[PATCH 10/20] KVM: arm64: Simplify FIXED_VALUE handling

[Fuad Tabba]

On Mon, 26 Jan 2026 at 12:17, Marc Zyngier <maz at kernel.org> wrote:
>
> The FIXED_VALUE qualifier (mostly used for HCR_EL2) is pointlessly
> complicated, as it tries to piggy-back on the previous RES0 handling
> while being done in a different phase, on different data.
>
> Instead, make it an integral part of the RESx computation, and allow
> it to directly set RESx bits. This is much easier to understand.
>
> It also paves the way for some additional changes to that will allow
> the full removal of the FIXED_VALUE handling.
>
> Signed-off-by: Marc Zyngier <maz at kernel.org>

The new code preserves the logic, and is easier to understand.

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

Cheers,
/fuad


> ---
>  arch/arm64/kvm/config.c | 67 ++++++++++++++---------------------------
>  1 file changed, 22 insertions(+), 45 deletions(-)
>
> diff --git a/arch/arm64/kvm/config.c b/arch/arm64/kvm/config.c
> index 39487182057a3..4fac04d3132c0 100644
> --- a/arch/arm64/kvm/config.c
> +++ b/arch/arm64/kvm/config.c
> @@ -37,7 +37,7 @@ struct reg_bits_to_feat_map {
>                         s8      lo_lim;
>                 };
>                 bool    (*match)(struct kvm *);
> -               bool    (*fval)(struct kvm *, u64 *);
> +               bool    (*fval)(struct kvm *, struct resx *);
>         };
>  };
>
> @@ -389,14 +389,12 @@ static bool feat_vmid16(struct kvm *kvm)
>         return kvm_has_feat_enum(kvm, ID_AA64MMFR1_EL1, VMIDBits, 16);
>  }
>
> -static bool compute_hcr_e2h(struct kvm *kvm, u64 *bits)
> +static bool compute_hcr_e2h(struct kvm *kvm, struct resx *bits)
>  {
> -       if (bits) {
> -               if (kvm_has_feat(kvm, FEAT_E2H0))
> -                       *bits &= ~HCR_EL2_E2H;
> -               else
> -                       *bits |= HCR_EL2_E2H;
> -       }
> +       if (kvm_has_feat(kvm, FEAT_E2H0))
> +               bits->res0 |= HCR_EL2_E2H;
> +       else
> +               bits->res1 |= HCR_EL2_E2H;
>
>         return true;
>  }
> @@ -1281,12 +1279,11 @@ static bool idreg_feat_match(struct kvm *kvm, const struct reg_bits_to_feat_map
>  }
>
>  static
> -struct resx __compute_fixed_bits(struct kvm *kvm,
> -                               const struct reg_bits_to_feat_map *map,
> -                               int map_size,
> -                               u64 *fixed_bits,
> -                               unsigned long require,
> -                               unsigned long exclude)
> +struct resx compute_resx_bits(struct kvm *kvm,
> +                             const struct reg_bits_to_feat_map *map,
> +                             int map_size,
> +                             unsigned long require,
> +                             unsigned long exclude)
>  {
>         struct resx resx = {};
>
> @@ -1299,14 +1296,18 @@ struct resx __compute_fixed_bits(struct kvm *kvm,
>                 if (map[i].flags & exclude)
>                         continue;
>
> -               if (map[i].flags & CALL_FUNC)
> -                       match = (map[i].flags & FIXED_VALUE) ?
> -                               map[i].fval(kvm, fixed_bits) :
> -                               map[i].match(kvm);
> -               else
> +               switch (map[i].flags & (CALL_FUNC | FIXED_VALUE)) {
> +               case CALL_FUNC | FIXED_VALUE:
> +                       map[i].fval(kvm, &resx);
> +                       continue;
> +               case CALL_FUNC:
> +                       match = map[i].match(kvm);
> +                       break;
> +               default:
>                         match = idreg_feat_match(kvm, &map[i]);
> +               }
>
> -               if (!match || (map[i].flags & FIXED_VALUE)) {
> +               if (!match) {
>                         if (map[i].flags & AS_RES1)
>                                 resx.res1 |= reg_feat_map_bits(&map[i]);
>                         else
> @@ -1317,17 +1318,6 @@ struct resx __compute_fixed_bits(struct kvm *kvm,
>         return resx;
>  }
>
> -static
> -struct resx compute_resx_bits(struct kvm *kvm,
> -                            const struct reg_bits_to_feat_map *map,
> -                            int map_size,
> -                            unsigned long require,
> -                            unsigned long exclude)
> -{
> -       return __compute_fixed_bits(kvm, map, map_size, NULL,
> -                                   require, exclude | FIXED_VALUE);
> -}
> -
>  static
>  struct resx compute_reg_resx_bits(struct kvm *kvm,
>                                  const struct reg_feat_map_desc *r,
> @@ -1368,16 +1358,6 @@ static u64 compute_fgu_bits(struct kvm *kvm, const struct reg_feat_map_desc *r)
>         return resx.res0 | resx.res1;
>  }
>
> -static
> -struct resx compute_reg_fixed_bits(struct kvm *kvm,
> -                                 const struct reg_feat_map_desc *r,
> -                                 u64 *fixed_bits, unsigned long require,
> -                                 unsigned long exclude)
> -{
> -       return __compute_fixed_bits(kvm, r->bit_feat_map, r->bit_feat_map_sz,
> -                                   fixed_bits, require | FIXED_VALUE, exclude);
> -}
> -
>  void compute_fgu(struct kvm *kvm, enum fgt_group_id fgt)
>  {
>         u64 val = 0;
> @@ -1417,7 +1397,6 @@ void compute_fgu(struct kvm *kvm, enum fgt_group_id fgt)
>
>  struct resx get_reg_fixed_bits(struct kvm *kvm, enum vcpu_sysreg reg)
>  {
> -       u64 fixed = 0, mask;
>         struct resx resx;
>
>         switch (reg) {
> @@ -1459,10 +1438,8 @@ struct resx get_reg_fixed_bits(struct kvm *kvm, enum vcpu_sysreg reg)
>                 resx.res1 |= __HCRX_EL2_RES1;
>                 break;
>         case HCR_EL2:
> -               mask = compute_reg_fixed_bits(kvm, &hcr_desc, &fixed, 0, 0).res0;
>                 resx = compute_reg_resx_bits(kvm, &hcr_desc, 0, 0);
> -               resx.res0 |= (mask & ~fixed);
> -               resx.res1 |= HCR_EL2_RES1 | (mask & fixed);
> +               resx.res1 |= HCR_EL2_RES1;
>                 break;
>         case SCTLR2_EL1:
>         case SCTLR2_EL2:
> --
> 2.47.3
>