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

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Message-Id: <C35A7DC7-EBFD-464F-975E-F89886D299A8@suse.de>
Date:	Thu, 6 Jan 2011 12:40:16 +0100
From:	Alexander Graf <agraf@...e.de>
To:	Zachary Amsden <zamsden@...hat.com>
Cc:	"kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [KVM TSC trapping / migration 2/2] Add TSC KHZ MSR


On 06.01.2011, at 12:27, Zachary Amsden wrote:

> On 01/06/2011 12:34 AM, Alexander Graf wrote:
>> Am 06.01.2011 um 11:10 schrieb Zachary Amsden<zamsden@...hat.com>:
>> 
>>   
>>> 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.
>>> 
>>> 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.
>>>     
>> Isn't there a real hw equivalent of such a register? It might make more sense to just implement that then.
>> 
>>   
> 
> Unfortunately, no.

Bleks. I couldn't find anything in AMD documentation either. Intel documentation is usually hard to find and incomplete anyways, so maybe something's hiding there - but if it's hidden so well it's no use to implement either.


Alex

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