linux-kernel - Re: [PATCH v2 08/10] Documentation: admin-guide: Add riscv sysctl_perf_user

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Message-ID: <CAOnJCU+sda3UaOvC3Si4TT3nvibUSEOwmx94eQK-9ouvG4=ovQ@mail.gmail.com>
Date:   Wed, 31 May 2023 10:08:56 -0700
From:   Atish Patra <atishp@...shpatra.org>
To:     Andrew Jones <ajones@...tanamicro.com>
Cc:     Alexandre Ghiti <alexghiti@...osinc.com>,
        Jonathan Corbet <corbet@....net>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...nel.org>,
        Namhyung Kim <namhyung@...nel.org>,
        Ian Rogers <irogers@...gle.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Anup Patel <anup@...infault.org>,
        Will Deacon <will@...nel.org>, Rob Herring <robh@...nel.org>,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-perf-users@...r.kernel.org, linux-riscv@...ts.infradead.org,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v2 08/10] Documentation: admin-guide: Add riscv sysctl_perf_user_access

On Wed, May 31, 2023 at 8:07 AM Andrew Jones <ajones@...tanamicro.com> wrote:
>
> On Fri, May 12, 2023 at 10:53:19AM +0200, Alexandre Ghiti wrote:
> > riscv now uses this sysctl so document its usage for this architecture.
> >
> > Signed-off-by: Alexandre Ghiti <alexghiti@...osinc.com>
> > ---
> >  Documentation/admin-guide/sysctl/kernel.rst | 24 +++++++++++++++++----
> >  1 file changed, 20 insertions(+), 4 deletions(-)
> >
> > diff --git a/Documentation/admin-guide/sysctl/kernel.rst b/Documentation/admin-guide/sysctl/kernel.rst
> > index 4b7bfea28cd7..93cd518ca94b 100644
> > --- a/Documentation/admin-guide/sysctl/kernel.rst
> > +++ b/Documentation/admin-guide/sysctl/kernel.rst
> > @@ -941,16 +941,32 @@ enabled, otherwise writing to this file will return ``-EBUSY``.
> >  The default value is 8.
> >
> >
> > -perf_user_access (arm64 only)
> > -=================================
> > +perf_user_access (arm64 and riscv only)
> > +=======================================
> > +
> > +Controls user space access for reading perf event counters.
> >
> > -Controls user space access for reading perf event counters. When set to 1,
> > -user space can read performance monitor counter registers directly.
> > +arm64
> > +=====
> >
> >  The default value is 0 (access disabled).
> > +When set to 1, user space can read performance monitor counter registers
> > +directly.
> >
> >  See Documentation/arm64/perf.rst for more information.
> >
> > +riscv
> > +=====
> > +
> > +When set to 0, user access is disabled.
> > +
> > +When set to 1, user space can read performance monitor counter registers
> > +directly only through perf, any direct access without perf intervention will
> > +trigger an illegal instruction.
> > +
> > +The default value is 2, which enables legacy mode (user space has direct
> > +access to cycle, time and insret CSRs only). Note that this legacy value
> > +is deprecated and will be removed once all userspace applications are fixed.
>
> All modes can access the time CSR so I'm not sure if it should be pointed
> out here as if it's an exception. Maybe we shouldn't point it out at all
> or we should point it out for all three?
>

Valid point. Thanks Drew.
In the future, we probably want to support prctl
(PR_SET_TSC/SECCOMP_MODE_STRICT) to disable even
time CSR access. I don't think there is any use case for it right now.

> Thanks,
> drew



-- 
Regards,
Atish