[PATCH] arm64: document the boot requirements for MTE

[Peter Collingbourne]

On Fri, Apr 22, 2022 at 6:42 AM Catalin Marinas <catalin.marinas at arm.com> wrote:
>
> 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).

Okay, I changed it in v2 to explicitly say FEAT_MTE2.

Peter