Re: [RFC PATCH net-next v2 2/2] virtio_net: Extend virtio to use VF datapath when available

[Jason Wang <jasowang@redhat.com>]

On 2018年01月24日 00:03, Samudrala, Sridhar wrote:
> On 1/23/2018 2:33 AM, Jason Wang wrote:
> > On 2018年01月12日 13:58, Sridhar Samudrala wrote:
> > >   static netdev_tx_t start_xmit(struct sk_buff *skb, struct 
> > > net_device *dev)
> > >   {
> > >       struct virtnet_info *vi = netdev_priv(dev);
> > >       int qnum = skb_get_queue_mapping(skb);
> > >       struct send_queue *sq = &vi->sq[qnum];
> > > +    struct net_device *vf_netdev;
> > >       int err;
> > >       struct netdev_queue *txq = netdev_get_tx_queue(dev, qnum);
> > >       bool kick = !skb->xmit_more;
> > >       bool use_napi = sq->napi.weight;
> > >   +    /* If VF is present and up then redirect packets
> > > +     * called with rcu_read_lock_bh
> > > +     */
> > > +    vf_netdev = rcu_dereference_bh(vi->vf_netdev);
> > > +    if (vf_netdev && netif_running(vf_netdev) &&
> > > +        !netpoll_tx_running(dev) &&
> > > +        is_unicast_ether_addr(eth_hdr(skb)->h_dest))
> > > +        return virtnet_vf_xmit(dev, vf_netdev, skb);
> > > +
> >
> > A question here.
> >
> > If I read the code correctly, all features were validated against 
> > virtio instead VF before transmitting. This assumes VF's feature is a 
> > superset of virtio, does this really work? Do we need to sanitize the 
> > feature before joining? (e.g at last NETIF_R_GSO_ROBUST needs to be 
> > removed).
>
> Actually, virtnet_vf_xmit() calls dev_queue_xmit() after updating 
> skb->dev to vf netdev.
> So the features get validated against VF features and the right tx 
> queue is selected
> before the real transmit.

I see, the the packet will go through qdiscs twice which is suboptimal. 
And the device may lose some ability of VF like tunnel GSO.

Thanks

> Thanks
> Sridhar