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/[email protected] 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
