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

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Message-ID: <4C3CDDD4.6010009@cisco.com>
Date:	Tue, 13 Jul 2010 15:42:44 -0600
From:	"David S. Ahern" <daahern@...co.com>
To:	Zachary Amsden <zamsden@...hat.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 07/13/10 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?

David

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