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

[Zachary Amsden]

On 04/19/2010 12:54 AM, Avi Kivity wrote:
> On 04/19/2010 01:51 PM, Peter Zijlstra wrote:
> > > > Right, so on x86 we have:
> > > >
> > > > X86_FEATURE_CONSTANT_TSC, which only states that TSC is frequency
> > > > independent, not that it doesn't stop in C states and similar fun 
> > > > stuff.
> > > >
> > > > X86_FEATURE_TSC_RELIABLE, which IIRC should indicate the TSC is 
> > > > constant
> > > > and synced between cores.
> > > Sockets and boards too?  (IOW, how reliable is TSC_RELIABLE)?
> > Not sure, IIRC we clear that when the TSC sync test fails, eg when we
> > mark the tsc clocksource unusable.
>
> Worrying.  By the time we detect this the guest may already have 
> gotten confused by clocks going backwards.

Upstream, we are marking the TSC unstable preemptively when hardware 
which will eventually sync test is detected, so this should be fine.

Zach
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