Re: [PATCH v2 1/9] perf: Add EVENT_ATTR_ID to simplify event attributes

On Wed, Jun 02, 2021 at 04:45:23PM +0800, liuqi (BA) wrote:

Hi Will,

Thanks for reviewing this patch.

On 2021/6/1 21:10, Will Deacon wrote:

quoted

On Wed, May 19, 2021 at 05:51:51PM +0800, Qi Liu wrote:

quoted

Similar EVENT_ATTR macros are defined in many PMU drivers,
like HiSilicon PMU driver, Arm PMU driver, Arm SMMU PMU
driver. So Add a generic macro to simplify code.

Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Arnaldo Carvalho de Melo <acme@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Signed-off-by: Qi Liu <redacted>
---
  include/linux/perf_event.h | 6 ++++++
  kernel/events/core.c       | 2 ++
  2 files changed, 8 insertions(+)

diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
index f5a6a2f..d0aa74e 100644
--- a/include/linux/perf_event.h
+++ b/include/linux/perf_event.h

@@ -1576,6 +1576,12 @@ static struct perf_pmu_events_attr _var = {				    \
  	.event_str	= _str,						    \
  };
+#define PMU_EVENT_ATTR_ID(_name, _id)					     \
+	(&((struct perf_pmu_events_attr[]) {				     \
+		{ .attr = __ATTR(_name, 0444, perf_event_sysfs_show, NULL),  \
+		  .id = _id, }						     \
+	})[0].attr.attr)
+
  #define PMU_FORMAT_ATTR(_name, _format)					\
  static ssize_t								\
  _name##_show(struct device *dev,					\

diff --git a/kernel/events/core.c b/kernel/events/core.c
index 0ac818b..330d9cc 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c

@@ -13295,6 +13295,8 @@ ssize_t perf_event_sysfs_show(struct device *dev, struct device_attribute *attr,
  	if (pmu_attr->event_str)
  		return sprintf(page, "%s\n", pmu_attr->event_str);
+	else
+		return sprintf(page, "config=%#llx\n", pmu_attr->id);

I think it's a really bad idea to hardcode this here. For example, I think
this patch series breaks user ABI for the SMMU PMU which used to print:

	"event=0x%02llx\n"

and by the looks of it many of the other conversions are unsound too.

Got it, so I'll use pmu_attr->event_str here, for example,
SMMU_EVENT_ATTR(cycles, "event=0x00")

You could, but honestly I don't really see this being any better than the
current code. The advantage of using "event=0x%02llx\n" is that things are
consistent by construction and therefore userspace can parse the information
easily. We lose that if we just hardcode the string and it's error-prone to
extend.

Will

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