netdev - XDP user interface conclusions

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Date:   Thu, 15 Sep 2016 20:13:47 +0200
From:   Jesper Dangaard Brouer <brouer@...hat.com>
To:     Brenden Blanco <bblanco@...mgrid.com>
Cc:     brouer@...hat.com,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Tom Herbert <tom@...bertland.com>,
        Daniel Borkmann <borkmann@...earbox.net>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "iovisor-dev@...ts.iovisor.org" <iovisor-dev@...ts.iovisor.org>
Subject: XDP user interface conclusions

Hi Brenden,

I don't quite understand the semantics of the XDP userspace interface.

We allow XDP programs to be (unconditionally) exchanged by another
program, this avoids taking the link down+up and avoids reallocating
RX ring resources (which is great).

We have two XDP samples programs in samples/bpf/ xdp1 and xdp2.  Now I
want to first load xdp1 and then to avoid the linkdown I load xdp2,
and then afterwards remove/stop program xdp1.

This does NOT work, because (in samples/bpf/xdp1_user.c) when xdp1
exits it unconditionally removes the running XDP program (loaded by xdp2)
via set_link_xdp_fd(ifindex, -1).  The xdp2 user program is still
running, and is unaware of its xdp/bpf program have been unloaded.

I find this userspace interface confusing. What this your intention?
Perhaps you can explain what the intended semantics or specification is?

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