linux-kernel - Re: [PATCH] Add Documentation/kvm/msr.txt

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Date:	Thu, 27 May 2010 10:13:12 -1000
From:	Zachary Amsden <zamsden@...hat.com>
To:	Glauber Costa <glommer@...hat.com>
CC:	Avi Kivity <avi@...hat.com>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Add Documentation/kvm/msr.txt

On 05/27/2010 06:02 AM, Glauber Costa wrote:
> On Thu, May 27, 2010 at 11:15:43AM +0300, Avi Kivity wrote:
>    
>> On 05/26/2010 09:04 PM, Glauber Costa wrote:
>>      
>>> This patch adds a file that documents the usage of KVM-specific
>>> MSRs.
>>>
>>>        
>> Looks good.  A few comments:
>>
>>      
>>> +
>>> +Custom MSR list
>>> +--------
>>> +
>>> +The current supported Custom MSR list is:
>>> +
>>> +MSR_KVM_WALL_CLOCK:  0x11
>>> +
>>> +	data: physical address of a memory area.
>>>        
>> Which must be in guest RAM (i.e., don't point it somewhere random
>> and expect the hypervisor to allocate it for you).
>>
>> Must be aligned to 4 bytes (we don't enforce it though).
>>      
> I don't see the reason for it.
>
> If this is a requirement, our own implementation
> is failing to meet it.
>    

It's so the atomic write actually is atomic.  Stating a 4 -byte 
alignment requirement prevents the wall clock from crossing a page boundary.

Zach

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