On Mon, Jul 06, 2026 at 07:03:28PM +0900, Akihiko Odaki wrote:
> Add internal state for PMUv3 emulation without programmable event
> counters. When fixed-counters-only mode is active, KVM reports no
> programmable counters and hides PMCEID, avoiding event-counter state
> whose behavior can depend on the selected hardware PMU.
>
> The cycle counter still uses a host perf event. Unlike the normal PMU
> path, fixed-counters-only mode may create that event from the hardware
> PMU attached to the VCPU's current pCPU. If the VCPU later loads on a
> pCPU that is not covered by the existing event's PMU, request a PMU
> reload so the cycle counter can be recreated against the new pCPU's PMU.
> Keep this affinity check limited to fixed-counters-only VMs; the normal
> programmable-counter mode continues to use the VM-wide PMU and does not
> need per-load reload decisions.
>
> Add a separate internal flag for explicit userspace PMU selection. The
> UAPI wiring added later will use it to keep explicit PMU selection and
> fixed-counters-only mode mutually exclusive while still allowing
> fixed-counters-only mode to replace the default PMU selected during
> KVM_ARM_VCPU_INIT.
>
> The UAPI wiring that sets the fixed-counters-only flag and records
> explicit PMU selection is added later in the series.
>
> Signed-off-by: Akihiko Odaki <[email protected]>
> ---
> arch/arm64/include/asm/kvm_host.h | 4 ++++
> arch/arm64/kvm/arm.c | 1 +
> arch/arm64/kvm/pmu-emul.c | 43
> +++++++++++++++++++++++++++++++++++++--
> include/kvm/arm_pmu.h | 2 ++
> 4 files changed, 48 insertions(+), 2 deletions(-)
>
> diff --git a/arch/arm64/include/asm/kvm_host.h
> b/arch/arm64/include/asm/kvm_host.h
> index 0c39d9db7d57..aa07b05b8231 100644
> --- a/arch/arm64/include/asm/kvm_host.h
> +++ b/arch/arm64/include/asm/kvm_host.h
> @@ -353,6 +353,10 @@ struct kvm_arch {
> #define KVM_ARCH_FLAG_WRITABLE_IMP_ID_REGS 10
> /* Unhandled SEAs are taken to userspace */
> #define KVM_ARCH_FLAG_EXIT_SEA 11
> + /* PMUv3 is emulated with an explicitly specified hardware PMU */
> +#define KVM_ARCH_FLAG_PMU_V3_EXPLICIT 12
> + /* PMUv3 is emulated without progammable event counters */
> +#define KVM_ARCH_FLAG_PMU_V3_FIXED_COUNTERS_ONLY 13
> unsigned long flags;
>
> /* VM-wide vCPU feature set */
> diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
> index 68767bb08285..1cc7754d5ace 100644
> --- a/arch/arm64/kvm/arm.c
> +++ b/arch/arm64/kvm/arm.c
> @@ -687,6 +687,7 @@ void kvm_arch_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
> if (has_vhe())
> kvm_vcpu_load_vhe(vcpu);
> kvm_arch_vcpu_load_fp(vcpu);
> + kvm_vcpu_load_pmu(vcpu);
> kvm_vcpu_pmu_restore_guest(vcpu);
> if (kvm_arm_is_pvtime_enabled(&vcpu->arch))
> kvm_make_request(KVM_REQ_RECORD_STEAL, vcpu);
> diff --git a/arch/arm64/kvm/pmu-emul.c b/arch/arm64/kvm/pmu-emul.c
> index e70628653e4b..40cad183376c 100644
> --- a/arch/arm64/kvm/pmu-emul.c
> +++ b/arch/arm64/kvm/pmu-emul.c
