Re: [Qemu-devel] [PATCH] Inter-VM shared memory PCI device

[Avi Kivity <avi@redhat.com>]

On 03/10/2010 06:38 AM, Cam Macdonell wrote:
> On Tue, Mar 9, 2010 at 5:03 PM, Paul Brook<paul@codesourcery.com>  wrote:
>    
>>>> In a cross environment that becomes extremely hairy.  For example the x86
>>>> architecture effectively has an implicit write barrier before every
>>>> store, and an implicit read barrier before every load.
>>>>          
>>> Btw, x86 doesn't have any implicit barriers due to ordinary loads.
>>> Only stores and atomics have implicit barriers, afaik.
>>>        
>> As of March 2009[1] Intel guarantees that memory reads occur in order (they
>> may only be reordered relative to writes). It appears AMD do not provide this
>> guarantee, which could be an interesting problem for heterogeneous migration..
>>
>> Paul
>>
>> [*] The most recent docs I have handy. Up to and including Core-2 Duo.
>>
>>      
> Interesting, but what ordering would cause problems that AMD would do
> but Intel wouldn't?  Wouldn't that ordering cause the same problems
> for POSIX shared memory in general (regardless of Qemu) on AMD?
>    

If some code was written for the Intel guarantees it would break if 
migrated to AMD.  Of course, it would also break if run on AMD in the 
first place.

> I think shared memory breaks migration anyway.
>    

Until someone implements distributed shared memory.

-- 
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