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Message-ID: <4BFEDDEB.9020700@redhat.com>
Date: Thu, 27 May 2010 11:02:35 -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 10:36 AM, Glauber Costa wrote:
> On Thu, May 27, 2010 at 10:13:12AM -1000, Zachary Amsden wrote:
>
>> 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.
>>
> Which atomic write? This is the wallclock, we do no atomic writes for
> querying it. Not to confuse with system time (the other msr).
>
>
>> Stating a 4 -byte
>> alignment requirement prevents the wall clock from crossing a page
>> boundary.
>>
> Yes, but why require it?
>
> reading the wallclock is not a hot path for anybody, is usually done
> just once, and crossing a page boundary here does not pose any correctness
> issue.
>
Little-endian non-atomic page crossing writes will write the small part
of the wallclock first, so another CPU may observe the following
wallclock sequence:
0x01ff .. 0x0100 .. 0x0200
Big-endian writes also have similar failure:
0x01ff .. 0x02ff .. 0x0200
This won't happen if there is a single instruction write of the wall
clock word.
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