Re: [PATCH v3 3/5] arm64: pmu: Add function implementation to update event index in userpage.

[Mark Rutland]

On Fri, Aug 16, 2019 at 01:59:32PM +0100, Raphael Gault wrote:
> In order to be able to access the counter directly for userspace,
> we need to provide the index of the counter using the userpage.
> We thus need to override the event_idx function to retrieve and
> convert the perf_event index to armv8 hardware index.
> 
> Since the arm_pmu driver can be used by any implementation, even
> if not armv8, two components play a role into making sure the
> behaviour is correct and consistent with the PMU capabilities:
> 
> * the ARMPMU_EL0_RD_CNTR flag which denotes the capability to access
> counter from userspace.
> * the event_idx call back, which is implemented and initialized by
> the PMU implementation: if no callback is provided, the default
> behaviour applies, returning 0 as index value.
> 
> Signed-off-by: Raphael Gault <raphael.gault@xxxxxxx>
> ---
>  arch/arm64/kernel/perf_event.c | 22 ++++++++++++++++++++++
>  include/linux/perf/arm_pmu.h   |  2 ++
>  2 files changed, 24 insertions(+)
> 
> diff --git a/arch/arm64/kernel/perf_event.c b/arch/arm64/kernel/perf_event.c
> index a0b4f1bca491..9fe3f6909513 100644
> --- a/arch/arm64/kernel/perf_event.c
> +++ b/arch/arm64/kernel/perf_event.c
> @@ -818,6 +818,22 @@ static void armv8pmu_clear_event_idx(struct pmu_hw_events *cpuc,
>  		clear_bit(idx - 1, cpuc->used_mask);
>  }
>  
> +static int armv8pmu_access_event_idx(struct perf_event *event)
> +{
> +	if (!(event->hw.flags & ARMPMU_EL0_RD_CNTR))
> +		return 0;
> +
> +	/*
> +	 * We remap the cycle counter index to 32 to
> +	 * match the offset applied to the rest of
> +	 * the counter indeces.

Typo: s/indeces/indices/

> +	 */
> +	if (event->hw.idx == ARMV8_IDX_CYCLE_COUNTER)
> +		return 32;
> +
> +	return event->hw.idx;
> +}
> +
>  /*
>   * Add an event filter to a given event.
>   */
> @@ -911,6 +927,9 @@ static int __armv8_pmuv3_map_event(struct perf_event *event,
>  	if (armv8pmu_event_is_64bit(event))
>  		event->hw.flags |= ARMPMU_EVT_64BIT;
>  
> +	if (!cpus_have_const_cap(ARM64_HAS_HETEROGENEOUS_PMU))
> +		event->hw.flags |= ARMPMU_EL0_RD_CNTR;
> +
>  	/* Only expose micro/arch events supported by this PMU */
>  	if ((hw_event_id > 0) && (hw_event_id < ARMV8_PMUV3_MAX_COMMON_EVENTS)
>  	    && test_bit(hw_event_id, armpmu->pmceid_bitmap)) {
> @@ -1031,6 +1050,8 @@ static int armv8_pmu_init(struct arm_pmu *cpu_pmu)
>  	cpu_pmu->set_event_filter	= armv8pmu_set_event_filter;
>  	cpu_pmu->filter_match		= armv8pmu_filter_match;
>  
> +	cpu_pmu->pmu.event_idx		= armv8pmu_access_event_idx;
> +
>  	return 0;
>  }
>  
> @@ -1209,6 +1230,7 @@ void arch_perf_update_userpage(struct perf_event *event,
>  	 */
>  	freq = arch_timer_get_rate();
>  	userpg->cap_user_time = 1;
> +	userpg->cap_user_rdpmc = !!(event->hw.flags & ARMPMU_EL0_RD_CNTR);

For bisectability, we should only expose this to userspace once we have
the code to enable/disable it, so the code exposing the index and
setting up the user page cap needs to be added after the context switch
code.

Thanks,
Mark.

>  
>  	clocks_calc_mult_shift(&userpg->time_mult, &shift, freq,
>  			NSEC_PER_SEC, 0);
> diff --git a/include/linux/perf/arm_pmu.h b/include/linux/perf/arm_pmu.h
> index 71f525a35ac2..1106a9ac00fd 100644
> --- a/include/linux/perf/arm_pmu.h
> +++ b/include/linux/perf/arm_pmu.h
> @@ -26,6 +26,8 @@
>   */
>  /* Event uses a 64bit counter */
>  #define ARMPMU_EVT_64BIT		1
> +/* Allow access to hardware counter from userspace */
> +#define ARMPMU_EL0_RD_CNTR		2
>  
>  #define HW_OP_UNSUPPORTED		0xFFFF
>  #define C(_x)				PERF_COUNT_HW_CACHE_##_x
> -- 
> 2.17.1
>