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

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Message-ID: <4BD1F43E.2080404@redhat.com>
Date:	Fri, 23 Apr 2010 22:25:50 +0300
From:	Avi Kivity <avi@...hat.com>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
CC:	Zachary Amsden <zamsden@...hat.com>,
	Glauber Costa <glommer@...hat.com>,
	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 04/23/2010 10:22 PM, Jeremy Fitzhardinge wrote:
> On 04/23/2010 02:34 AM, Avi Kivity wrote:
>    
>>> 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.
>>      
> I presume the clobbers are needed for cpuid; if you use [lm]fence then
> they shouldn't be needed, right?
>
>    

Right.  But I'm not sure lfence/mfence are sufficient - from what I 
understand rdtsc can pass right through them.

-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.

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