netdev - Re: [PATCH net-next 0/2] virtio-net: re enable XDP

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Date:   Thu, 1 Mar 2018 11:35:31 +0100
From:   Jesper Dangaard Brouer <brouer@...hat.com>
To:     Jason Wang <jasowang@...hat.com>
Cc:     mst@...hat.com, virtualization@...ts.linux-foundation.org,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        john.fastabend@...il.com, brouer@...hat.com,
        Alexei Starovoitov <alexei.starovoitov@...il.com>
Subject: Re: [PATCH net-next 0/2] virtio-net: re enable XDP_REDIRECT for
 mergeable buffer

On Thu, 1 Mar 2018 17:23:37 +0800
Jason Wang <jasowang@...hat.com> wrote:

> On 2018年03月01日 17:10, Jesper Dangaard Brouer wrote:
> > On Thu,  1 Mar 2018 11:19:03 +0800
> > Jason Wang <jasowang@...hat.com> 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.

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).

-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  LinkedIn: http://www.linkedin.com/in/brouer