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

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Date:	Mon, 03 Dec 2012 14:05:27 +0800
From:	Jason Wang <jasowang@...hat.com>
To:	Rusty Russell <rusty@...tcorp.com.au>
Cc:	mst@...hat.com, 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 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.
> 
> > +static int virtnet_set_queues(struct virtnet_info *vi)
> > +{
> > +	struct scatterlist sg;
> > +	struct virtio_net_ctrl_rfs s;
> > +	struct net_device *dev = vi->dev;
> > +
> > +	s.virtqueue_pairs = vi->curr_queue_pairs;
> > +	sg_init_one(&sg, &s, sizeof(s));
> > +
> > +	if (!vi->has_cvq)
> > +		return -EINVAL;
> > +
> > +	if (!virtnet_send_command(vi, VIRTIO_NET_CTRL_RFS,
> > +				  VIRTIO_NET_CTRL_RFS_VQ_PAIRS_SET, &sg, 1, 0)){
> > +		dev_warn(&dev->dev, "Fail to set the number of queue pairs to"
> > +			 " %d\n", vi->curr_queue_pairs);
> > +		return -EINVAL;
> > +	}
> 
> Where do we check the VIRTIO_NET_F_RFS bit?

Yes, we need this check. Will let the caller does the check and add a comment 
and check in the caller.
> 
> >  static int virtnet_probe(struct virtio_device *vdev)
> >  {
> > 
> > -	int err;
> > +	int i, err;
> > 
> >  	struct net_device *dev;
> >  	struct virtnet_info *vi;
> > 
> > +	u16 curr_queue_pairs;
> > +
> > +	/* Find if host supports multiqueue virtio_net device */
> > +	err = virtio_config_val(vdev, VIRTIO_NET_F_RFS,
> > +				offsetof(struct virtio_net_config,
> > +				max_virtqueue_pairs), &curr_queue_pairs);
> > +
> > +	/* We need at least 2 queue's */
> > +	if (err)
> > +		curr_queue_pairs = 1;
> 
> Huh?  Just call this queue_pairs.  It's not curr_ at all...
> 
> > +	if (virtio_has_feature(vdev, VIRTIO_NET_F_CTRL_VQ))
> > +		vi->has_cvq = true;
> > +
> > +	/* Use single tx/rx queue pair as default */
> > +	vi->curr_queue_pairs = 1;
> > +	vi->max_queue_pairs = curr_queue_pairs;
> 
> See...

Right, will use max_queue_pairs then.

Thanks
> 
> Cheers,
> Rusty.
> --
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