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

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Date:	Mon, 19 Apr 2010 08:35:45 -1000
From:	Zachary Amsden <zamsden@...hat.com>
To:	Avi Kivity <avi@...hat.com>
CC:	Peter Zijlstra <peterz@...radead.org>,
	Glauber Costa <glommer@...hat.com>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	Marcelo Tosatti <mtosatti@...hat.com>
Subject: Re: [PATCH 1/5] Add a global synchronization point for pvclock

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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