Re: [PATCH v6 11/19] KVM: arm64: Context swap Partitioned PMU guest registers

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

On 09/02/2026 10:14 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/arm64/kvm/arm.c        |   2 +
>   arch/arm64/kvm/pmu-direct.c | 123 ++++++++++++++++++++++++++++++++++++
>   include/kvm/arm_pmu.h       |   4 ++
>   3 files changed, 129 insertions(+)
> 
> diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
> index 620a465248d1b..adbe79264c032 100644
> --- a/arch/arm64/kvm/arm.c
> +++ b/arch/arm64/kvm/arm.c
> @@ -635,6 +635,7 @@ 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);
>   
> @@ -676,6 +677,7 @@ 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 f2e6b1eea8bd6..b07b521543478 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
> @@ -163,3 +164,125 @@ u8 kvm_pmu_hpmn(struct kvm_vcpu *vcpu)
>   
>   	return *host_data_ptr(nr_event_counters);
>   }
> +
> +/**
> + * kvm_pmu_load() - Load untrapped PMU registers
> + * @vcpu: Pointer to struct kvm_vcpu
> + *
> + * Load all untrapped PMU registers from the VCPU into the PCPU. Mask
> + * to only bits belonging to guest-reserved counters and leave
> + * host-reserved counters alone in bitmask registers.
> + */
> +void kvm_pmu_load(struct kvm_vcpu *vcpu)
> +{
> +	struct arm_pmu *pmu;
> +	unsigned long guest_counters;
> +	u64 mask;
> +	u8 i;
> +	u64 val;
> +
> +	/*
> +	 * If we aren't guest-owned then we know the guest isn't using
> +	 * the PMU anyway, so no need to bother with the swap.
> +	 */
> +	if (!kvm_vcpu_pmu_is_partitioned(vcpu))
> +		return;
> +
> +	preempt_disable();
> +
> +	pmu = vcpu->kvm->arch.arm_pmu;
> +	guest_counters = kvm_pmu_guest_counter_mask(pmu);
> +
> +	for_each_set_bit(i, &guest_counters, ARMPMU_MAX_HWEVENTS) {
> +		val = __vcpu_sys_reg(vcpu, PMEVCNTR0_EL0 + i);
> +
> +		write_sysreg(i, pmselr_el0);
> +		write_sysreg(val, pmxevcntr_el0);

This needs to have a special case for ARMV8_PMU_CYCLE_IDX because you 
can't use pmxevcntr_el0 to read or write PMCCNTR_EL0:

D24.5.22:

   SEL 0b11111      Select the cycle counter, PMCCNTR_EL0:

                    MRS and MSR of PMXEVCNTR_EL0 are CONSTRAINED
                    UNPREDICTABLE.

There are 3 separate instances of the same thing in the patches. I was 
getting undefined instruction errors on my Radxa O6 board until they 
were all fixed.