Re: [PATCH 09/18] Robust TSC compensation

[Zachary Amsden]

On 07/13/2010 11:42 AM, David S. Ahern wrote:
> On 07/13/10 15:15, Zachary Amsden wrote:
>
>    
> > > What prevents a vcpu from seeing its TSC go backwards, in case the first
> > > write in the 5 second window is smaller than the victim vcpu's last
> > > visible TSC value ?
> > >
> > >        
> > Nothing, unfortunately.  However, the TSC would already have to be out
> > of sync in order for the problem to occur.  It can never happen in
> > normal circumstances on a stable hardware TSC except in one case;
> > migration.  During the CPU state transfer phase of migration, however,
> >      
> What about across processor sockets? Aren't CPUs brought up at different
> points such that their TSCs start at different times?
>    

Yes, that's called an unsynchronized TSC.  In that case, the 
compensation does the best it can based on time since the first TSC 
write, but it will never be exact.
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