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

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Message-ID: <CAMzpN2gsMdry_v1BKk=e2x1A2Z3W+caaiFzGm7mkotr4SihrYA@mail.gmail.com>
Date: Wed, 21 Jan 2026 06:49:16 -0500
From: Brian Gerst <brgerst@...il.com>
To: "H. Peter Anvin" <hpa@...or.com>
Cc: linux-kernel@...r.kernel.org, 
	tip-bot2 for Uros Bizjak <tip-bot2@...utronix.de>, linux-tip-commits@...r.kernel.org, 
	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 Wed, Jan 21, 2026 at 2:29 AM H. Peter Anvin <hpa@...or.com> wrote:
>
> 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.

You would have to be really careful to only use short (16-bit)
variables, because it will not zero-extend with a memory operand.