Re: [PATCH v5 19/24] KVM: arm64: Implement lazy PMU context swaps

Colton Lewis
2025-12-12T22:25:45Z

Oliver Upton <oupton@kernel.org> writes:

> On Tue, Dec 09, 2025 at 08:51:16PM +0000, Colton Lewis wrote:
>> +enum vcpu_pmu_register_access {
>> + VCPU_PMU_ACCESS_UNSET,
>> + VCPU_PMU_ACCESS_VIRTUAL,
>> + VCPU_PMU_ACCESS_PHYSICAL,
>> +};

> This is confusing. Even when the guest is accessing registers directly
> on the CPU I'd still call that "hardware assisted virtualization" and
> not "physical".

It was what I thought described the access pattern. Do you have another
naming suggestion?

>> +#endif /* _ASM_ARM64_KVM_TYPES_H */
>> diff --git a/arch/arm64/kvm/debug.c b/arch/arm64/kvm/debug.c
>> index 0ab89c91e19cb..c2cf6b308ec60 100644
>> --- a/arch/arm64/kvm/debug.c
>> +++ b/arch/arm64/kvm/debug.c
>> @@ -34,7 +34,7 @@ static int cpu_has_spe(u64 dfr0)
>> * - Self-hosted Trace Filter controls (MDCR_EL2_TTRF)
>> * - Self-hosted Trace (MDCR_EL2_TTRF/MDCR_EL2_E2TB)
>> */
>> -static void kvm_arm_setup_mdcr_el2(struct kvm_vcpu *vcpu)
>> +void kvm_arm_setup_mdcr_el2(struct kvm_vcpu *vcpu)
>> {
>> int hpmn = kvm_pmu_hpmn(vcpu);

>> diff --git a/arch/arm64/kvm/hyp/include/hyp/switch.h
>> b/arch/arm64/kvm/hyp/include/hyp/switch.h
>> index bde79ec1a1836..ea288a712bb5d 100644
>> --- a/arch/arm64/kvm/hyp/include/hyp/switch.h
>> +++ b/arch/arm64/kvm/hyp/include/hyp/switch.h
>> @@ -963,6 +963,8 @@ static bool kvm_hyp_handle_pmu_regs(struct kvm_vcpu
>> *vcpu)
>> if (ret)
>> __kvm_skip_instr(vcpu);

>> + kvm_pmu_set_physical_access(vcpu);
>> +
>> return ret;
>> }

>> diff --git a/arch/arm64/kvm/pmu-direct.c b/arch/arm64/kvm/pmu-direct.c
>> index 8d0d6d1a0d851..c5767e2ebc651 100644
>> --- a/arch/arm64/kvm/pmu-direct.c
>> +++ b/arch/arm64/kvm/pmu-direct.c
>> @@ -73,6 +73,7 @@ bool kvm_vcpu_pmu_use_fgt(struct kvm_vcpu *vcpu)
>> u8 hpmn = vcpu->kvm->arch.nr_pmu_counters;

>> return kvm_vcpu_pmu_is_partitioned(vcpu) &&
>> + vcpu->arch.pmu.access == VCPU_PMU_ACCESS_PHYSICAL &&
>> cpus_have_final_cap(ARM64_HAS_FGT) &&
>> (hpmn != 0 || cpus_have_final_cap(ARM64_HAS_HPMN0));
>> }
>> @@ -92,6 +93,26 @@ u64 kvm_pmu_fgt2_bits(void)
>> | HDFGRTR2_EL2_nPMICNTR_EL0;
>> }

>> +/**
>> + * kvm_pmu_set_physical_access()
>> + * @vcpu: Pointer to vcpu struct
>> + *
>> + * Reconfigure the guest for physical access of PMU hardware if
>> + * allowed. This means reconfiguring mdcr_el2 and loading the vCPU
>> + * state onto hardware.
>> + *
>> + */
>> +
>> +void kvm_pmu_set_physical_access(struct kvm_vcpu *vcpu)
>> +{
>> + if (kvm_vcpu_pmu_is_partitioned(vcpu)
>> + && vcpu->arch.pmu.access == VCPU_PMU_ACCESS_VIRTUAL) {
>> + vcpu->arch.pmu.access = VCPU_PMU_ACCESS_PHYSICAL;
>> + kvm_arm_setup_mdcr_el2(vcpu);
>> + kvm_pmu_load(vcpu);
>> + }

> It isn't immediately obvious how this guards against preemption.

> Also, the general approach for these context-loading situations is to do
> a full load/put on the vCPU rather than a directed load.

Understood. Will fix.

>> +static void kvm_pmu_register_init(struct kvm_vcpu *vcpu)
>> +{
>> + if (vcpu->arch.pmu.access == VCPU_PMU_ACCESS_UNSET)
>> + vcpu->arch.pmu.access = VCPU_PMU_ACCESS_VIRTUAL;

> This is confusing. The zero value of the enum should be consistent with
> the "unloaded" state.

That's the way I initially wrote it but it had a problem on a different
kernel. I forget the exact issue, but I never saw the problem on
upstream so I'm happy to go back to it.

>> diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
>> index f2ae761625a66..d73218706b834 100644
>> --- a/arch/arm64/kvm/sys_regs.c
>> +++ b/arch/arm64/kvm/sys_regs.c
>> @@ -1197,6 +1197,8 @@ static bool access_pmu_evtyper(struct kvm_vcpu
>> *vcpu, struct sys_reg_params *p,
>> p->regval = __vcpu_sys_reg(vcpu, reg);
>> }

>> + kvm_pmu_set_physical_access(vcpu);
>> +
>> return true;
>> }

>> @@ -1302,6 +1304,8 @@ static bool access_pmovs(struct kvm_vcpu *vcpu,
>> struct sys_reg_params *p,
>> p->regval = __vcpu_sys_reg(vcpu, PMOVSSET_EL0);
>> }

>> + kvm_pmu_set_physical_access(vcpu);
>> +
>> return true;
>> }

> Aren't there a ton of other registers the guest may access before
> these two? Having generic PMU register accessors would allow you to
> manage residence of PMU registers from a single spot.

Yes but these are the only two that use the old trap handlers. I also
call set_physical_access from my fast path handler that handles all the
other registers when partitioned.

Agree on having some generic accessors which you mention in an earlier
patch.