linux-kernel - Re: [PATCH 08/10] KVM: x86: Refactor REX prefix handling in instruction emulation

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Message-ID: <dcc0eb50-bb85-486f-aa21-8a06e4a43251@intel.com>
Date: Mon, 17 Nov 2025 11:56:54 -0800
From: "Chang S. Bae" <chang.seok.bae@...el.com>
To: Paolo Bonzini <pbonzini@...hat.com>, <linux-kernel@...r.kernel.org>,
	<kvm@...r.kernel.org>
CC: <kbusch@...nel.org>
Subject: Re: [PATCH 08/10] KVM: x86: Refactor REX prefix handling in
 instruction emulation

On 11/13/2025 4:36 PM, Paolo Bonzini wrote:
> From: "Chang S. Bae" <chang.seok.bae@...el.com>
> 
> Restructure how to represent and interpret REX fields, preparing
> for handling of both REX2 and VEX.
> 
> 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 extends 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 was used.
> 
> 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 default. - Paolo]
> Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>

Thanks for the incorporation!