Re: [PATCH net-next 0/2] virtio-net: re enable XDP_REDIRECT for mergeable buffer

[Jason Wang]

On 2018å03æ01æ 18:35, Jesper Dangaard Brouer wrote:
> On Thu, 1 Mar 2018 17:23:37 +0800
> Jason Wang <jasowang@xxxxxxxxxx> wrote:
>
> > On 2018å03æ01æ 17:10, Jesper Dangaard Brouer wrote:
> > > On Thu,  1 Mar 2018 11:19:03 +0800
> > > Jason Wang <jasowang@xxxxxxxxxx> wrote:
> > >   
> > > > This series tries to re-enable XDP_REDIRECT for mergeable buffer which
> > > > was removed since commit 7324f5399b06 ("virtio_net: disable
> > > > XDP_REDIRECT in receive_mergeable() case"). Main concerns are:
> > > >
> > > > - not enough tailroom was reserved which breaks cpumap
> > > To address this at a more fundamental level, I would suggest that we/you
> > > instead extend XDP to know it's buffers "frame" size/end.  (The
> > > assumption use to be, xdp_buff->data_hard_start + PAGE_SIZE, but
> > > ixgbe+virtio_net broke that assumption).
> > >
> > > It should actually be fairly easy to implement:
> > >    * Simply extend xdp_buff with a "data_hard_end" pointer.
> > Right, and then cpumap can warn and drop packets with insufficient
> > tailroom.
> >
> > But it should be a patch on top of this I think.
> Hmmm, not really.  If we/you instead fix the issue of XDP doesn't know
> the end/size of the frame, then we don't need this mixed XDP
> generic/native code path mixing.

I know this but I'm still a little bit confused. According to the commit 
log of 7324f5399b06 ("virtio_net: disable XDP_REDIRECT in 
receive_mergeable() case"), you said:

"""
ÂÂÂ The longer explaination is that receive_mergeable() tries to
ÂÂÂ work-around and satisfy these XDP requiresments e.g. by having a
ÂÂÂ function xdp_linearize_page() that allocates and memcpy RX buffers
 around (in case packet is scattered across multiple rx buffers). This
ÂÂÂ does currently satisfy XDP_PASS, XDP_DROP and XDP_TX (but only because
ÂÂÂ we have not implemented bpf_xdp_adjust_tail yet).
"""

So I consider the tailroom is a must for the (future) tail adjustment.

> You could re-enable native redirect, and push the responsibility to
> cpumap for detecting this too-small frame "missing tailroom" (and avoid
> crashing...). (If we really want to support this, cpumap could fallback
> to dev_alloc_skb, and handle it gracefully).

Right but it will be slower than build_skb().

Thanks