU is set.
   * Documentation and -ENXIO ordering notes for the PMU device
     attributes (SET_PMU must precede INIT/FILTER).
 - New patch (Oliver): ignore userspace writes to PMCR_EL0.N under the
   strict flag and require the KVM_ARM_VCPU_PMU_V3_SET_NR_COUNTERS
   attribute, since PMCR_EL0.N is context-dependent under NV.
 - Advertise the KVM_CAP_ARM_PMU_V3_STRICT capability.
 - Documentation fixes in api.rst (the flag Requires KVM_ARM_VCPU_PMU_V3).

Congkai Tan (3):
  KVM: arm64: Expose PMMIR_EL1.SLOTS under strict PMUv3 UAPI
  KVM: arm64: Advertise STALL_SLOT* in PMCEID1 under strict PMUv3 UAPI
  KVM: arm64: Add KVM_ARM_VCPU_PMU_V3_STRICT vCPU feature

Oliver Upton (1):
  KVM: arm64: Ignore writes to PMCR_EL0.N when using strict UAPI

 Documentation/virt/kvm/api.rst                | 11 ++++
 Documentation/virt/kvm/devices/vcpu.rst       | 11 +++-
 arch/arm64/include/asm/kvm_host.h             |  5 +-
 arch/arm64/include/uapi/asm/kvm.h             |  1 +
 arch/arm64/kvm/arm.c                          | 19 ++++--
 arch/arm64/kvm/pmu-emul.c                     | 54 ++++++++++++----
 arch/arm64/kvm/sys_regs.c                     | 64 ++++++++++++++++++-
 include/kvm/arm_pmu.h                         |  4 ++
 include/uapi/linux/kvm.h                      |  1 +
 .../selftests/kvm/arm64/get-reg-list.c        |  1 +
 10 files changed, 149 insertions(+), 22 deletions(-)


base-commit: 8cdeaa50eae8dad34885515f62559ee83e7e8dda
-- 
2.50.1