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

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Message-ID: <2216d4f6-cdb3-4403-9685-5622830a0177@intel.com>
Date: Thu, 13 Nov 2025 15:26:06 -0800
From: "Chang S. Bae" <chang.seok.bae@...el.com>
To: Paolo Bonzini <pbonzini@...hat.com>, <kvm@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>
CC: <seanjc@...gle.com>, <chao.gao@...el.com>, <zhao1.liu@...el.com>
Subject: Re: [PATCH RFC v1 12/20] KVM: x86: Support REX2-extended register
 index in the decoder

On 11/11/2025 8:53 AM, Paolo Bonzini wrote:
> 
> Replying here for both patches 10 and 12, because I think you can merge 
> them in one, 

Done

> 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.

Neat! This looks much better. Thanks for the suggestion.