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

Avi Kivity
2010-04-23T19:26:00Z

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.