linux-kernel - [tip:x86/apic] x86/asm/entry/64: Use shorter MOVs from segment registers

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Date:	Sat, 16 May 2015 23:04:05 -0700
From:	tip-bot for Denys Vlasenko <tipbot@...or.com>
To:	linux-tip-commits@...r.kernel.org
Cc:	peterz@...radead.org, mingo@...nel.org, hpa@...or.com,
	brgerst@...il.com, ast@...mgrid.com, luto@...capital.net,
	fweisbec@...il.com, torvalds@...ux-foundation.org, bp@...en8.de,
	linux-kernel@...r.kernel.org, dvlasenk@...hat.com,
	rostedt@...dmis.org, oleg@...hat.com, wad@...omium.org,
	tglx@...utronix.de, keescook@...omium.org
Subject: [tip:x86/apic] x86/asm/entry/64:
  Use shorter MOVs from segment registers

Commit-ID:  adeb5537849d9db428fe0ddc3562e5a765a347e2
Gitweb:     http://git.kernel.org/tip/adeb5537849d9db428fe0ddc3562e5a765a347e2
Author:     Denys Vlasenko <dvlasenk@...hat.com>
AuthorDate: Fri, 15 May 2015 22:39:06 +0200
Committer:  Ingo Molnar <mingo@...nel.org>
CommitDate: Sun, 17 May 2015 07:57:54 +0200

x86/asm/entry/64: Use shorter MOVs from segment registers

The "movw %ds,%cx" instruction needs a 0x66 prefix, while
"movl %ds,%ecx" does not.

The difference is that latter form (on 64-bit CPUs)
overwrites the entire %ecx, not only its lower half.

But subsequent code doesn't depend on the value of upper
half of %ecx, so we can safely use the shorter instruction.

The new code is also faster than the old one - now we don't
depend on the old value of %ecx, but this code fragment is
not performance-critical so it does not matter much.

Signed-off-by: Denys Vlasenko <dvlasenk@...hat.com>
Cc: Alexei Starovoitov <ast@...mgrid.com>
Cc: Andy Lutomirski <luto@...capital.net>
Cc: Borislav Petkov <bp@...en8.de>
Cc: Brian Gerst <brgerst@...il.com>
Cc: Frederic Weisbecker <fweisbec@...il.com>
Cc: H. Peter Anvin <hpa@...or.com>
Cc: Kees Cook <keescook@...omium.org>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Oleg Nesterov <oleg@...hat.com>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Steven Rostedt <rostedt@...dmis.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Will Drewry <wad@...omium.org>
Link: http://lkml.kernel.org/r/1431722346-26585-1-git-send-email-dvlasenk@redhat.com
Signed-off-by: Ingo Molnar <mingo@...nel.org>
---
 arch/x86/kernel/entry_64.S | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/arch/x86/kernel/entry_64.S b/arch/x86/kernel/entry_64.S
index 09c3f9e..47b9581 100644
--- a/arch/x86/kernel/entry_64.S
+++ b/arch/x86/kernel/entry_64.S
@@ -1190,17 +1190,17 @@ ENTRY(xen_failsafe_callback)
 	/*CFI_REL_OFFSET ds,DS*/
 	CFI_REL_OFFSET r11,8
 	CFI_REL_OFFSET rcx,0
-	movw %ds,%cx
+	movl %ds,%ecx
 	cmpw %cx,0x10(%rsp)
 	CFI_REMEMBER_STATE
 	jne 1f
-	movw %es,%cx
+	movl %es,%ecx
 	cmpw %cx,0x18(%rsp)
 	jne 1f
-	movw %fs,%cx
+	movl %fs,%ecx
 	cmpw %cx,0x20(%rsp)
 	jne 1f
-	movw %gs,%cx
+	movl %gs,%ecx
 	cmpw %cx,0x28(%rsp)
 	jne 1f
 	/* All segments match their saved values => Category 2 (Bad IRET). */
--
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