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Date: Tue, 11 Dec 2007 19:40:43 -0600
From: Anthony Liguori <anthony@codemonkey.ws>
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Subject: Re: [Qemu-devel] Re: [PATCH 2/3] virtio network device
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To: Rusty Russell <rusty@rustcorp.com.au>
Cc: Anthony Liguori <aliguori@us.ibm.com>, qemu-devel@nongnu.org, Paul Brook <paul@codesourcery.com>

Rusty Russell wrote:
> On Sunday 09 December 2007 09:02:48 Anthony Liguori wrote:
>   
>> If QEMU ever got true SMP support, then virtio would not work as it
>> requires 16-bit atomic writes which AFAIK is not possible on a number of
>> non-x86 architectures.
>>     
>
> Hmm?  Where is this requirement coming from?
>
> I think everyone should stop using the word "atomic" in virtio discussions; 
> it's confusing.
>   

The virtio ring queue indices are 16-bit and are readable to one end 
while writable on the other end.  To ensure that this can be done in a 
lock-less way, it's necessary to atomically update the index.  Atomic is 
the right word here because if the 16-bit write gets converted into two 
8-bit writes, then very bad things could happen with SMP.

Regards,

Anthony Liguori

> Rusty.
>
>