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

[Gabriel Brookman <brookmangabriel@gmail.com>]

Gustavo,

Thanks for the review! It makes sense.

On Thu, Nov 06, 2025 at 12:44:23PM +0100, Gustavo Romero wrote:
> Gabriel, given what Peter explained above, although the patch
> is correct, the best to move forward here is to embedded this
> patch into the additional series that implements the others
> "subfeatures" and, as a last step, after the patches that
> implement the features, you enable all the features in 'max'.

Once I finish FEAT_MTE_STORE_ONLY I'll send a new series, which will
include the changes from this series, the STORE_ONLY implementation,
and also include patches for the updated documentation and tests for
FEAT_MTE_TAGGED_FAR as you requested. As I continue to write patches
for MTE4 features, I'll add them to that patchset and we can merge it
all at once.

Thanks,
Gabriel

On Thu, Nov 6, 2025 at 6:44 AM Gustavo Romero <gustavo.romero@linaro.org> wrote:
>
> Hi Peter,
>
> On 11/6/25 11:31, Peter Maydell wrote:
> > 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.
>
> ah, true. I forget that when we do that we enable them in 'max'
> as a last step.
>
>
> > (We don't always adhere strictly to the architecture's
> > "feature X implies Y exists" rules,
>
> Thanks for confirming it ;)
>
>
> > 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.)
>
> Gabriel, given what Peter explained above, although the patch
> is correct, the best to move forward here is to embedded this
> patch into the additional series that implements the others
> "subfeatures" and, as a last step, after the patches that
> implement the features, you enable all the features in 'max'.
>
>
> Cheers,
> Gustavo