> @@ -96,6 +96,11 @@ u64 kvm_pmu_evtyper_mask(struct kvm *kvm)
> return mask;
> }
>
> +static bool kvm_pmu_fixed_counters_only(struct kvm *kvm)
> +{
> + return test_bit(KVM_ARCH_FLAG_PMU_V3_FIXED_COUNTERS_ONLY,
> &kvm->arch.flags);
> +}
> +
> /**
> * kvm_pmc_is_64bit - determine if counter is 64bit
> * @pmc: counter context
> @@ -343,7 +348,11 @@ u64 kvm_pmu_implemented_counter_mask(struct kvm_vcpu
> *vcpu)
>
> static void kvm_pmc_enable_perf_event(struct kvm_pmc *pmc)
> {
> - if (!pmc->perf_event) {
> + struct kvm_vcpu *vcpu = kvm_pmc_to_vcpu(pmc);
> +
> + if (!pmc->perf_event ||
> + (kvm_pmu_fixed_counters_only(vcpu->kvm) &&
> + !cpumask_test_cpu(vcpu->cpu,
> &to_arm_pmu(pmc->perf_event->pmu)->supported_cpus))) {
> kvm_pmu_create_perf_event(pmc);
> return;
> }
> @@ -720,6 +729,12 @@ static void kvm_pmu_create_perf_event(struct kvm_pmc
> *pmc)
> int eventsel;
> u64 evtreg;
>
> + if (kvm_pmu_fixed_counters_only(vcpu->kvm)) {
> + arm_pmu = kvm_pmu_probe_armpmu(vcpu->cpu);
> + if (WARN_ON_ONCE(!arm_pmu))
> + return;
> + }
> +
> evtreg = kvm_pmc_read_evtreg(pmc);
>
> kvm_pmu_stop_counter(pmc);
> @@ -748,7 +763,7 @@ static void kvm_pmu_create_perf_event(struct kvm_pmc *pmc)
> * Don't create an event if we're running on hardware that requires
> * PMUv3 event translation and we couldn't find a valid mapping.
> */
> - eventsel = kvm_map_pmu_event(vcpu->kvm->arch.arm_pmu, eventsel);
> + eventsel = kvm_map_pmu_event(arm_pmu, eventsel);
> if (eventsel < 0)
> return;
>
> @@ -878,6 +893,9 @@ u64 kvm_pmu_get_pmceid(struct kvm_vcpu *vcpu, bool
> pmceid1)
> u64 val, mask = 0;
> int base, i, nr_events;
>
> + if (kvm_pmu_fixed_counters_only(vcpu->kvm))
> + return 0;
> +
Even if we advertise bits in PMCEID, does it matter? There's no PMC that
the guest could use to count it.
I understand it isn't aesthetic but I really want to minimize the
special-casing that has to be done for this feature.
> +void kvm_vcpu_load_pmu(struct kvm_vcpu *vcpu)
> +{
> + /*
> + * ARMV8_PMU_INSTR_IDX will need the same check once
> + * FEAT_PMUv3_ICNTR is supported.
> + */
> + struct kvm_pmc *pmc = kvm_vcpu_idx_to_pmc(vcpu, ARMV8_PMU_CYCLE_IDX);
> + struct arm_pmu *cpu_pmu;
> +
> + if (!kvm_pmu_fixed_counters_only(vcpu->kvm) ||
> + !kvm_pmu_counter_is_enabled(pmc) || !pmc->perf_event)
> + return;
> +
> + cpu_pmu = to_arm_pmu(pmc->perf_event->pmu);
> + if (!cpumask_test_cpu(vcpu->cpu, &cpu_pmu->supported_cpus))
> + kvm_make_request(KVM_REQ_RELOAD_PMU, vcpu);
Just detect the changing PMU implementation here, KVM_REQ_RELOAD_PMU
will need to detect the PMCs that require an update anyway. Stash the
last cpu in kvm_arch_vcpu_load() and pass it to this:
void kvm_vcpu_load_pmu(struct kvm_vcpu *vcpu, int last_cpu)
{
if (!kvm_pmu_fixed_counters_only(vcpu->kvm) || vcpu->cpu == last_cpu)
return;
if (kvm_pmu_probe_armpmu(vcpu->cpu) != kvm_pmu_probe_armpmu(last_cpu))
kvm_make_request(KVM_REQ_RELOAD_PMU);
}
Thanks,
Oliver
