Subject: Re: [net-next rfc v7 2/3] virtio_net: multiqueue support

From: "Michael S. Tsirkin" <[email-masked]>
Date: Mon, 3 Dec 2012 13:11:18 +0200

On Mon, Dec 03, 2012 at 06:01:58PM +0800, Jason Wang wrote:
> On 12/03/2012 05:47 PM, Michael S. Tsirkin wrote:
> > On Mon, Dec 03, 2012 at 02:05:27PM +0800, Jason Wang wrote:
> >> On Monday, December 03, 2012 12:34:08 PM Rusty Russell wrote:
> >>> Jason Wang <[email-masked]> writes:
> >>>> +static const struct ethtool_ops virtnet_ethtool_ops;
> >>>> +
> >>>> +/*
> >>>> + * Converting between virtqueue no. and kernel tx/rx queue no.
> >>>> + * 0:rx0 1:tx0 2:cvq 3:rx1 4:tx1 ... 2N+1:rxN 2N+2:txN
> >>>> + */
> >>>> +static int vq2txq(struct virtqueue *vq)
> >>>> +{
> >>>> +	int index = virtqueue_get_queue_index(vq);
> >>>> +	return index == 1 ? 0 : (index - 2) / 2;
> >>>> +}
> >>>> +
> >>>> +static int txq2vq(int txq)
> >>>> +{
> >>>> +	return txq ? 2 * txq + 2 : 1;
> >>>> +}
> >>>> +
> >>>> +static int vq2rxq(struct virtqueue *vq)
> >>>> +{
> >>>> +	int index = virtqueue_get_queue_index(vq);
> >>>> +	return index ? (index - 1) / 2 : 0;
> >>>> +}
> >>>> +
> >>>> +static int rxq2vq(int rxq)
> >>>> +{
> >>>> +	return rxq ? 2 * rxq + 1 : 0;
> >>>> +}
> >>>> +
> >>> I thought MST changed the proposed spec to make the control queue always
> >>> the last one, so this logic becomes trivial.
> >> But it may break the support of legacy guest. If we boot a legacy single queue 
> >> guest on a 2 queue virtio-net device. It may think vq 2 is cvq which is indeed 
> >> rx1.
> > Legacy guyest support should be handled by host using feature
> > bits in the usual way: host should detect legacy guest
> > by checking the VIRTIO_NET_F_RFS feature.
> >
> > If VIRTIO_NET_F_RFS is acked, cvq is vq max_virtqueue_pairs * 2.
> > If it's not acked, cvq is vq 2.
> >
> 
> We could, but we didn't gain much from this.

It just seems cleaner and easier to understand.

> Furthermore, we need also
> do the dynamic creation/destroying of virtqueues during feature
> negotiation which seems not supported in qemu now.

It's not *done* in qemu now, but it seems easy: just call
virtio_add_queue for vq2 and on from virtio_net_set_features.
As features can be modified multiple times, we
should add virtio_del_queue and call that beforehand
to get to the known state (two vqs).

-- 
MST