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

On 1/12/23 19:24, Mark Rutland wrote:

On Thu, Jan 05, 2023 at 08:40:36AM +0530, Anshuman Khandual wrote:

quoted

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 <redacted>
---
 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;

Something like this would work, but should we use __u32 instead of unsigned int
to ensure 32 bit width ?

-       bool            secure_access; /* 32-bit ARM only */
+       unsigned int    secure_access   : 1, /* 32-bit ARM only */
+                       has_branch_stack: 1,
+                       reserved        : 31;

... 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.

Right, the changed code also builds on arm32 without any code change.

quoted

@@ -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?

This additional 'private' attribute structure sticking out from struct arm_pmu
should be at the end. But is there any benefit moving this earlier ?

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

Sure, will add the following comment, and a space in between.

diff --git a/include/linux/perf/arm_pmu.h b/include/linux/perf/arm_pmu.h
index f60f7e01acae..c0a090ff7991 100644
--- a/include/linux/perf/arm_pmu.h
+++ b/include/linux/perf/arm_pmu.h

@@ -130,6 +130,8 @@ struct arm_pmu {
 
        /* Only to be used by ACPI probing code */
        unsigned long acpi_cpuid;
+
+       /* Implementation specific attributes */
        void            *private;
 };

quoted

 };
 
+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;
}

Right, will change this helper as required.

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