linux-kernel - Re: [tip: x86/cleanups] x86/segment: Use MOVL when reading segment registers

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Message-ID: <3A54B6DC-2462-41DA-8C34-D38CCD2A9A2B@zytor.com>
Date: Tue, 20 Jan 2026 04:38:00 -0800
From: "H. Peter Anvin" <hpa@...or.com>
To: linux-kernel@...r.kernel.org,
        tip-bot2 for Uros Bizjak <tip-bot2@...utronix.de>,
        linux-tip-commits@...r.kernel.org
CC: Uros Bizjak <ubizjak@...il.com>, "Borislav Petkov (AMD)" <bp@...en8.de>,
        Michael Kelley <mhklinux@...look.com>, x86@...nel.org
Subject: Re: [tip: x86/cleanups] x86/segment: Use MOVL when reading segment registers

On January 20, 2026 4:08:01 AM PST, tip-bot2 for Uros Bizjak <tip-bot2@...utronix.de> wrote:
>The following commit has been merged into the x86/cleanups branch of tip:
>
>Commit-ID:     53ed3d91a141f5c8b3bce45b0004fbbfefe77956
>Gitweb:        https://git.kernel.org/tip/53ed3d91a141f5c8b3bce45b0004fbbfefe77956
>Author:        Uros Bizjak <ubizjak@...il.com>
>AuthorDate:    Mon, 05 Jan 2026 10:02:32 +01:00
>Committer:     Borislav Petkov (AMD) <bp@...en8.de>
>CommitterDate: Tue, 20 Jan 2026 12:34:58 +01:00
>
>x86/segment: Use MOVL when reading segment registers
>
>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.
>
>Example:
>
>  4e4:       66 8c c0                mov    %es,%ax
>  4e7:       66 89 83 80 0b 00 00    mov    %ax,0xb80(%rbx)
>
>  4e4:       8c c0                   mov    %es,%eax
>  4e6:       66 89 83 80 0b 00 00    mov    %ax,0xb80(%rbx)
>
>Also, use the %k0 modifier which generates the SImode (signed integer)
>register name for the target register.
>
>  [ bp: Extend and clarify commit message. ]
>
>Signed-off-by: Uros Bizjak <ubizjak@...il.com>
>Signed-off-by: Borislav Petkov (AMD) <bp@...en8.de>
>Reviewed-by: H. Peter Anvin (Intel) <hpa@...or.com>
>Tested-by: Michael Kelley <mhklinux@...look.com>
>Link: https://patch.msgid.link/20260105090422.6243-1-ubizjak@gmail.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 f59ae71..9f5be2b 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__ */
>

Incidentally, why aren't we using =rm here? Segment moves support memory operands.