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

Peter Zijlstra
2010-04-19T10:49:20Z

On Mon, 2010-04-19 at 12:46 +0200, Peter Zijlstra wrote:
> On Sat, 2010-04-17 at 21:48 +0300, Avi Kivity wrote:
> > > After this patch is applied, I don't see a single warp in time during 5 days
> > > of execution, in any of the machines I saw them before.
> > >
> > >
> >
> > Please define a cpuid bit that makes this optional. When we eventually
> > enable it in the future, it will allow a wider range of guests to enjoy it.
>
> 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.

Fun, we also have:

X86_FEATURE_NONSTOP_TSC, which states the thing doesn't stop in C
states.