Re: [PATCH 1/5] Add a global synchronization point for pvclock

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On 04/22/2010 11:34 PM, Avi Kivity wrote:
On 04/23/2010 04:44 AM, Zachary Amsden wrote:
   Or apply this patch.
time-warp.patch


diff -rup a/time-warp-test.c b/time-warp-test.c
--- a/time-warp-test.c    2010-04-15 16:30:13.955981607 -1000
+++ b/time-warp-test.c    2010-04-15 16:35:37.777982377 -1000
@@ -91,7 +91,7 @@ static inline unsigned long long __rdtsc
  {
      DECLARE_ARGS(val, low, high);

-    asm volatile("cpuid; rdtsc" : EAX_EDX_RET(val, low, high));
+ asm volatile("cpuid; rdtsc" : EAX_EDX_RET(val, low, high) :: "ebx", "ecx");


Plus, replace cpuid by lfence/mfence.  cpuid will trap.


Does lfence / mfence actually serialize? I thought there was some great confusion about that not being the case on all AMD processors, and possibly not at all on Intel.

A trap, however is a great way to serialize.

I think, there is no serializing instruction which can be used from userspace which does not trap, at least, I don't know one off the top of my head.

Zach
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