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

On Thu, 1 Mar 2018 21:15:36 +0800
Jason Wang [off-list ref] wrote:

On 2018年03月01日 18:35, Jesper Dangaard Brouer wrote:

quoted

On Thu, 1 Mar 2018 17:23:37 +0800
Jason Wang [off-list ref] wrote:
 

quoted

On 2018年03月01日 17:10, Jesper Dangaard Brouer wrote:  

quoted

On Thu,  1 Mar 2018 11:19:03 +0800
Jason Wang [off-list ref] wrote:
    

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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:

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

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

That is true, BUT implementing the "data_hard_end" extension is a
pre-requisite.  It will also be to catch the issue of too little
tail-room if/when implementing bpf_xdp_adjust_tail().

It is of-cause a "nice-to-have", to fix this virtio_net driver's
receive_mergeable() call to have enough tail-room, but I don't see it
as a solution to the fundamental problem.

quoted

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().

True, but bad argument in this context, as you are already using a
similar function call napi_alloc_skb().  And it will be even slower to
call generic-XDP code path.

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