linux-kernel - Re: [perf] why is /proc/sys/kernel/perf_user

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Date:   Fri, 17 Jun 2022 16:11:19 -0400 (EDT)
From:   Vince Weaver <vincent.weaver@...ne.edu>
To:     Rob Herring <robh@...nel.org>
cc:     Vince Weaver <vincent.weaver@...ne.edu>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        linux-perf-users <linux-perf-users@...r.kernel.org>,
        Will Deacon <will@...nel.org>,
        Mark Rutland <mark.rutland@....com>
Subject: Re: [perf] why is /proc/sys/kernel/perf_user_access ARM64 only?

On Wed, 15 Jun 2022, Rob Herring wrote:

> On Wed, Jun 15, 2022 at 11:57 AM Vince Weaver <vincent.weaver@...ne.edu> wrote:
> > It turns out ARM64 has added a custom
> >         "/proc/sys/kernel/perf_user_access"
> > to control rdpmc access, but only on ARM64.
> >         e2012600810c9ded81f6f63a8d04781be3c300ad
> >
> > Why is this ARM64-only?  Why isn't this generic perf infrastructure?
> 
> Adding it on x86 would break users at least if default off.

Well in theory the file could have been added on x86 but default to on.
Some of the low-level perf feature detection is bad enough as is, without
having architecture-dependent proc files to worry about too.

> > How is this different from the existing
> >         /sys/bus/event_source/devices/cpu/rdpmc
> > tooling?
> 
> big.LITTLE

interesting.  I wonder how x86 Alder Lake deals with things.

In any case I guess it's really too late to do anything about this.  I 
should check to make sure the manpage gets updated.

Vince