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Date:   Wed, 6 May 2020 05:54:46 -0400
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Jason Wang <jasowang@...hat.com>
Cc:     virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, bpf@...r.kernel.org,
        Jesper Dangaard Brouer <brouer@...hat.com>
Subject: Re: [PATCH net-next 1/2] virtio-net: don't reserve space for vnet
 header for XDP

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 <brouer@...hat.com>
> > > Signed-off-by: Jason Wang <jasowang@...hat.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

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