On Wed, May 06, 2020 at 04:19:40PM +0800, Jason Wang wrote:
>
> On 2020/5/6 下午3:53, 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
Then what's the problem?
>
> >
> >
> > > Cc: Jesper Dangaard Brouer <bro...@redhat.com>
> > > Signed-off-by: Jason Wang <jasow...@redhat.com>
> > > ---
> > > 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
