linux-kernel - [PATCH 2/2] x86/asm/entry/32: Remove unnecessary optimization in stub32

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Message-Id: <1433339930-20880-2-git-send-email-dvlasenk@redhat.com>
Date:	Wed,  3 Jun 2015 15:58:50 +0200
From:	Denys Vlasenko <dvlasenk@...hat.com>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Denys Vlasenko <dvlasenk@...hat.com>,
	Josh Triplett <josh@...htriplett.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Borislav Petkov <bp@...en8.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	Andy Lutomirski <luto@...capital.net>,
	Oleg Nesterov <oleg@...hat.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Alexei Starovoitov <ast@...mgrid.com>,
	Will Drewry <wad@...omium.org>,
	Kees Cook <keescook@...omium.org>, x86@...nel.org,
	linux-kernel@...r.kernel.org
Subject: [PATCH 2/2] 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.

Signed-off-by: Denys Vlasenko <dvlasenk@...hat.com>
CC: Josh Triplett <josh@...htriplett.org>
CC: Linus Torvalds <torvalds@...ux-foundation.org>
CC: Steven Rostedt <rostedt@...dmis.org>
CC: Ingo Molnar <mingo@...nel.org>
CC: Borislav Petkov <bp@...en8.de>
CC: "H. Peter Anvin" <hpa@...or.com>
CC: Andy Lutomirski <luto@...capital.net>
CC: Oleg Nesterov <oleg@...hat.com>
CC: Frederic Weisbecker <fweisbec@...il.com>
CC: Alexei Starovoitov <ast@...mgrid.com>
CC: Will Drewry <wad@...omium.org>
CC: Kees Cook <keescook@...omium.org>
CC: x86@...nel.org
CC: linux-kernel@...r.kernel.org
---

This is a resend.

There was a patch by Josh Triplett
"x86: Opt into HAVE_COPY_THREAD_TLS, for both 32-bit and 64-bit"
sent on May 11,
which does the same thing as part of a bigger cleanup.
He was supportive of this patch because of comments.
He will simply have to drop one hunk from his patch.

 arch/x86/ia32/ia32entry.S | 6 ++----
 1 file changed, 2 insertions(+), 4 deletions(-)

diff --git a/arch/x86/ia32/ia32entry.S b/arch/x86/ia32/ia32entry.S
index 8e72256..0c302d0 100644
--- a/arch/x86/ia32/ia32entry.S
+++ b/arch/x86/ia32/ia32entry.S
@@ -567,11 +567,9 @@ GLOBAL(stub32_clone)
 	 * 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:
+	 * We need to swap args here:
 	 */
-	mov	%r8, %rcx
+	xchg	%r8, %rcx
 	jmp	ia32_ptregs_common
 
 	ALIGN
-- 
1.8.1.4

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