On Wed, May 06, 2020 at 04:34:36PM +0800, Jason Wang wrote: > > On 2020/5/6 下午4:21, Jesper Dangaard Brouer wrote: > > On Wed, 6 May 2020 14:16:32 +0800 > > Jason Wang <jasowang@xxxxxxxxxx> 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 > > IMHO above statements are wrong. XDP cannot access memory before > > xdp.data_hard_start. Thus, it is safe to store a vnet headers before > > xdp.data_hard_start. (The sfc driver also use this "before" area). > > > The problem is if we place vnet header before data_hard_start, virtio-net > will fail any header adjustment. > > Or do you mean to copy vnet header before data_hard_start before processing > XDP? > > > > > > > there's no way for XDP to know the existence of the vnet header currently. > > It is true that XDP is unaware of this area, which is the way it > > should be. Currently the area will survive after calling BPF/XDP. > > After your change it will be overwritten in xdp_frame cases. > > > > > > > So let's just not reserve space for vnet header in this case. > > I think this is a wrong approach! > > > > We are working on supporting GRO multi-buffer for XDP. The vnet header > > contains GRO information (see pahole below sign). > > > Another note is that since we need reserve room for skb_shared_info, GRO for > XDP may probably lead more frag list. > > > > It is currently not > > used in the XDP case, but we will be working towards using it. > > > Good to know that, but I think it can only work when the packet is not > modified by XDP? > > > > There > > are a lot of unanswered questions on how this will be implemented. > > Thus, I cannot layout how we are going to leverage this info yet, but > > your patch are killing this info, which IHMO is going in the wrong > > direction. > > > I can copy vnet header ahead of data_hard_start, does it work for you? > > Thanks That's likely to be somewhat expensive. > > > > > > > > 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; > > > >
