[PATCH v2] x86: Use xorl %0,%0 in __get_user_asm

[PATCH v2] x86: Use xorl %0,%0 in __get_user_asm

[Uros Bizjak]

xorl %0,%0 is equivalent to xorq %0,%0 as both will zero the
entire register.  Use xorl %0,%0 for all operand sizes to avoid
REX prefix byte when legacy registers are used and to avoid size
prefix byte when 16bit registers are used.

Zeroing the full register is OK in this use case.  xorl %0,%0 also
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.

Changes since v1:
- Rewrite commit message.

Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: "H. Peter Anvin" <hpa@zytor.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

Re: [PATCH v2] x86: Use xorl %0,%0 in __get_user_asm

[H. Peter Anvin]

On 2020-08-27 11:09, Uros Bizjak wrote:
> xorl %0,%0 is equivalent to xorq %0,%0 as both will zero the
> entire register.  Use xorl %0,%0 for all operand sizes to avoid
> REX prefix byte when legacy registers are used and to avoid size
> prefix byte when 16bit registers are used.
> 
> Zeroing the full register is OK in this use case.  xorl %0,%0 also
> 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.
> 
> Changes since v1:
> - Rewrite commit message.
> 
> Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: Borislav Petkov <bp@alien8.de>
> Cc: "H. Peter Anvin" <hpa@zytor.com>

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

Re: [PATCH v2] x86: Use xorl %0,%0 in __get_user_asm

[Al Viro]

On Thu, Aug 27, 2020 at 08:09:04PM +0200, Uros Bizjak wrote:
> xorl %0,%0 is equivalent to xorq %0,%0 as both will zero the
> entire register.  Use xorl %0,%0 for all operand sizes to avoid
> REX prefix byte when legacy registers are used and to avoid size
> prefix byte when 16bit registers are used.
> 
> Zeroing the full register is OK in this use case.  xorl %0,%0 also
> breaks register dependency chains, avoiding potential partial
> register stalls with 8 and 16bit operands.

No objections, but talking about stalls is more than slightly
ridiculous - we'd just taken a #PF, failed there, flipped
pt_regs %rip to fixup section, returned from fault and are
about to fail whatever syscall that had been; a stall here
is really not an issue...

Re: [PATCH v2] x86: Use xorl %0,%0 in __get_user_asm

[Uros Bizjak]

On Thu, Aug 27, 2020 at 10:14 PM Al Viro <viro@zeniv.linux.org.uk> wrote:
>
> On Thu, Aug 27, 2020 at 08:09:04PM +0200, Uros Bizjak wrote:
> > xorl %0,%0 is equivalent to xorq %0,%0 as both will zero the
> > entire register.  Use xorl %0,%0 for all operand sizes to avoid
> > REX prefix byte when legacy registers are used and to avoid size
> > prefix byte when 16bit registers are used.
> >
> > Zeroing the full register is OK in this use case.  xorl %0,%0 also
> > breaks register dependency chains, avoiding potential partial
> > register stalls with 8 and 16bit operands.
>
> No objections, but talking about stalls is more than slightly
> ridiculous - we'd just taken a #PF, failed there, flipped
> pt_regs %rip to fixup section, returned from fault and are
> about to fail whatever syscall that had been; a stall here
> is really not an issue...

Should I submit a v3 with the offending sentence removed, or could I
just ask a committer to remove it on the fly?

Uros.

[tip: x86/asm] x86/uaccess: Use XORL %0,%0 in __get_user_asm()

[tip-bot2 for Uros Bizjak]

The following commit has been merged into the x86/asm branch of tip:

Commit-ID:     767ec7289e83721fee205a13b459f12fb2cf922f
Gitweb:        https://git.kernel.org/tip/767ec7289e83721fee205a13b459f12fb2cf922f
Author:        Uros Bizjak <ubizjak@gmail.com>
AuthorDate:    Thu, 27 Aug 2020 20:09:04 +02:00
Committer:     Borislav Petkov <bp@suse.de>
CommitterDate: Thu, 03 Sep 2020 22:49:03 +02:00

x86/uaccess: Use XORL %0,%0 in __get_user_asm()

XORL %0,%0 is equivalent to XORQ %0,%0 as both will zero the entire
register. Use XORL %0,%0 for all operand sizes to avoid REX prefix byte
when legacy registers are used and to avoid size prefix byte when 16bit
registers are used.

Zeroing the full register is OK in this use case.

As a result, the size of the .fixup section decreases by 20 bytes.

 [ bp: Massage commit message. ]

Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: H. Peter Anvin (Intel) <hpa@zytor.com>
Link: https://lkml.kernel.org/r/20200827180904.96399-1-ubizjak@gmail.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 ecefaff..2bffba2 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)				\