linux-kernel - Re: [PATCH V3] arm64: Disable EL2 traps for BRBE instructions executed in EL1

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Message-ID: <20230605140038.GB21212@willie-the-truck>
Date:   Mon, 5 Jun 2023 15:00:39 +0100
From:   Will Deacon <will@...nel.org>
To:     Anshuman Khandual <anshuman.khandual@....com>
Cc:     linux-arm-kernel@...ts.infradead.org,
        Catalin Marinas <catalin.marinas@....com>,
        Mark Brown <broonie@...nel.org>, Marc Zyngier <maz@...nel.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH V3] arm64: Disable EL2 traps for BRBE instructions
 executed in EL1

On Tue, May 16, 2023 at 02:00:15PM +0530, Anshuman Khandual wrote:
> This disables EL2 traps for BRBE instructions executed in EL1. This would
> enable BRBE to be configured and used successfully in the guest kernel.
> While here, this updates Documentation/arm64/booting.rst as well.

Is it safe to give the guest access to BRBE if the hypervisor doesn't
switch the state? I'm assuming the architecture defaults to "trap" for a
reason and disabling the trap alone isn't sufficient.

If it _is_ safe, please can you explain that in the commit message? If not,
then this should be at the end of the series adding hypervisor support.

Will