Re: [PATCH 1/2] KVM: arm64: Iterate arm_pmus list to probe for default PMU

[Oliver Upton <oliver.upton@linux.dev>]

On Thu, May 25, 2023 at 09:27:21PM +0000, Oliver Upton wrote:
> To date KVM has relied on using a perf event to probe the core PMU at
> the time of vPMU initialization. Behind the scenes perf_event_init()

Oops:

s/perf_event_init/perf_init_event/

> would iteratively walk the PMUs of the system and return the PMU that
> could handle the event. However, an upcoming change in perf core will
> drop the iterative walk, thereby breaking the fragile dance we do on the
> KVM side.
> 
> Avoid the problem altogether by iterating over the list of supported
> PMUs maintained in KVM, returning the core PMU that matches the CPU
> we were called on.
> 
> Tested-by: Nathan Chancellor <nathan@kernel.org>
> Signed-off-by: Oliver Upton <oliver.upton@linux.dev>
> ---
>  arch/arm64/kvm/pmu-emul.c | 46 ++++++++++-----------------------------
>  1 file changed, 12 insertions(+), 34 deletions(-)
> 
> diff --git a/arch/arm64/kvm/pmu-emul.c b/arch/arm64/kvm/pmu-emul.c
> index 45727d50d18d..5deddc49e745 100644
> --- a/arch/arm64/kvm/pmu-emul.c
> +++ b/arch/arm64/kvm/pmu-emul.c
> @@ -694,45 +694,23 @@ void kvm_host_pmu_init(struct arm_pmu *pmu)
>  
>  static struct arm_pmu *kvm_pmu_probe_armpmu(void)
>  {
> -	struct perf_event_attr attr = { };
> -	struct perf_event *event;
> -	struct arm_pmu *pmu = NULL;
> -
> -	/*
> -	 * Create a dummy event that only counts user cycles. As we'll never
> -	 * leave this function with the event being live, it will never
> -	 * count anything. But it allows us to probe some of the PMU
> -	 * details. Yes, this is terrible.
> -	 */
> -	attr.type = PERF_TYPE_RAW;
> -	attr.size = sizeof(attr);
> -	attr.pinned = 1;
> -	attr.disabled = 0;
> -	attr.exclude_user = 0;
> -	attr.exclude_kernel = 1;
> -	attr.exclude_hv = 1;
> -	attr.exclude_host = 1;
> -	attr.config = ARMV8_PMUV3_PERFCTR_CPU_CYCLES;
> -	attr.sample_period = GENMASK(63, 0);
> +	struct arm_pmu *tmp, *pmu = NULL;
> +	struct arm_pmu_entry *entry;
> +	int cpu;
>  
> -	event = perf_event_create_kernel_counter(&attr, -1, current,
> -						 kvm_pmu_perf_overflow, &attr);
> +	mutex_lock(&arm_pmus_lock);
>  
> -	if (IS_ERR(event)) {
> -		pr_err_once("kvm: pmu event creation failed %ld\n",
> -			    PTR_ERR(event));
> -		return NULL;
> -	}
> +	cpu = smp_processor_id();
> +	list_for_each_entry(entry, &arm_pmus, entry) {
> +		tmp = entry->arm_pmu;
>  
> -	if (event->pmu) {
> -		pmu = to_arm_pmu(event->pmu);
> -		if (pmu->pmuver == ID_AA64DFR0_EL1_PMUVer_NI ||
> -		    pmu->pmuver == ID_AA64DFR0_EL1_PMUVer_IMP_DEF)
> -			pmu = NULL;
> +		if (cpumask_test_cpu(cpu, &tmp->supported_cpus)) {
> +			pmu = tmp;
> +			break;
> +		}
>  	}
>  
> -	perf_event_disable(event);
> -	perf_event_release_kernel(event);
> +	mutex_unlock(&arm_pmus_lock);
>  
>  	return pmu;
>  }
> -- 
> 2.41.0.rc0.172.g3f132b7071-goog
> 

-- 
Thanks,
Oliver