Re: [PATCH RFC v1 12/20] KVM: x86: Support REX2-extended register index in the decoder

Paolo Bonzini
2025-11-11T16:53:24Z

On 11/10/25 19:01, Chang S. Bae wrote:
> Update register index decoding to account for the additional bit fields
> introduced by the REX2 prefix.
>
> Both ModR/M and opcode register decoding paths now consider the extended
> index bits (R4, X4, B4) in addition to the legacy REX bits (R3, X3, B3).
>
> Signed-off-by: Chang S. Bae <chang.seok.bae@intel.com>

Replying here for both patches 10 and 12, because I think you can merge
them in one, I'd prefer to avoid bitfields.

You only need a single enum:

enum {
REX_B = 1,
REX_X = 2,
REX_R = 4,
REX_W = 8,
REX_M = 0x80,
};

for REX_W/REX_M you access them directly, while for RXB you go through a
function:

static inline rex_get_rxb(u8 rex, u8 fld)
{
BUILD_BUG_ON(!__builtin_constant_p(fld));
BUILD_BUG_ON(fld != REX_B && fld != REX_X && fld != REX_R);

rex >>= ffs(fld) - 1; // bits 0+4
return (rex & 1) + (rex & 0x10 ? 8 : 0);
}

> } else {
> reg = (ctxt->b & 7) |
> - (ctxt->rex.bits.b3 * BIT(3));
> + (ctxt->rex.bits.b3 * BIT(3)) |
> + (ctxt->rex.bits.b4 * BIT(4));

+ rex_get_rxb(ctxt->rex, REX_B);

and likewise everywhere else.

> }
>
> if (ctxt->d & Sse) {
> @@ -1124,9 +1125,12 @@ static int decode_modrm(struct x86_emulate_ctxt *ctxt,
> int rc = X86EMUL_CONTINUE;
> ulong modrm_ea = 0;
>
> - ctxt->modrm_reg = ctxt->rex.bits.r3 * BIT(3);
> - index_reg = ctxt->rex.bits.x3 * BIT(3);
> - base_reg = ctxt->rex.bits.b3 * BIT(3);
> + ctxt->modrm_reg = (ctxt->rex.bits.r3 * BIT(3)) |
> + (ctxt->rex.bits.r4 * BIT(4));
> + index_reg = (ctxt->rex.bits.x3 * BIT(3)) |
> + (ctxt->rex.bits.x4 * BIT(4));
> + base_reg = (ctxt->rex.bits.b3 * BIT(3)) |
> + (ctxt->rex.bits.b4 * BIT(4));
>
> ctxt->modrm_mod = (ctxt->modrm & 0xc0) >> 6;
> ctxt->modrm_reg |= (ctxt->modrm & 0x38) >> 3;