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

Mark Rutland
2023-01-12T13:55:27Z

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@arm.com>
> Cc: Will Deacon <will@kernel.org>
> Cc: Mark Rutland <mark.rutland@arm.com>
> Cc: linux-arm-kernel@lists.infradead.org
> Cc: linux-kernel@vger.kernel.org
> Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
> ---
> 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