linux-kernel - [PATCH] x86: Use XORL r32,r32 in __get_user

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Date:   Thu, 27 Aug 2020 18:49:05 +0200
From:   Uros Bizjak <ubizjak@...il.com>
To:     x86@...nel.org, linux-kernel@...r.kernel.org
Cc:     Uros Bizjak <ubizjak@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>
Subject: [PATCH] x86: Use XORL r32,r32 in __get_user_asm

Use XORL r32,r32 for all operand sizes. x86_64 zero extends 32bit
operations, so for 64bit operands, XORL r32,r32 is functionally
equal to XORL r64,r64, but avoids a REX prefix byte when legacy
registers are used.

32bit operation also avoids 0x66 size prefix for 16bit operands
and REX prefix when %sil, %dil and %bpl 8bit registers are used.

As a bonus, 32bit XORL breaks register dependency chains, avoiding
potential partial register stalls with 8 and 16bit operands.

The patch lowers the size of .fixup section by 20 bytes.

Signed-off-by: Uros Bizjak <ubizjak@...il.com>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Ingo Molnar <mingo@...hat.com>
Cc: Borislav Petkov <bp@...en8.de>
Cc: "H. Peter Anvin" <hpa@...or.com>
---
 arch/x86/include/asm/uaccess.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/uaccess.h b/arch/x86/include/asm/uaccess.h
index ecefaffd15d4..2bffba2a1b23 100644
--- a/arch/x86/include/asm/uaccess.h
+++ b/arch/x86/include/asm/uaccess.h
@@ -343,7 +343,7 @@ do {									\
 		     "2:\n"						\
 		     ".section .fixup,\"ax\"\n"				\
 		     "3:	mov %[efault],%[errout]\n"		\
-		     "	xor"itype" %[output],%[output]\n"		\
+		     "	xorl %k[output],%k[output]\n"			\
 		     "	jmp 2b\n"					\
 		     ".previous\n"					\
 		     _ASM_EXTABLE_UA(1b, 3b)				\
-- 
2.26.2