[PATCH 16/20] KVM: arm64: Simplify handling of full register invalid constraint

[Fuad Tabba]

On Mon, 26 Jan 2026 at 12:17, Marc Zyngier <maz at kernel.org> wrote:
>
> Now that we embed the RESx bits in the register description, it becomes
> easier to deal with registers that are simply not valid, as their
> existence is not satisfied by the configuration (SCTLR2_ELx without
> FEAT_SCTLR2, for example). Such registers essentially become RES0 for
> any bit that wasn't already advertised as RESx.
>
> Signed-off-by: Marc Zyngier <maz at kernel.org>

Reviewed-by: Fuad Tabba <tabba at google.com>

Cheers,
/fuad


> ---
>  arch/arm64/kvm/config.c | 15 +++++++++------
>  1 file changed, 9 insertions(+), 6 deletions(-)
>
> diff --git a/arch/arm64/kvm/config.c b/arch/arm64/kvm/config.c
> index 28e534f2850ea..0c037742215ac 100644
> --- a/arch/arm64/kvm/config.c
> +++ b/arch/arm64/kvm/config.c
> @@ -1332,7 +1332,7 @@ struct resx compute_reg_resx_bits(struct kvm *kvm,
>                                  const struct reg_feat_map_desc *r,
>                                  unsigned long require, unsigned long exclude)
>  {
> -       struct resx resx, tmp;
> +       struct resx resx;
>
>         resx = compute_resx_bits(kvm, r->bit_feat_map, r->bit_feat_map_sz,
>                                  require, exclude);
> @@ -1342,11 +1342,14 @@ struct resx compute_reg_resx_bits(struct kvm *kvm,
>                 resx.res1 |= r->feat_map.masks->res1;
>         }
>
> -       tmp = compute_resx_bits(kvm, &r->feat_map, 1, require, exclude);
> -
> -       resx.res0 |= tmp.res0;
> -       resx.res0 |= ~reg_feat_map_bits(&r->feat_map);
> -       resx.res1 |= tmp.res1;
> +       /*
> +        * If the register itself was not valid, all the non-RESx bits are
> +        * now considered RES0 (this matches the behaviour of registers such
> +        * as SCTLR2 and TCR2). Weed out any potential (though unlikely)
> +        * overlap with RES1 bits coming from the previous computation.
> +        */
> +       resx.res0 |= compute_resx_bits(kvm, &r->feat_map, 1, require, exclude).res0;
> +       resx.res1 &= ~resx.res0;
>
>         return resx;
>  }
> --
> 2.47.3
>