Subject: Re: [PATCH bpf-next] xsk: support AF_PACKET

From: Jesper Dangaard Brouer <[email-masked]>
Date: Fri, 28 May 2021 11:50:03 +0200

On Fri, 28 May 2021 17:02:01 +0800
Xuan Zhuo <[email-masked]> wrote:

> On Fri, 28 May 2021 10:55:58 +0200, Toke Høiland-Jørgensen <[email-masked]> wrote:
> > Xuan Zhuo <[email-masked]> writes:
> >  
> > > In xsk mode, users cannot use AF_PACKET(tcpdump) to observe the current
> > > rx/tx data packets. This feature is very important in many cases. So
> > > this patch allows AF_PACKET to obtain xsk packages.  
> >
> > You can use xdpdump to dump the packets from the XDP program before it
> > gets redirected into the XSK:
> > https://github.com/xdp-project/xdp-tools/tree/master/xdp-dump  
> 
> Wow, this is a good idea.

Yes, it is rather cool (credit to Eelco).  Notice the extra info you
can capture from 'exit', like XDP return codes, if_index, rx_queue.

The tool uses the perf ring-buffer to send/copy data to userspace.
This is actually surprisingly fast, but I still think AF_XDP will be
faster (but it usually 'steals' the packet).

Another (crazy?) idea is to extend this (and xdpdump), is to leverage
Hangbin's recent XDP_REDIRECT extension e624d4ed4aa8 ("xdp: Extend
xdp_redirect_map with broadcast support").  We now have a
xdp_redirect_map flag BPF_F_BROADCAST, what if we create a
BPF_F_CLONE_PASS flag?

The semantic meaning of BPF_F_CLONE_PASS flag is to copy/clone the
packet for the specified map target index (e.g AF_XDP map), but
afterwards it does like veth/cpumap and creates an SKB from the
xdp_frame (see __xdp_build_skb_from_frame()) and send to netstack.
(Feel free to kick me if this doesn't make any sense)

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