linux-kernel - Re: [RFCv1 4/4] perf: arm_spe: Dynamically switch PID tracing to contextidr

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Date:   Fri, 22 Oct 2021 10:09:35 +0800
From:   Leo Yan <leo.yan@...aro.org>
To:     Kees Cook <keescook@...omium.org>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Ard Biesheuvel <ardb@...nel.org>,
        Sami Tolvanen <samitolvanen@...gle.com>,
        Nicholas Piggin <npiggin@...il.com>,
        James Morse <james.morse@....com>,
        Marc Zyngier <maz@...nel.org>, Joey Gouly <joey.gouly@....com>,
        Peter Collingbourne <pcc@...gle.com>,
        Vincenzo Frascino <vincenzo.frascino@....com>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Stephane Eranian <eranian@...gle.com>,
        James Clark <james.clark@....com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [RFCv1 4/4] perf: arm_spe: Dynamically switch PID tracing to
 contextidr

Hi Kees,

On Thu, Oct 21, 2021 at 08:49:46AM -0700, Kees Cook wrote:
> On Thu, Oct 21, 2021 at 09:45:30PM +0800, Leo Yan wrote:
> > Now Arm64 provides API for enabling and disable PID tracing, Arm SPE
> > driver invokes these functions to dynamically enable it during
> > profiling when the program runs in root PID name space, and disable PID
> > tracing when the perf event is stopped.
> > 
> > Device drivers should not depend on CONFIG_PID_IN_CONTEXTIDR for PID
> > tracing, so this patch uses the consistent condition for setting bit
> > EL1_CX for PMSCR.
> 
> My own preference here would be to not bother with the new
> enable/disable helpers, but just open code it right here. (Save a patch
> and is the only user.) But I defer to the taste of arm64 maintainers. :)

Yes, with your reminding I recognize that we can avoid the new helpers.
Just remind, tracing PID in contextidr is not only used by Arm SPE
driver, it will be used in Arm CoreSight driver as well.  I plan to use
a separate patch set to address Arm CoreSight (CoreSight driver misses
to checking root PID namespace so need firstly fix that issue).

Just give more info, so you and arm64 maintainers could judge we
should use helpers or directly access static key.

Thanks for your review!

Leo