linux-kernel - [PATCH v2 1/3] x86: drop mfence in favor of lock+addl

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Date:	Wed, 13 Jan 2016 00:10:14 +0200
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Davidlohr Bueso <dave@...olabs.net>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	the arch/x86 maintainers <x86@...nel.org>,
	Davidlohr Bueso <dbueso@...e.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	virtualization <virtualization@...ts.linux-foundation.org>,
	Ingo Molnar <mingo@...hat.com>, Arnd Bergmann <arnd@...db.de>,
	Andy Lutomirski <luto@...nel.org>,
	Borislav Petkov <bp@...e.de>,
	Andrey Konovalov <andreyknvl@...gle.com>
Subject: [PATCH v2 1/3] x86: drop mfence in favor of lock+addl

mfence appears to be way slower than a locked instruction - let's use
lock+add unconditionally, same as we always did on old 32-bit.

Signed-off-by: Michael S. Tsirkin <mst@...hat.com>
---
 arch/x86/include/asm/barrier.h | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/arch/x86/include/asm/barrier.h b/arch/x86/include/asm/barrier.h
index a584e1c..7f99726 100644
--- a/arch/x86/include/asm/barrier.h
+++ b/arch/x86/include/asm/barrier.h
@@ -15,11 +15,12 @@
  * Some non-Intel clones support out of order store. wmb() ceases to be a
  * nop for these.
  */
-#define mb() alternative("lock; addl $0,0(%%esp)", "mfence", X86_FEATURE_XMM2)
+
+#define mb() asm volatile("lock; addl $0,0(%%esp)" ::: "memory")
 #define rmb() alternative("lock; addl $0,0(%%esp)", "lfence", X86_FEATURE_XMM2)
 #define wmb() alternative("lock; addl $0,0(%%esp)", "sfence", X86_FEATURE_XMM)
 #else
-#define mb() 	asm volatile("mfence":::"memory")
+#define mb() asm volatile("lock; addl $0,0(%%rsp)" ::: "memory")
 #define rmb()	asm volatile("lfence":::"memory")
 #define wmb()	asm volatile("sfence" ::: "memory")
 #endif
-- 
MST