Re: [RFC V1 07/11] arm64/perf: Add BRBE driver

[James Clark <james.clark@arm.com>]

On 24/01/2022 04:30, Anshuman Khandual wrote:
> This adds a BRBE driver which implements all the required helper functions
> for struct arm_pmu. Following functions are defined by this driver which
> will configure, enable, capture, reset and disable BRBE buffer HW as and
> when requested via perf branch stack sampling framework.
> 
> - arm64_pmu_brbe_filter()
> - arm64_pmu_brbe_enable()
> - arm64_pmu_brbe_disable()
> - arm64_pmu_brbe_read()
> - arm64_pmu_brbe_probe()
> - arm64_pmu_brbe_reset()
> - arm64_pmu_brbe_supported()
> 
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: Arnaldo Carvalho de Melo <acme@kernel.org>
> Cc: Mark Rutland <mark.rutland@arm.com>
> Cc: Will Deacon <will@kernel.org>
> Cc: Catalin Marinas <catalin.marinas@arm.com>
> Cc: linux-arm-kernel@lists.infradead.org
> Cc: linux-perf-users@vger.kernel.org
> Cc: linux-kernel@vger.kernel.org
> Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
> ---
[...]
> +bool arm64_pmu_brbe_supported(struct perf_event *event)
> +{
> +	struct arm_pmu *armpmu = to_arm_pmu(event->pmu);
> +	struct pmu_hw_events *hw_events = per_cpu_ptr(armpmu->hw_events, event->cpu);
> +
> +	if (event->attr.branch_sample_type & PERF_SAMPLE_BRANCH_KERNEL) {
> +		if (!perfmon_capable()) {
> +			pr_warn_once("does not have permission for kernel branch filter\n");
> +			return false;

Why not check perf_event_paranoid too? I would expect kernel sampling to be allowed for non
root users with paranoid <= 0.