[PATCH v2 12/20] KVM: arm64: Add RES1_WHEN_E2Hx constraints as configuration flags

[Fuad Tabba]

Hi Marc,

On Mon, 2 Feb 2026 at 18:43, Marc Zyngier <maz at kernel.org> wrote:
>
> "Thanks" to VHE, SCTLR_EL2 radically changes shape depending on the
> value of HCR_EL2.E2H, as a lot of the bits that didn't have much
> meaning with E2H=0 start impacting EL0 with E2H=1.
>
> This has a direct impact on the RESx behaviour of these bits, and
> we need a way to express them.
>
> For this purpose, introduce two new constaints that, when the

nit: constants

> controlling feature is not present, force the field to RES1 depending
> on the value of E2H. Note that RES0 is still implicit,

nit: , -> .
>
> This allows diverging RESx values depending on the value of E2H,
> something that is required by a bunch of SCTLR_EL2 bits.
>
> Signed-off-by: Marc Zyngier <maz at kernel.org>

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

Cheers,
/fuad


> ---
>  arch/arm64/kvm/config.c | 12 +++++++++---
>  1 file changed, 9 insertions(+), 3 deletions(-)
>
> diff --git a/arch/arm64/kvm/config.c b/arch/arm64/kvm/config.c
> index 7d133954ae01b..7e8e42c1cee4a 100644
> --- a/arch/arm64/kvm/config.c
> +++ b/arch/arm64/kvm/config.c
> @@ -26,6 +26,8 @@ struct reg_bits_to_feat_map {
>  #define        MASKS_POINTER   BIT(3)  /* Pointer to fgt_masks struct instead of bits */
>  #define        AS_RES1         BIT(4)  /* RES1 when not supported */
>  #define        REQUIRES_E2H1   BIT(5)  /* Add HCR_EL2.E2H RES1 as a pre-condition */
> +#define        RES1_WHEN_E2H0  BIT(6)  /* RES1 when E2H=0 and not supported */
> +#define        RES1_WHEN_E2H1  BIT(7)  /* RES1 when E2H=1 and not supported */
>
>         unsigned long   flags;
>
> @@ -1297,10 +1299,14 @@ static struct resx compute_resx_bits(struct kvm *kvm,
>                         match &= !e2h0;
>
>                 if (!match) {
> -                       if (map[i].flags & AS_RES1)
> -                               resx.res1 |= reg_feat_map_bits(&map[i]);
> +                       u64 bits = reg_feat_map_bits(&map[i]);
> +
> +                       if ((map[i].flags & AS_RES1)                    ||
> +                           (e2h0 && (map[i].flags & RES1_WHEN_E2H0))   ||
> +                           (!e2h0 && (map[i].flags & RES1_WHEN_E2H1)))
> +                               resx.res1 |= bits;
>                         else
> -                               resx.res0 |= reg_feat_map_bits(&map[i]);
> +                               resx.res0 |= bits;
>                 }
>         }
>
> --
> 2.47.3
>