linux-kernel - [tip:x86/asm] x86/asm/entry/32: Remove unnecessary optimization in stub32

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Date:	Sun, 7 Jun 2015 01:32:49 -0700
From:	tip-bot for Denys Vlasenko <tipbot@...or.com>
To:	linux-tip-commits@...r.kernel.org
Cc:	linux-kernel@...r.kernel.org, rostedt@...dmis.org,
	wad@...omium.org, akpm@...ux-foundation.org, tglx@...utronix.de,
	brgerst@...il.com, josh@...htriplett.org, hpa@...or.com,
	torvalds@...ux-foundation.org, dvlasenk@...hat.com,
	luto@...capital.net, keescook@...omium.org, mingo@...nel.org,
	fweisbec@...il.com, peterz@...radead.org, ast@...mgrid.com,
	bp@...en8.de, oleg@...hat.com
Subject: [tip:x86/asm] x86/asm/entry/32:
  Remove unnecessary optimization in stub32_clone

Commit-ID:  7a5a9824c18f93415944c997dc6bb8eecfddd2e7
Gitweb:     http://git.kernel.org/tip/7a5a9824c18f93415944c997dc6bb8eecfddd2e7
Author:     Denys Vlasenko <dvlasenk@...hat.com>
AuthorDate: Wed, 3 Jun 2015 15:58:50 +0200
Committer:  Ingo Molnar <mingo@...nel.org>
CommitDate: Fri, 5 Jun 2015 13:41:28 +0200

x86/asm/entry/32: Remove unnecessary optimization in stub32_clone

Really swap arguments #4 and #5 in stub32_clone instead of
"optimizing" it into a move.

Yes, tls_val is currently unused. Yes, on some CPUs XCHG is a
little bit more expensive than MOV. But a cycle or two on an
expensive syscall like clone() is way below noise floor, and
this optimization is simply not worth the obfuscation of logic.

[ There's also ongoing work on the clone() ABI by Josh Triplett
  that will depend on this change later on. ]

Signed-off-by: Denys Vlasenko <dvlasenk@...hat.com>
Cc: Alexei Starovoitov <ast@...mgrid.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>
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: Josh Triplett <josh@...htriplett.org>
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/1433339930-20880-2-git-send-email-dvlasenk@redhat.com
Signed-off-by: Ingo Molnar <mingo@...nel.org>
---
 arch/x86/entry/ia32entry.S | 13 ++++++-------
 1 file changed, 6 insertions(+), 7 deletions(-)

diff --git a/arch/x86/entry/ia32entry.S b/arch/x86/entry/ia32entry.S
index d0c7b28..9558dac 100644
--- a/arch/x86/entry/ia32entry.S
+++ b/arch/x86/entry/ia32entry.S
@@ -529,14 +529,13 @@ GLOBAL(\label)
 GLOBAL(stub32_clone)
 	leaq	sys_clone(%rip), %rax
 	/*
-	 * 32-bit clone API is clone(..., int tls_val, int *child_tidptr).
-	 * 64-bit clone API is clone(..., int *child_tidptr, int tls_val).
-	 * Native 64-bit kernel's sys_clone() implements the latter.
-	 * We need to swap args here. But since tls_val is in fact ignored
-	 * by sys_clone(), we can get away with an assignment
-	 * (arg4 = arg5) instead of a full swap:
+	 * The 32-bit clone ABI is: clone(..., int tls_val, int *child_tidptr).
+	 * The 64-bit clone ABI is: clone(..., int *child_tidptr, int tls_val).
+	 *
+	 * The native 64-bit kernel's sys_clone() implements the latter,
+	 * so we need to swap arguments here before calling it:
 	 */
-	mov	%r8, %rcx
+	xchg	%r8, %rcx
 	jmp	ia32_ptregs_common
 
 	ALIGN
--
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