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

[James Clark <james.clark@linaro.org>]

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.