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Date:   Fri, 24 Mar 2023 11:40:46 +0000
From:   Mark Brown <broonie@kernel.org>
To:     Anshuman Khandual <anshuman.khandual@arm.com>
Cc:     linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org,
        will@kernel.org, catalin.marinas@arm.com, mark.rutland@arm.com,
        James Clark <james.clark@arm.com>,
        Rob Herring <robh@kernel.org>, Marc Zyngier <maz@kernel.org>,
        Suzuki Poulose <suzuki.poulose@arm.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        linux-perf-users@vger.kernel.org
Subject: Re: [PATCH V9 00/10] arm64/perf: Enable branch stack sampling
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On Fri, Mar 24, 2023 at 08:50:32AM +0530, Anshuman Khandual wrote:
> On 3/23/23 18:24, Mark Brown wrote:
> > On Thu, Mar 23, 2023 at 09:55:47AM +0530, Anshuman Khandual wrote:

> >> By default entire SYS_HFGITR_EL2 is set as cleared during init and tha=
t would
> >> prevent a guest from using BRBE.

> > It should prevent the host as well shouldn't it?=20

> In a EL2 host environment, BRBE is being enabled either in EL2 (kernel/hv=
) or
> in EL0 (user space), it never gets enabled on EL1. Moreover BRBIALL/BRBINJ
> instructions are always executed while being inside EL2 (kernel/hv). Henc=
e how
> could these instructions cause trap in EL2 ?

Ah, I see - I didn't realise this couldn't run at EL1.

> > Yes, looks roughly what I'd expect.

> I could send an stand alone patch after your latest series [1], which dis=
ables
> BRBINJ/BRBIALL instruction trap in EL2 to enable BRBE usage in the guest.

Sounds resaonable enough to me.

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