Re: [PATCH v6 06/19] perf: arm_pmuv3: Keep out of guest counter partition

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

On 09/02/2026 10:14 pm, Colton Lewis wrote:
> If the PMU is partitioned, keep the driver out of the guest counter
> partition and only use the host counter partition.
> 
> Define some functions that determine whether the PMU is partitioned
> and construct mutually exclusive bitmaps for testing which partition a
> particular counter is in. Note that despite their separate position in
> the bitmap, the cycle and instruction counters are always in the guest
> partition.
> 
> Signed-off-by: Colton Lewis <coltonlewis@google.com>
> ---
>   arch/arm/include/asm/arm_pmuv3.h | 18 +++++++
>   arch/arm64/kvm/pmu-direct.c      | 86 ++++++++++++++++++++++++++++++++
>   drivers/perf/arm_pmuv3.c         | 40 +++++++++++++--
>   include/kvm/arm_pmu.h            | 24 +++++++++
>   4 files changed, 164 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/arm/include/asm/arm_pmuv3.h b/arch/arm/include/asm/arm_pmuv3.h
> index 154503f054886..bed4dfa755681 100644
> --- a/arch/arm/include/asm/arm_pmuv3.h
> +++ b/arch/arm/include/asm/arm_pmuv3.h
> @@ -231,6 +231,24 @@ static inline bool kvm_set_pmuserenr(u64 val)
>   }
>   
>   static inline void kvm_vcpu_pmu_resync_el0(void) {}
> +static inline void kvm_pmu_host_counters_enable(void) {}
> +static inline void kvm_pmu_host_counters_disable(void) {}
> +
> +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;
> +}
> +
> +static inline u64 kvm_pmu_guest_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/pmu-direct.c b/arch/arm64/kvm/pmu-direct.c
> index 74e40e4915416..05ac38ec3ea20 100644
> --- a/arch/arm64/kvm/pmu-direct.c
> +++ b/arch/arm64/kvm/pmu-direct.c
> @@ -5,6 +5,8 @@
>    */
>   
>   #include <linux/kvm_host.h>
> +#include <linux/perf/arm_pmu.h>
> +#include <linux/perf/arm_pmuv3.h>
>   
>   #include <asm/arm_pmuv3.h>
>   
> @@ -20,3 +22,87 @@ bool has_host_pmu_partition_support(void)
>   	return has_vhe() &&
>   		system_supports_pmuv3();
>   }
> +
> +/**
> + * kvm_pmu_is_partitioned() - Determine if given PMU is partitioned
> + * @pmu: Pointer to arm_pmu struct
> + *
> + * Determine if given PMU is partitioned by looking at hpmn field. The
> + * PMU is partitioned if this field is less than the number of
> + * counters in the system.
> + *
> + * Return: True if the PMU is partitioned, false otherwise
> + */
> +bool kvm_pmu_is_partitioned(struct arm_pmu *pmu)
> +{
> +	if (!pmu)
> +		return false;
> +
> +	return pmu->max_guest_counters >= 0 &&
> +		pmu->max_guest_counters <= *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 ARMV8_PMU_CNT_MASK_ALL;
> +
> +	return GENMASK(nr_counters - 1, pmu->max_guest_counters);
> +}
> +
> +/**
> + * 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)
> +{
> +	return ARMV8_PMU_CNT_MASK_C & GENMASK(pmu->max_guest_counters - 1, 0);

This should be an | instead of an & otherwise it's always zero. None of 
the passed through counters count anything with it like this, although 
the cycle counter always worked even with this issue.

I'm not sure if the selftests that you added catch this? I didn't try 
running them but seems like checking for non zero counter values is a 
very easy thing to test.