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Message-ID: <YmKwrs3dJ09ybBJa@arm.com>
Date: Fri, 22 Apr 2022 14:42:06 +0100
From: Catalin Marinas <catalin.marinas@....com>
To: Mark Brown <broonie@...nel.org>
Cc: Peter Collingbourne <pcc@...gle.com>,
Will Deacon <will@...nel.org>,
Jonathan Corbet <corbet@....net>,
linux-arm-kernel@...ts.infradead.org, linux-doc@...r.kernel.org,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH] arm64: document the boot requirements for MTE
On Tue, Apr 05, 2022 at 09:04:04AM +0100, Mark Brown wrote:
> On Mon, Apr 04, 2022 at 02:18:58PM -0700, Peter Collingbourne wrote:
>
> > + For CPUs with the Memory Tagging Extension feature:
> > +
> > + - If EL3 is present:
> > +
> > + - SCR_EL3.ATA (bit 26) must be initialised to 0b1.
> > +
> > + - If the kernel is entered at EL1 and EL2 is present:
> > +
> > + - HCR_EL2.ATA (bit 56) must be initialised to 0b1.
>
> Very nitpicky but this is only required for FEAT_MTE2 and above, plain
> FEAT_MTE doesn't have these traps. I don't know that this is a thing
> that anyone's actually implemented
I think that's a valid point. CPUs may implement FEAT_MTE2 but downgrade
it to FEAT_MTE if the SoC does not provide allocation tag storage. So we
should make it clear here that only from FEAT_MTE2 we should set those
bits (ID_AA64PFR1_EL1.MTE >= 2), otherwise they should be 0 or
hyp/firmware risks the OS triggering random external aborts.
> and from v8.7 on it's not permitted but the above isn't strictly true
> if someone did for some reason have the most basic version.
The wording is tricky: "This feature is mandatory from Armv8.7 when
FEAT_MTE2 is implemented". So one can still implement FEAT_MTE (or none
at all).
--
Catalin
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