Re: [PATCH V7 3/6] arm64/perf: Add branch stack support in struct arm_pmu

[Mark Rutland]

On Thu, Jan 05, 2023 at 08:40:36AM +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
> Signed-off-by: Anshuman Khandual <anshuman.khandual@xxxxxxx>
> ---
>  drivers/perf/arm_pmu.c       | 3 +--
>  include/linux/perf/arm_pmu.h | 9 +++++++++
>  2 files changed, 10 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/perf/arm_pmu.c b/drivers/perf/arm_pmu.c
> index 14a3ed3bdb0b..a85b2d67022e 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 2a9d07cee927..64e1b2594025 100644
> --- a/include/linux/perf/arm_pmu.h
> +++ b/include/linux/perf/arm_pmu.h
> @@ -80,11 +80,14 @@ enum armpmu_attr_groups {
>  	ARMPMU_NR_ATTR_GROUPS
>  };
>  
> +#define ARM_PMU_BRANCH_STACK	BIT(0)
> +
>  struct arm_pmu {
>  	struct pmu	pmu;
>  	cpumask_t	supported_cpus;
>  	char		*name;
>  	int		pmuver;
> +	int		features;
>  	irqreturn_t	(*handle_irq)(struct arm_pmu *pmu);
>  	void		(*enable)(struct perf_event *event);
>  	void		(*disable)(struct perf_event *event);

Hmm, we already have the secure_access field separately. How about we fold that
in and go with:

	unsigned int	secure_access    : 1,
			has_branch_stack : 1;

... that way we have one way to manage flags, we don't need to allocate the
bits, and the bulk of the existing code for secure_access can stay as-is.

> @@ -119,8 +122,14 @@ struct arm_pmu {
>  
>  	/* Only to be used by ACPI probing code */
>  	unsigned long acpi_cpuid;
> +	void		*private;

Does this need to be on the end of struct arm_pmu, or can it be placed earlier?

The line spacing makes it look like the ACPI comment applies to 'private',
which isn't the case.

>  };
>  
> +static inline bool arm_pmu_branch_stack_supported(struct arm_pmu *armpmu)
> +{
> +	return armpmu->features & ARM_PMU_BRANCH_STACK;
> +}

With the above, this would become:

static inline bool arm_pmu_branch_stack_supported(struct arm_pmu *armpmu)
{
	return armpmu->has_branch_stack;
}

Thanks,
Mark.

> +
>  #define to_arm_pmu(p) (container_of(p, struct arm_pmu, pmu))
>  
>  u64 armpmu_event_update(struct perf_event *event);
> -- 
> 2.25.1
>