linux-kernel - Re: [net-next rfc v7 2/3] virtio

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Date:	Mon, 3 Dec 2012 11:47:35 +0200
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Jason Wang <jasowang@...hat.com>
Cc:	Rusty Russell <rusty@...tcorp.com.au>, krkumar2@...ibm.com,
	virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
	bhutchings@...arflare.com, jwhan@...ewood.snu.ac.kr,
	shiyer@...hat.com
Subject: Re: [net-next rfc v7 2/3] virtio_net: multiqueue support

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 <jasowang@...hat.com> 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.

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