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

Avi Kivity
2010-04-24T09:29:13Z

On 04/24/2010 12:31 AM, Zachary Amsden wrote:
> 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.

They don't.

>
> 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.

iret.

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