Re: [Qemu-devel] Re: [PATCH 2/3] virtio network device

[Anthony Liguori <anthony@codemonkey.ws>]

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.
>
>