Re: [PATCH net-next 1/2] virtio-net: don't reserve space for vnet header for XDP

[Jason Wang]

On 2020/5/6 äå3:53, Michael S. Tsirkin wrote:
> On Wed, May 06, 2020 at 02:16:32PM +0800, Jason Wang wrote:
> > We tried to reserve space for vnet header before
> > xdp.data_hard_start. But this is useless since the packet could be
> > modified by XDP which may invalidate the information stored in the
> > header and there's no way for XDP to know the existence of the vnet
> > header currently.
> What do you mean? Doesn't XDP_PASS use the header in the buffer?


We don't, see 436c9453a1ac0 ("virtio-net: keep vnet header zeroed after 
processing XDP")

If there's other place, it should be a bug.


> > So let's just not reserve space for vnet header in this case.
> In any case, we can find out XDP does head adjustments
> if we need to.


But XDP program can modify the packets without adjusting headers.

Thanks


> > Cc: Jesper Dangaard Brouer <brouer@xxxxxxxxxx>
> > Signed-off-by: Jason Wang <jasowang@xxxxxxxxxx>
> > ---
> >   drivers/net/virtio_net.c | 6 +++---
> >   1 file changed, 3 insertions(+), 3 deletions(-)
> >
> > diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> > index 11f722460513..98dd75b665a5 100644
> > --- a/drivers/net/virtio_net.c
> > +++ b/drivers/net/virtio_net.c
> > @@ -684,8 +684,8 @@ static struct sk_buff *receive_small(struct net_device *dev,
> >   			page = xdp_page;
> >   		}
> >   
> > -		xdp.data_hard_start = buf + VIRTNET_RX_PAD + vi->hdr_len;
> > -		xdp.data = xdp.data_hard_start + xdp_headroom;
> > +		xdp.data_hard_start = buf + VIRTNET_RX_PAD;
> > +		xdp.data = xdp.data_hard_start + xdp_headroom + vi->hdr_len;
> >   		xdp.data_end = xdp.data + len;
> >   		xdp.data_meta = xdp.data;
> >   		xdp.rxq = &rq->xdp_rxq;
> > @@ -845,7 +845,7 @@ static struct sk_buff *receive_mergeable(struct net_device *dev,
> >   		 * the descriptor on if we get an XDP_TX return code.
> >   		 */
> >   		data = page_address(xdp_page) + offset;
> > -		xdp.data_hard_start = data - VIRTIO_XDP_HEADROOM + vi->hdr_len;
> > +		xdp.data_hard_start = data - VIRTIO_XDP_HEADROOM;
> >   		xdp.data = data + vi->hdr_len;
> >   		xdp.data_end = xdp.data + (len - vi->hdr_len);
> >   		xdp.data_meta = xdp.data;
> > --
> > 2.20.1