[PATCH 0/1] Return EINVAL when precise_ip perf events are requested on Arm

James Clark
2020-01-15T10:59:18Z

Hi,

Since we're adding support for SPE in user space Perf, we've encountered an issue
where we would like some more feedback if SPE isn't available for an event.

At the moment there is a patch for perf where you can enable SPE by doing this:

perf record -r branch-misses:p ...

Perf will have a hard coded list of events that can use SPE when ":p" is specified
and open the SPE PMU instead of the specified one. But if the event isn't in that
list, then Perf will attempt to open the normal event with precise_ip = 1.
That will succeed at the moment, but we'd like the kernel to say it's not supported
so there is a chance of showing a warning to the user.

This isn't just relevant to Perf though, there may be other tools that are already
setting this.

Therefore I'm looking for feedback on whether this would break backwards
compatibility with user space tools that are already setting precise_ip and
expecting it to not error out on Arm.

This change would also be beneficial for the case where if in the (distant) future
we do add some kind of precise support, there will be a chance of userspace
determining what is supported and what isn't.

Thanks
James

Cc: Will Deacon <will@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Arnaldo Carvalho de Melo <acme@kernel.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Tan Xiaojun <tanxiaojun@huawei.com>
Cc: Al Grant <al.grant@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-kernel@vger.kernel.org

James Clark (1):
Return EINVAL when precise_ip perf events are requested on Arm

drivers/perf/arm_pmu.c | 3 +++
include/uapi/linux/perf_event.h | 4 ++--
2 files changed, 5 insertions(+), 2 deletions(-)

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2.24.0