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

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Message-ID: <20260121140857.3544d19e@pumpkin>
Date: Wed, 21 Jan 2026 14:08:57 +0000
From: David Laight <david.laight.linux@...il.com>
To: Brian Gerst <brgerst@...il.com>
Cc: "H. Peter Anvin" <hpa@...or.com>, 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, 21 Jan 2026 06:49:16 -0500
Brian Gerst <brgerst@...il.com> wrote:

> 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.
> 

It would be much safer to have something that returned the value
of the segment register (zero extended to 32 bits).

	David