Re: [PATCH v9 25/30] KVM: arm64: Expose SME to nested guests

[Mark Brown]

On Tue, Jan 13, 2026 at 02:37:57PM +0000, Fuad Tabba wrote:
> On Tue, 23 Dec 2025 at 01:23, Mark Brown <broonie@xxxxxxxxxx> wrote:

> >         case SYS_ID_AA64PFR1_EL1:
> > -               /* Only support BTI, SSBS, CSV2_frac */
> > +               /* Only support BTI, SME, SSBS, CSV2_frac */
> >                 val &= ~(ID_AA64PFR1_EL1_PFAR           |
> >                          ID_AA64PFR1_EL1_MTEX           |
> >                          ID_AA64PFR1_EL1_THE            |
> >                          ID_AA64PFR1_EL1_GCS            |
> >                          ID_AA64PFR1_EL1_MTE_frac       |
> >                          ID_AA64PFR1_EL1_NMI            |
> > -                        ID_AA64PFR1_EL1_SME            |

> Should we also limit this to SME2, i.e.

> + val = ID_REG_LIMIT_FIELD_ENUM(val, ID_AA64PFR1_EL1, SME, SME2);

> That said, we don't do anything similar to SVE, but it might also be
> worth doing that there.

This feels like a general approach issue with these registers that's out
of scope for this series, it's not just the vector extensions which
could introduce new state or anything else that requires explicit
support.  AIUI the theory here is that we bootstrap from the host's
sanitised registers so the time to add any required limits on future
values would be when enabling them for the host kernel, assuming KVM
support isn't added simultaneously.

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