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    SubjectRe: [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@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
    > Tested-by: James Clark <james.clark@arm.com>
    > Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.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@arm.com>

    Mark.

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