linux-kernel - Re: [PATCH] arm64: perf: Ensure EL0 access is disabled at reset

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Date:   Tue, 27 Apr 2021 09:05:37 -0500
From:   Rob Herring <robh@...nel.org>
To:     Mark Rutland <mark.rutland@....com>
Cc:     Will Deacon <will@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.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>
Subject: Re: [PATCH] arm64: perf: Ensure EL0 access is disabled at reset

On Tue, Apr 27, 2021 at 8:55 AM Mark Rutland <mark.rutland@....com> wrote:
>
> On Tue, Apr 27, 2021 at 08:48:52AM -0500, Rob Herring wrote:
> > The ER, SW, and EN bits in the PMUSERENR_EL0 register are UNKNOWN at
> > reset and the register is never initialized, so EL0 access could be
> > enabled by default on some implementations. Let's initialize
> > PMUSERENR_EL0 to a known state with EL0 access disabled.
>
> We reset PMUSERENR_EL0 via the reset_pmuserenr_el0 macro, called from
> __cpu_setup when a CPU is onlined and from cpu_do_resume() when a CPU
> returns from a context-destructive idle state. We do it there so that
> it's handled even if a kernel isn't built with perf support.

Indeed.

> AFAICT, that *should* do the right thing -- are you seeing UNKNOWN
> values, or was this found by inspection?

Inspection. Sorry for the noise.

Rob