Re: [PATCH v7 10/20] KVM: arm64: Context swap Partitioned PMU guest registers

[Colton Lewis <coltonlewis@google.com>]

James Clark <james.clark@linaro.org> writes:

> On 04/05/2026 10:18 pm, Colton Lewis wrote:
>> Save and restore newly untrapped registers that can be directly
>> accessed by the guest when the PMU is partitioned.

>> * PMEVCNTRn_EL0
>> * PMCCNTR_EL0
>> * PMSELR_EL0
>> * PMCR_EL0
>> * PMCNTEN_EL0
>> * PMINTEN_EL1

>> If we know we are not partitioned (that is, using the emulated vPMU),
>> then return immediately. A later patch will make this lazy so the
>> context swaps don't happen unless the guest has accessed the PMU.

>> PMEVTYPER is handled in a following patch since we must apply the KVM
>> event filter before writing values to hardware.

>> PMOVS guest counters are cleared to avoid the possibility of
>> generating spurious interrupts when PMINTEN is written. This is fine
>> because the virtual register for PMOVS is always the canonical value.

>> Signed-off-by: Colton Lewis <coltonlewis@google.com>
>> ---
>>    arch/arm/include/asm/arm_pmuv3.h |   4 +
>>    arch/arm64/kvm/arm.c             |   2 +
>>    arch/arm64/kvm/pmu-direct.c      | 169 +++++++++++++++++++++++++++++++
>>    include/kvm/arm_pmu.h            |  16 +++
>>    4 files changed, 191 insertions(+)

>> diff --git a/arch/arm/include/asm/arm_pmuv3.h  
>> b/arch/arm/include/asm/arm_pmuv3.h
>> index 42d62aa48d0a6..eebc89bdab7a1 100644
>> --- a/arch/arm/include/asm/arm_pmuv3.h
>> +++ b/arch/arm/include/asm/arm_pmuv3.h
>> @@ -235,6 +235,10 @@ static inline bool kvm_pmu_is_partitioned(struct  
>> arm_pmu *pmu)
>>    {
>>    	return false;
>>    }
>> +static inline u64 kvm_pmu_host_counter_mask(struct arm_pmu *pmu)
>> +{
>> +	return ~0;
>> +}

>>    /* PMU Version in DFR Register */
>>    #define ARMV8_PMU_DFR_VER_NI        0
>> diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
>> index 410ffd41fd73a..a942f2bc13fc4 100644
>> --- a/arch/arm64/kvm/arm.c
>> +++ b/arch/arm64/kvm/arm.c
>> @@ -680,6 +680,7 @@ void kvm_arch_vcpu_load(struct kvm_vcpu *vcpu, int  
>> cpu)
>>    		kvm_vcpu_load_vhe(vcpu);
>>    	kvm_arch_vcpu_load_fp(vcpu);
>>    	kvm_vcpu_pmu_restore_guest(vcpu);
>> +	kvm_pmu_load(vcpu);
>>    	if (kvm_arm_is_pvtime_enabled(&vcpu->arch))
>>    		kvm_make_request(KVM_REQ_RECORD_STEAL, vcpu);

>> @@ -721,6 +722,7 @@ void kvm_arch_vcpu_put(struct kvm_vcpu *vcpu)
>>    	kvm_timer_vcpu_put(vcpu);
>>    	kvm_vgic_put(vcpu);
>>    	kvm_vcpu_pmu_restore_host(vcpu);
>> +	kvm_pmu_put(vcpu);
>>    	if (vcpu_has_nv(vcpu))
>>    		kvm_vcpu_put_hw_mmu(vcpu);
>>    	kvm_arm_vmid_clear_active();
>> diff --git a/arch/arm64/kvm/pmu-direct.c b/arch/arm64/kvm/pmu-direct.c
>> index 63ac72910e4b5..360d022d918d5 100644
>> --- a/arch/arm64/kvm/pmu-direct.c
>> +++ b/arch/arm64/kvm/pmu-direct.c
>> @@ -9,6 +9,7 @@
>>    #include <linux/perf/arm_pmuv3.h>

>>    #include <asm/arm_pmuv3.h>
>> +#include <asm/kvm_emulate.h>

>>    /**
>>     * has_host_pmu_partition_support() - Determine if partitioning is  
>> possible
>> @@ -98,3 +99,171 @@ u8 kvm_pmu_hpmn(struct kvm_vcpu *vcpu)

>>    	return *host_data_ptr(nr_event_counters);
>>    }
>> +
>> +/**
>> + * kvm_pmu_host_counter_mask() - Compute bitmask of host-reserved  
>> counters
>> + * @pmu: Pointer to arm_pmu struct
>> + *
>> + * Compute the bitmask that selects the host-reserved counters in the
>> + * {PMCNTEN,PMINTEN,PMOVS}{SET,CLR} registers. These are the counters
>> + * in HPMN..N
>> + *
>> + * Return: Bitmask
>> + */
>> +u64 kvm_pmu_host_counter_mask(struct arm_pmu *pmu)
>> +{
>> +	u8 nr_counters = *host_data_ptr(nr_event_counters);
>> +
>> +	if (kvm_pmu_is_partitioned(pmu))
>> +		return GENMASK(nr_counters - 1, pmu->max_guest_counters);
>> +
>> +	return ARMV8_PMU_CNT_MASK_ALL;
>> +}
>> +
>> +/**
>> + * kvm_pmu_guest_counter_mask() - Compute bitmask of guest-reserved  
>> counters
>> + * @pmu: Pointer to arm_pmu struct
>> + *
>> + * Compute the bitmask that selects the guest-reserved counters in the
>> + * {PMCNTEN,PMINTEN,PMOVS}{SET,CLR} registers. These are the counters
>> + * in 0..HPMN and the cycle and instruction counters.
>> + *
>> + * Return: Bitmask
>> + */
>> +u64 kvm_pmu_guest_counter_mask(struct arm_pmu *pmu)
>> +{
>> +	if (kvm_pmu_is_partitioned(pmu))
>> +		return ARMV8_PMU_CNT_MASK_C | GENMASK(pmu->max_guest_counters - 1, 0);
>> +
>> +	return 0;
>> +}

> Minor nit: slightly inconsistent use of types. Returns a u64 but doesn't
> use GENMASK_ULL and is also usually saved into a long when it's called.

Will fix