On Tue, 2021-06-22 at 15:51 +0800, Jason Wang wrote:
> > @@ -2388,14 +2406,18 @@ static int tun_xdp_one(struct tun_struct *tun,
> >           skb_reserve(skb, xdp->data - xdp->data_hard_start);
> >           skb_put(skb, xdp->data_end - xdp->data);
> >    
> > -       if (virtio_net_hdr_to_skb(skb, gso, tun_is_little_endian(tun))) {
> > +       if (!gso)
> > +               skb_reset_mac_header(skb);
> > +       else if (virtio_net_hdr_to_skb(skb, gso, tun_is_little_endian(tun))) {
> >                   atomic_long_inc(&tun->rx_frame_errors);
> >                   kfree_skb(skb);
> 
> 
> This should work as well.

Actually there's no need for the skb_reset_mac_header() as that's going
to happen anyway just a few lines down — either from eth_type_trans()
or explicitly in the IFF_TUN code path that I added in my first patch.

I also stripped out a lot more code from vhost_net_build_xdp() in the
!sock_hlen case where it was pointless.

https://git.infradead.org/users/dwmw2/linux.git/shortlog/vhost-net

I'll repost the whole series after I take look at whether I can get it
to work without *either* side doing the vnet header, and add a test
case based on my userspace code in 
https://gitlab.com/openconnect/openconnect/-/blob/vhost/vhost.c