netdev - Re: [PATCH bpf-next 0/9] xdp: Support multiple programs on a single interface through chain calls

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Date:   Wed, 2 Oct 2019 19:49:07 +0000
From:   Song Liu <songliubraving@...com>
To:     Toke Høiland-Jørgensen <toke@...hat.com>
CC:     Daniel Borkmann <daniel@...earbox.net>,
        Alexei Starovoitov <ast@...nel.org>, Martin Lau <kafai@...com>,
        Yonghong Song <yhs@...com>,
        Marek Majkowski <marek@...udflare.com>,
        Lorenz Bauer <lmb@...udflare.com>,
        David Miller <davem@...emloft.net>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "bpf@...r.kernel.org" <bpf@...r.kernel.org>
Subject: Re: [PATCH bpf-next 0/9] xdp: Support multiple programs on a single
 interface through chain calls



> On Oct 2, 2019, at 12:23 PM, Toke Høiland-Jørgensen <toke@...hat.com> wrote:
> 
> Song Liu <songliubraving@...com> writes:
> 
>>> On Oct 2, 2019, at 6:30 AM, Toke Høiland-Jørgensen <toke@...hat.com> wrote:
>>> 
>>> This series adds support for executing multiple XDP programs on a single
>>> interface in sequence, through the use of chain calls, as discussed at the Linux
>>> Plumbers Conference last month:
>>> 
>>> https://urldefense.proofpoint.com/v2/url?u=https-3A__linuxplumbersconf.org_event_4_contributions_460_&d=DwIDaQ&c=5VD0RTtNlTh3ycd41b3MUw&r=dR8692q0_uaizy0jkrBJQM5k2hfm4CiFxYT8KaysFrg&m=YXqqHTC51zXBviPBEk55y-fQjFQwcXWFlH0IoOqm2KU&s=NF4w3eSPmNhSpJr1-0FLqqlqfgEV8gsCQb9YqWQ9p-k&e= 
>>> 
>>> # HIGH-LEVEL IDEA
>>> 
>>> The basic idea is to express the chain call sequence through a special map type,
>>> which contains a mapping from a (program, return code) tuple to another program
>>> to run in next in the sequence. Userspace can populate this map to express
>>> arbitrary call sequences, and update the sequence by updating or replacing the
>>> map.
>>> 
>>> The actual execution of the program sequence is done in bpf_prog_run_xdp(),
>>> which will lookup the chain sequence map, and if found, will loop through calls
>>> to BPF_PROG_RUN, looking up the next XDP program in the sequence based on the
>>> previous program ID and return code.
>>> 
>>> An XDP chain call map can be installed on an interface by means of a new netlink
>>> attribute containing an fd pointing to a chain call map. This can be supplied
>>> along with the XDP prog fd, so that a chain map is always installed together
>>> with an XDP program.
>> 
>> Interesting work!
>> 
>> Quick question: can we achieve the same by adding a "retval to
>> call_tail_next" map to each program?
> 
> Hmm, that's an interesting idea; I hadn't thought of that. As long as
> that map can be manipulated outside of the program itself, it may work.
> I wonder how complex it gets to modify the call sequence, though; say
> you want to change A->B->C to A->C->B - how do you do that without
> interrupting the sequence while you're modifying things? Or is it OK if
> that is not possible?

We can always load another copy of B and C, say D == B, and E == C. And 
make it A->E->D.