[PATCH v3 4/4] x86: drop mfence in favor of lock+addl

Michael S. Tsirkin
2016-01-13T20:12:53Z

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

Just poking at SP would be the most natural, but if we
then read the value from SP, we get a false dependency
which will slow us down.

This was noted in this article:
http://shipilev.net/blog/2014/on-the-fence-with-dependencies/

And is easy to reproduce by sticking a barrier in a small non-inline
function.

So let's use a negative offset - which avoids this problem since we
build with the red zone disabled.

Update rmb/wmb on 32 bit to use the negative offset, too, for
consistency.

Suggested-by: Andy Lutomirski <luto@amacapital.net>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
---
arch/x86/include/asm/barrier.h | 13 ++++++-------
1 file changed, 6 insertions(+), 7 deletions(-)

diff --git a/arch/x86/include/asm/barrier.h b/arch/x86/include/asm/barrier.h
index bfb28ca..9a2d257 100644
--- a/arch/x86/include/asm/barrier.h
+++ b/arch/x86/include/asm/barrier.h
@@ -11,16 +11,15 @@
*/

#ifdef CONFIG_X86_32
-#define mb() asm volatile(ALTERNATIVE("lock; addl $0,0(%%esp)", "mfence", \
- X86_FEATURE_XMM2) ::: "memory", "cc")
-#define rmb() asm volatile(ALTERNATIVE("lock; addl $0,0(%%esp)", "lfence", \
+#define mb() asm volatile("lock; addl $0,-4(%%esp)" ::: "memory", "cc")
+#define rmb() asm volatile(ALTERNATIVE("lock; addl $0,-4(%%esp)", "lfence", \
X86_FEATURE_XMM2) ::: "memory", "cc")
-#define wmb() asm volatile(ALTERNATIVE("lock; addl $0,0(%%esp)", "sfence", \
+#define wmb() asm volatile(ALTERNATIVE("lock; addl $0,-4(%%esp)", "sfence", \
X86_FEATURE_XMM2) ::: "memory", "cc")
#else
-#define mb() asm volatile("mfence":::"memory")
-#define rmb() asm volatile("lfence":::"memory")
-#define wmb() asm volatile("sfence" ::: "memory")
+#define mb() asm volatile("lock; addl $0,-4(%%rsp)" ::: "memory", "cc")
+#define rmb() asm volatile("lfence" ::: "memory")
+#define wmb() asm volatile("sfence" ::: "memory")
#endif

#ifdef CONFIG_X86_PPRO_FENCE
--
MST