linux-kernel - Re: [PATCH RESEND v2 1/3] x86: Use MOVL when reading segment registers

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Message-ID: <9f05741d-584c-48a3-a09a-b8d78b230de4@zytor.com>
Date: Mon, 19 Jan 2026 08:22:33 -0800
From: "H. Peter Anvin" <hpa@...or.com>
To: Uros Bizjak <ubizjak@...il.com>, linux-hyperv@...r.kernel.org,
        x86@...nel.org, linux-kernel@...r.kernel.org
Cc: Michael Kelley <mhklinux@...look.com>,
        Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...nel.org>,
        Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>
Subject: Re: [PATCH RESEND v2 1/3] x86: Use MOVL when reading segment
 registers

On 2026-01-05 01:02, Uros Bizjak wrote:
> Use MOVL when reading segment registers to avoid 0x66 operand-size
> override insn prefix. The segment value is always 16-bit and gets
> zero-extended to the full 32-bit size.
> 
> Signed-off-by: Uros Bizjak <ubizjak@...il.com>
> Tested-by: Michael Kelley <mhklinux@...look.com>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Ingo Molnar <mingo@...nel.org>
> Cc: Borislav Petkov <bp@...en8.de>
> Cc: Dave Hansen <dave.hansen@...ux.intel.com>
> Cc: "H. Peter Anvin" <hpa@...or.com>
> ---
>  arch/x86/include/asm/segment.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/x86/include/asm/segment.h b/arch/x86/include/asm/segment.h
> index f59ae7186940..9f5be2bbd291 100644
> --- a/arch/x86/include/asm/segment.h
> +++ b/arch/x86/include/asm/segment.h
> @@ -348,7 +348,7 @@ static inline void __loadsegment_fs(unsigned short value)
>   * Save a segment register away:
>   */
>  #define savesegment(seg, value)				\
> -	asm("mov %%" #seg ",%0":"=r" (value) : : "memory")
> +	asm("movl %%" #seg ",%k0" : "=r" (value) : : "memory")
>  
>  #endif /* !__ASSEMBLER__ */
>  #endif /* __KERNEL__ */

Looks good to me.

Reviewed-by: H. Peter Anvin (Intel) <hpa@...or.com>