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/[email protected] v1: https://lore.kernel.org/r/[email protected] 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
