Re: [PATCH] target/arm: add support for FEAT_MTE_TAGGED_FAR

[Peter Maydell <peter.maydell@linaro.org>]

On Wed, 5 Nov 2025 at 17:49, Gustavo Romero <gustavo.romero@linaro.org> wrote:
>
> Hi Gabriel,
>
> Thanks for your contribution.
>
> On 11/4/25 21:50, Gabriel Brookman wrote:
> > FEAT_MTE_TAGGED_FAR is a feature required for MTE4. The feature
> > guarantees that the full address (including tag bits) is reported after
> > a SEGV_MTESERR, and advertises itself in the ID_AA64PFR2_EL1 system
> > register. QEMU was already reporting the full address, so this commit
> > simply advertises the feature by setting that register, and unsets the
> > register if MTE is disabled.
> >
> > Signed-off-by: Gabriel Brookman <brookmangabriel@gmail.com>
> > ---
> > This patch is the first step toward implementing ARM's Enhanced Memory
> > Tagging Extension (MTE4). MTE4 guarantees the presence of several
> > subfeatures: FEAT_MTE_CANONICAL_TAGS, FEAT_MTE_TAGGED_FAR,
> > FEAT_MTE_STORE_ONLY, FEAT_MTE_NO_ADDRESS_TAGS, and FEAT_MTE_PERM,
> > none of which are currently implemented in QEMU.
> >
> > According to the ARM ARM, the presence of any of these features (except
> > FEAT_MTE_PERM) implies the presence of all the others. For simplicity
> > and ease of review, I plan to introduce them one at a time. This first
> > patch focuses on FEAT_MTE_TAGGED_FAR.
>
> I think it's ok to add these "subfeatures" separately.

We can add the implementation of the subfeatures separately,
but we should not enable them in 'max' until they're all
present. (We don't always adhere strictly to the architecture's
"feature X implies Y exists" rules, but in this case
because they're really a tightly linked bundle that add
up to "MTE4" I think that presnting only a subset to guests
is likely to result in them not behaving correctly.)

thanks
-- PMM