[PATCH v2 0/3] KVM: arm64: Expose PMMIR_EL1.SLOTS to guests

[Congkai Tan <congkai@amazon.com>]

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 adds the new flag KVM_ARM_VCPU_PMU_V3_STRICT. 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.
- Patch 2 exposes PMMIR_EL1.SLOTS of the selected PMU under the flag, and
  adds userspace get/set for PMMIR_EL1 so that SLOTS can be reset to 0
  for backward compatibility.
- Patch 3 stops masking STALL_SLOT* in PMCEID1 under the flag.

When the flag is not set, behaviors are unchanged.

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

v1 -> v2 changes:
 - Gate the whole feature behind a new KVM_ARM_VCPU_PMU_V3_STRICT vCPU
   feature flag, instead of unconditionally exposing PMMIR_EL1.SLOTS.
 - When the flag is set, skip creating a default PMU during vCPU init.
 - Split the PMCEID1 unmask into its own patch, also gated by the flag.
 - Snapshot SLOTS into a new field pmmir_slots in kvm_arch during the
   handling of KVM_ARM_VCPU_PMU_V3_SET_PMU when the flag is set;
   access_pmmir()/get_pmmir() return it and set_pmmir() only accepts the
   SLOTS field (rejecting other bits with -EINVAL).
 - Add get_user and set_user for PMMIR_EL1 to support setting the SLOTS
   back to 0, and add PMMIR_EL1 to the get-reg-list selftest.

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

 Documentation/virt/kvm/api.rst                   |  5 ++
 arch/arm64/include/asm/kvm_host.h                |  5 +-
 arch/arm64/include/uapi/asm/kvm.h                |  1 +
 arch/arm64/kvm/arm.c                             | 18 +++++--
 arch/arm64/kvm/pmu-emul.c                        | 50 ++++++++++++++-----
 arch/arm64/kvm/sys_regs.c                        | 63 +++++++++++++++++++++++-
 include/kvm/arm_pmu.h                            |  4 ++
 tools/arch/arm64/include/uapi/asm/kvm.h          |  1 +
 tools/testing/selftests/kvm/arm64/get-reg-list.c |  1 +
 9 files changed, 128 insertions(+), 20 deletions(-)


base-commit: 1702da76e017ae0fbe1a92b07bc332972c293e89
--
2.50.1