Re: [PATCH v4 6/6] perf mem record: Use the IBS swfilt filter when available

[Ravi Bangoria <ravi.bangoria@amd.com>]

Hi Arnaldo,

> IBS events with exclude_{user,kernel} bits, as used for per-thread
> recording when kernel samples are not allowed, are rejected on hardware
> without the privilege filter, so per-thread 'perf mem record' fails on
> AMD:
> 
>   $ perf mem record -o /dev/null -- true
>   Failure to open event 'ibs_op/ldlat=0/u' on PMU 'ibs_op' which will be removed.
> 
> Kernel v6.14 added swfilt, a software privilege filter exposed as the
> 'swfilt' format term, making those events usable per-thread.  Give the
> ibs_op memory events extra tables with the term, selected in
> perf_pmu__arch_init() when the PMU exposes it, keeping the names that
> need system wide mode otherwise; the knowledge that IBS needs this
> stays in the arch code.
> 
> Suggested-by: Namhyung Kim <namhyung@kernel.org>
> Suggested-by: Ravi Bangoria <ravi.bangoria@amd.com>
> Assisted-by: LLM
> Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

Reviewed-by: Ravi Bangoria <ravi.bangoria@amd.com>

> + * ... Keep it even when
> + * the event has no exclude bits yet, 'perf record' adds them on the
> + * first open failure and retries with the same name.

Currently, 'perf record' fails rather than silently retrying with
/swfilt=1/. The reason to not transparently add /swfilt=1/ is to
make the user aware that software filtering is being used, and it
is not overhead free.

Thanks,
Ravi