linux-kernel - Re: [PATCH V11 03/10] arm64/perf: Add branch stack support in struct arm

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Message-ID: <ZH2VmOUuZobZMcc/@FVFF77S0Q05N>
Date:   Mon, 5 Jun 2023 08:58:16 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     Anshuman Khandual <anshuman.khandual@....com>
Cc:     linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        will@...nel.org, catalin.marinas@....com,
        Mark Brown <broonie@...nel.org>,
        James Clark <james.clark@....com>,
        Rob Herring <robh@...nel.org>, Marc Zyngier <maz@...nel.org>,
        Suzuki Poulose <suzuki.poulose@....com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        linux-perf-users@...r.kernel.org
Subject: Re: [PATCH V11 03/10] arm64/perf: Add branch stack support in struct
 arm_pmu

On Wed, May 31, 2023 at 09:34:21AM +0530, Anshuman Khandual wrote:
> This updates 'struct arm_pmu' for branch stack sampling support later. This
> adds a new 'features' element in the structure to track supported features,
> and another 'private' element to encapsulate implementation attributes on a
> given 'struct arm_pmu'. These updates here will help in tracking any branch
> stack sampling support, which is being added later. This also adds a helper
> arm_pmu_branch_stack_supported().
> 
> This also enables perf branch stack sampling event on all 'struct arm pmu',
> supporting the feature but after removing the current gate that blocks such
> events unconditionally in armpmu_event_init(). Instead a quick probe can be
> initiated via arm_pmu_branch_stack_supported() to ascertain the support.
> 
> Cc: Catalin Marinas <catalin.marinas@....com>
> Cc: Will Deacon <will@...nel.org>
> Cc: Mark Rutland <mark.rutland@....com>
> Cc: linux-arm-kernel@...ts.infradead.org
> Cc: linux-kernel@...r.kernel.org
> Tested-by: James Clark <james.clark@....com>
> Signed-off-by: Anshuman Khandual <anshuman.khandual@....com>
> ---
>  drivers/perf/arm_pmu.c       |  3 +--
>  include/linux/perf/arm_pmu.h | 12 +++++++++++-
>  2 files changed, 12 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/perf/arm_pmu.c b/drivers/perf/arm_pmu.c
> index aada47e3b126..d4a4f2bd89a5 100644
> --- a/drivers/perf/arm_pmu.c
> +++ b/drivers/perf/arm_pmu.c
> @@ -510,8 +510,7 @@ static int armpmu_event_init(struct perf_event *event)
>  		!cpumask_test_cpu(event->cpu, &armpmu->supported_cpus))
>  		return -ENOENT;
>  
> -	/* does not support taken branch sampling */
> -	if (has_branch_stack(event))
> +	if (has_branch_stack(event) && !arm_pmu_branch_stack_supported(armpmu))
>  		return -EOPNOTSUPP;
>  
>  	return __hw_perf_event_init(event);
> diff --git a/include/linux/perf/arm_pmu.h b/include/linux/perf/arm_pmu.h
> index f7fbd162ca4c..0da745eaf426 100644
> --- a/include/linux/perf/arm_pmu.h
> +++ b/include/linux/perf/arm_pmu.h
> @@ -102,7 +102,9 @@ struct arm_pmu {
>  	int		(*map_event)(struct perf_event *event);
>  	void		(*sched_task)(struct perf_event_pmu_context *pmu_ctx, bool sched_in);
>  	int		num_events;
> -	bool		secure_access; /* 32-bit ARM only */
> +	unsigned int	secure_access	: 1, /* 32-bit ARM only */
> +			has_branch_stack: 1, /* 64-bit ARM only */
> +			reserved	: 30;
>  #define ARMV8_PMUV3_MAX_COMMON_EVENTS		0x40
>  	DECLARE_BITMAP(pmceid_bitmap, ARMV8_PMUV3_MAX_COMMON_EVENTS);
>  #define ARMV8_PMUV3_EXT_COMMON_EVENT_BASE	0x4000
> @@ -118,8 +120,16 @@ struct arm_pmu {
>  
>  	/* Only to be used by ACPI probing code */
>  	unsigned long acpi_cpuid;
> +
> +	/* Implementation specific attributes */
> +	void		*private;
>  };
>  
> +static inline bool arm_pmu_branch_stack_supported(struct arm_pmu *armpmu)
> +{
> +	return armpmu->has_branch_stack;
> +}

Since this is a trivial test, and we already access the 'secure_access' field
directly, I'd prefer we removed this helper and directly accessesed
arm_pmu::has_branch_stack, e.g. with the logic in armpmu_event_init() being:

	if (has_branch_stack(event) && !armpmu->has_branch_stack)
		return -EOPNOTSUPP;

With that:

Acked-by: Mark Rutland <mark.rutland@....com>

Mark.