Messages in this thread | | | Subject | Re: [PATCH net-next 1/2] virtio-net: don't reserve space for vnet header for XDP | From | Jason Wang <> | Date | Wed, 6 May 2020 16:34:36 +0800 |
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On 2020/5/6 下午4:21, Jesper Dangaard Brouer wrote: > On Wed, 6 May 2020 14:16:32 +0800 > Jason Wang <jasowang@redhat.com> 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
> > >> Cc: Jesper Dangaard Brouer <brouer@redhat.com> >> Signed-off-by: Jason Wang <jasowang@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; > >
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