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Date:   Mon, 31 Oct 2016 09:19:55 +0100
From:   Thomas Graf <tgraf@...g.ch>
To:     Tom Herbert <tom@...bertland.com>
Cc:     "David S. Miller" <davem@...emloft.net>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        roopa <roopa@...ulusnetworks.com>
Subject: Re: [PATCH net-next 3/4] bpf: BPF for lightweight tunnel
 encapsulation

On 10/30/16 at 06:28pm, Tom Herbert wrote:
> On Sun, Oct 30, 2016 at 2:47 PM, Thomas Graf <tgraf@...g.ch> wrote:
> > Instead of building complex logic, we can allow the program to return
> > a code to indicate when to perform another route lookup just as we do
> > for the redirect case. Just because the destination address has
> > changed may not require another lookup in all cases. A typical example
> > would be a program rewriting addresses for the default route to other
> > address which are always handled by the default route as well. An
> > unconditional lookup would hurt performance in many cases.
> 
> Right, that's why we rely on a dst cache. Any use of LWT that
> encapsulates or tunnels to a fixed destination (ILA, VXLAN, IPIP,
> etc.) would want to use the dst cache optimization to avoid the second
> lookup. The ILA LWT code used to call orig output and that worked as
> long as we could set the default router as the gateway "via". It was
> something we were able to deploy, but not a general solution.
> Integrating properly with routing gives a much better solution IMO.
> Note that David Lebrun's latest LWT Segment Routing patch does the
> second lookup with the dst cache to try to avoid it.

Yes, I saw both ILA and SR dst_cache. I was planning on addressing
the conditional reroute in a second step but it looks fairly simple
actually so I'm fine adding this in a v2 based on a return code. I
will limit lwt-bpf to AF_INET && AF_INET6 though.

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