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

[Avi Kivity]

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.

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