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SubjectRe: [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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