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

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Date:	Mon, 19 Apr 2010 12:51:13 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Avi Kivity <avi@...hat.com>
Cc:	Glauber Costa <glommer@...hat.com>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	Zachary Amsden <zamsden@...hat.com>
Subject: Re: [PATCH 1/5] Add a global synchronization point for pvclock

On Mon, 2010-04-19 at 13:49 +0300, Avi Kivity wrote:
> On 04/19/2010 01:46 PM, 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.
> >
> >    
> 
> 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.

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