Re: [PATCH v8 5/7] KVM: arm64: PMU: Implement fixed-counters-only emulation

On 2026/07/07 20:23, Akihiko Odaki wrote:
> On 2026/07/07 3:23, Oliver Upton wrote:
>> 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.
> 
> This strictly ensures that sysregs are stable after migrating across 
> physical CPUs, which may have different PMCEID values.
> 
>>
>>> +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);
>> }
> 
> It is a nice way to simplify the code and to avoid hardcoding 
> ARMV8_PMU_INSTR_IDX. I'll use the code for the next version.

I tried this but unfortunately it doesn't seem to work. 
kvm_arch_vcpu_put() sets vcpu->cpu to -1 so we cannot simply read it to 
get the last cpu in kvm_arch_vcpu_load().

Regards,
Akihiko Odaki