linux-kernel - Re: [PATCH 09/18] Robust TSC compensation

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Date:	Tue, 13 Jul 2010 13:32:46 -1000
From:	Zachary Amsden <zamsden@...hat.com>
To:	"David S. Ahern" <daahern@...co.com>
CC:	Marcelo Tosatti <mtosatti@...hat.com>, KVM <kvm@...r.kernel.org>,
	Avi Kivity <avi@...hat.com>,
	Glauber Costa <glommer@...hat.com>,
	Linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 09/18] Robust TSC compensation

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?
>    

It depends on the platform.  But it doesn't matter.  The definition we 
use is different start TSCs == out of sync.  Some systems have 
synchronized TSCs, some do not.

See patch 18/18 - "Timekeeping documentation" for details.
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