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

From: Michael S. Tsirkin
Date: Wed May 06 2020 - 05:47:09 EST


On Wed, May 06, 2020 at 04:34:36PM +0800, Jason Wang wrote:
>
> On 2020/5/6 äå4:21, 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;
> >
> >