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

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Message-ID: <30d95df2-c3b-b3e3-d65e-c6be0355fb1@maine.edu>
Date:   Wed, 15 Jun 2022 13:57:03 -0400 (EDT)
From:   Vince Weaver <vincent.weaver@...ne.edu>
To:     linux-kernel@...r.kernel.org
cc:     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@...r.kernel.org, Will Deacon <will@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Rob Herring <robh@...nel.org>
Subject: [perf] why is /proc/sys/kernel/perf_user_access ARM64 only?


Just wasted a lot of time tracking down why rdpmc() event reading wasn't 
working on an ARM64 machine.

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

Also, when user events are disabled, why is the ARMv8 PMU not disabling
the cap_user_rdpmc bit in the perf mmap() page?

rdpmc was trouble before, but now it's an even bigger 
architecture-dependent mess just trying to figure out if the feature is 
enabled or not.

Vince