Re: [Qemu-devel] [QA-virtio]:Why vring size is limited to 1024?

[Avi Kivity <avi@cloudius-systems.com>]

On 10/08/2014 12:15 PM, Michael S. Tsirkin wrote:
> On Wed, Oct 08, 2014 at 10:43:07AM +0300, Avi Kivity wrote:
>> On 09/30/2014 12:33 PM, Michael S. Tsirkin wrote:
>>> a single descriptor might use all of
>>> the virtqueue. In this case we wont to be able to pass the
>>> descriptor directly to linux as a single iov, since
>>>
>> You could separate maximum request scatter/gather list size from the
>> virtqueue size.  They are totally unrelated - even now you can have a larger
>> request by using indirect descriptors.
> We could add a feature to have a smaller or larger S/G length limit.
> Is this something useful?
>

Having a larger ring size is useful, esp. with zero-copy transmit, and 
you would need the sglist length limit in order to not require 
linearization on linux hosts.  So the limit is not useful in itself, 
only indirectly.

Google cloud engine exposes virtio ring sizes of 16384.

Even more useful is getting rid of the desc array and instead passing 
descs inline in avail and used.