linux-kernel - Re: [PATCH 8/11] Add a CPU KHZ calibration function to paravirt-ops

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Message-ID: <45C8F92C.9050605@vmware.com>
Date:	Tue, 06 Feb 2007 13:54:52 -0800
From:	Zachary Amsden <zach@...are.com>
To:	Andi Kleen <ak@....de>
CC:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...l.org>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	Chris Wright <chrisw@...s-sol.org>
Subject: Re: [PATCH 8/11] Add a CPU KHZ calibration function to paravirt-ops

Andi Kleen wrote:
> On Mon, Feb 05, 2007 at 07:53:23PM -0800, Zachary Amsden wrote:
>   
>> Provide a paravirtualized way to get the CPU clock frequency; this allows much
>> of the code in tsc.c to be shared between all paravirt implementations.
>>     
>
> Is this really needed? 
>   

Honestly, no.  But it is better and more accurate to do it.

> What worries me somewhat of your patches is that you seem
> to be quick at adding new hooks. But I would like to keep
> paravirtops as minimal as possible with new hooks only
> added when there is a very good justification.
>
> I don't see it here.
>   

We can calibrate CPU frequency pretty accurately, despite being in a VM 
where time does not operate in the normal way.  Many other hypervisors 
won't be able to do such a good job, or might not implement the required 
hardware.

Adding this hook allows everyone to just re-use all the frequency 
scaling / recalibration code in tsc.c.

The primary justification is allowing code re-use, which is why I 
allowed myself to add a hook.  I also want paravirt-ops to be as minimal 
as possible.

Zach
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