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

[Zachary Amsden <zamsden@redhat.com>]

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.