[Qemu-devel] [PATCH v6] x86: use lock+addl for smp_mb()

[Michael S. Tsirkin]

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

Results:
perf stat -r 10 -- ./virtio_ring_0_9 --sleep --host-affinity 0 --guest-affinity 0
Before:
       0.922565990 seconds time elapsed                                         
 ( +-  1.15% )
After:
       0.578667024 seconds time elapsed                                         
 ( +-  1.21% )

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.

For userspace, use an address just below the redzone.

The one difference between lock+add and mfence is that lock+addl does
not affect clflush, previous patches converted all uses of clflush to
call mb(), such that changes to smp_mb won't affect it.

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

As a follow-up, it might be worth considering switching users
of clflush to another API (e.g. clflush_mb?) - we will
then be able to convert mb to smp_mb.

Also arguably, gcc should switch to use lock+add for __sync_synchronize.
This might be worth pursuing separately.

Suggested-by: Andy Lutomirski <address@hidden>
Signed-off-by: Michael S. Tsirkin <address@hidden>
---
 arch/x86/include/asm/barrier.h | 12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

Changes from v5:
- ringtest update
- document mb() interaction with clflush
- add micro-benchmark results


diff --git a/arch/x86/include/asm/barrier.h b/arch/x86/include/asm/barrier.h
index bfb28ca..3c6ba1e 100644
--- a/arch/x86/include/asm/barrier.h
+++ b/arch/x86/include/asm/barrier.h
@@ -11,11 +11,11 @@
  */
 
 #ifdef CONFIG_X86_32
-#define mb() asm volatile(ALTERNATIVE("lock; addl $0,0(%%esp)", "mfence", \
+#define mb() asm volatile(ALTERNATIVE("lock; addl $0,-4(%%esp)", "mfence", \
                                      X86_FEATURE_XMM2) ::: "memory", "cc")
-#define rmb() asm volatile(ALTERNATIVE("lock; addl $0,0(%%esp)", "lfence", \
+#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")
@@ -30,7 +30,11 @@
 #endif
 #define dma_wmb()      barrier()
 
-#define __smp_mb()     mb()
+#ifdef CONFIG_X86_32
+#define __smp_mb()     asm volatile("lock; addl $0,-4(%%esp)" ::: "memory", 
"cc")
+#else
+#define __smp_mb()     asm volatile("lock; addl $0,-4(%%rsp)" ::: "memory", 
"cc")
+#endif
 #define __smp_rmb()    dma_rmb()
 #define __smp_wmb()    barrier()
 #define __smp_store_mb(var, value) do { (void)xchg(&var, value); } while (0)
diff --git a/tools/virtio/ringtest/main.h b/tools/virtio/ringtest/main.h
index 90b0133..5706e07 100644
--- a/tools/virtio/ringtest/main.h
+++ b/tools/virtio/ringtest/main.h
@@ -110,11 +110,15 @@ static inline void busy_wait(void)
                barrier();
 } 
 
+#if defined(__x86_64__) || defined(__i386__)
+#define smp_mb()     asm volatile("lock; addl $0,-128(%%rsp)" ::: "memory", 
"cc")
+#else
 /*
  * Not using __ATOMIC_SEQ_CST since gcc docs say they are only synchronized
  * with other __ATOMIC_SEQ_CST calls.
  */
 #define smp_mb() __sync_synchronize()
+#endif
 
 /*
  * This abuses the atomic builtins for thread fences, and
-- 
MST