Re: [PATCH V11 03/10] arm64/perf: Add branch stack support in struct arm_pmu

[Anshuman Khandual]

On 6/5/23 13:28, Mark Rutland wrote:
> 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@xxxxxxx>
>> Cc: Will Deacon <will@xxxxxxxxxx>
>> Cc: Mark Rutland <mark.rutland@xxxxxxx>
>> Cc: linux-arm-kernel@xxxxxxxxxxxxxxxxxxx
>> Cc: linux-kernel@xxxxxxxxxxxxxxx
>> Tested-by: James Clark <james.clark@xxxxxxx>
>> Signed-off-by: Anshuman Khandual <anshuman.khandual@xxxxxxx>
>> ---
>>  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;

Sure, will drop the helper and change as suggested in all the call sites.

> 
> With that:
> 
> Acked-by: Mark Rutland <mark.rutland@xxxxxxx>
> 
> Mark.