linux-kernel - Re: [KVM TSC trapping / migration 2/2] Add TSC KHZ MSR

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Message-ID: <20110107104859.GA12056@amt.cnet>
Date:	Fri, 7 Jan 2011 08:48:59 -0200
From:	Marcelo Tosatti <mtosatti@...hat.com>
To:	Zachary Amsden <zamsden@...hat.com>
Cc:	kvm@...r.kernel.org, Avi Kivity <avi@...hat.com>,
	Glauber Costa <glommer@...hat.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [KVM TSC trapping / migration 2/2] Add TSC KHZ MSR

On Thu, Jan 06, 2011 at 12:10:45AM -1000, Zachary Amsden wrote:
> Use an MSR to allow "soft" migration to hosts which do not support
> TSC trapping.  Rather than make this a required element of any
> migration protocol, we allow the TSC rate to be exported as a data
> field (useful in its own right), but we also allow a one time write
> of the MSR during VM creation.  The result is that for the common
> use case, no protocol change is required to communicate TSC rate
> to the receiving host.

Migration to hosts which do not support the feature can be achieved by
saving/restoring the TSC rate + flags in a subsection. A subsection
seems more appropriate than an MSR for this.

> This allows administrative tools to configure migration policy
> as they see appropriate.  Rather than dictate this policy with the
> KVM implementation, we properly allow migration to hosts which both
> do and do not support setting of the TSC rate on the receiving end.
> If it is wished to not support migration to a host which lacks
> support for the TSC rate feature, that can be coordinated externally.
> 
> Signed-off-by: Zachary Amsden <zamsden@...hat.com>
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