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

Chang S. Bae
2025-11-13T23:23:42Z

On 11/11/2025 10:17 AM, Paolo Bonzini wrote:
>
> commit fc8aa5c45d558393069a1c89b7a64e059b8f9418
> Author: Chang S. Bae <chang.seok.bae@intel.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@intel.com>
>     Message-ID: <20251110180131.28264-11-chang.seok.bae@intel.com>
>     [Extracted from APX series; removed bitfields and REX2-specific
>      definitions. - Paolo]
>     Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

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