linux-kernel - Re: [PATCH RFC v1 10/20] KVM: x86: Refactor REX prefix handling in instruction emulation

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Message-ID: <cc45ccba-c29a-4eaa-ad62-5a5c7bb2a773@intel.com>
Date: Thu, 13 Nov 2025 15:23:37 -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 10/20] KVM: x86: Refactor REX prefix handling in
 instruction emulation

On 11/11/2025 10:17 AM, Paolo Bonzini wrote:
> 
> commit fc8aa5c45d558393069a1c89b7a64e059b8f9418
> Author: Chang S. Bae <chang.seok.bae@...el.com>
> Date:   Mon Nov 10 18:01:21 2025 +0000
> 
>      KVM: x86: Refactor REX prefix handling in instruction emulation
>      Restructure how to represent and interpret REX fields, preparing
>      for handling of VEX and REX2.
>      REX uses the upper four bits of a single byte as a fixed identifier,
>      and the lower four bits containing the data. VEX and REX2 extend 
> this so
>      that the first byte identifies the prefix and the rest encode 
> additional
>      bits; and while VEX only has the same four data bits as REX, eight 
> zero
>      bits are a valid value for the data bits of REX2.  So, stop storing 
> the
>      REX byte as-is.  Instead, store only the low bits of the REX prefix 
> and
>      track separately whether a REX-like prefix wasused.
>      No functional changes intended.
>      Signed-off-by: Chang S. Bae <chang.seok.bae@...el.com>
>      Message-ID: <20251110180131.28264-11-chang.seok.bae@...el.com>
>      [Extracted from APX series; removed bitfields and REX2-specific
>       definitions. - Paolo]
>      Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>

Sure, I think it's good to have consistent handling like this for these
extended prefixes across the board. Thanks!