linux-kernel - Re: [PATCH V9 00/10] arm64/perf: Enable branch stack sampling

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Message-ID: <655ff114-99d2-4612-9167-cc8688f2b6b2@sirena.org.uk>
Date:   Tue, 21 Mar 2023 19:02:40 +0000
From:   Mark Brown <broonie@...nel.org>
To:     Anshuman Khandual <anshuman.khandual@....com>
Cc:     linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        will@...nel.org, catalin.marinas@....com, mark.rutland@....com,
        James Clark <james.clark@....com>,
        Rob Herring <robh@...nel.org>, Marc Zyngier <maz@...nel.org>,
        Suzuki Poulose <suzuki.poulose@....com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        linux-perf-users@...r.kernel.org
Subject: Re: [PATCH V9 00/10] arm64/perf: Enable branch stack sampling

On Wed, Mar 15, 2023 at 10:44:34AM +0530, Anshuman Khandual wrote:
> This series enables perf branch stack sampling support on arm64 platform
> via a new arch feature called Branch Record Buffer Extension (BRBE). All
> relevant register definitions could be accessed here.
> 
> https://developer.arm.com/documentation/ddi0601/2021-12/AArch64-Registers

While looking at another feature I noticed that HFGITR_EL2 has two traps
for BRBE instructions, nBRBINJ and nBRBIALL which trap BRB INJ and BRB
IALL.  Even if we don't use those right now does it make sense to
document a requirement for those traps to be disabled now in case we
need them later, and do so during EL2 setup for KVM guests?  That could
always be done incrementally.

I've got a patch adding the definition of that register to sysreg which
I should be sending shortly, no need to duplicate that effort.

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