Re: Network Device virtqueue numbering

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, May 18, 2016 at 10:38:13AM +0200, Ladi Prosek wrote:
> Hello virtio-comment,
> 
> This is a comment on section "5.1.2 Network Device: virtqueues" of
> virtio 1.0 spec CS 04, copied below.
> 
> The numbers don't add up. For 1-based N, receiveqN is at index 2N-2,
> transmitqN at index 2N-1 and controlq (if present) at index 2N.
> Specifically for the N=1 special case mentioned in the paragraph,
> controlq would be at index 2 and not 2*1+2 = 4.
> 
> This could be fixed either by using the correct queue indices [2N-2,
> 2N-1, 2N] instead of [2N, 2N+1, 2N+2] or by switching to 0-based
> receiveq/transmitq numbering.
> 
> Thanks!
> Ladi

Yes- that't a mistake.  Numbers go 0 to 2N-1, 2N is the control queue.


> 
> 5.1.2 Virtqueues
> 
> 0
>     receiveq1
> 1
>     transmitq1
> …
> 2N
>     receiveqN
> 2N+1
>     transmitqN
> 2N+2
>     controlq
> 
> N=1 if VIRTIO_NET_F_MQ is not negotiated, otherwise N is set by
> max_virtqueue_pairs.
> 
> controlq only exists if VIRTIO_NET_F_CTRL_VQ set.