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

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Message-ID: <4BD211B9.5080707@redhat.com>
Date:	Fri, 23 Apr 2010 11:31:37 -1000
From:	Zachary Amsden <zamsden@...hat.com>
To:	Avi Kivity <avi@...hat.com>
CC:	Glauber Costa <glommer@...hat.com>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	Peter Zijlstra <peterz@...radead.org>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org, Marcelo Tosatti <mtosatti@...hat.com>
Subject: Re: [PATCH 1/5] Add a global synchronization point for pvclock

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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