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 18:54:55 +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 11:38 AM, Song Liu <songliubraving@...com> wrote:
> 
> 
> 
>> 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? I think one issue is how to avoid loop like A->B->C->A, 
> but this should be solvable? 

Also, could you please share a real word example? I saw the example from LPC
slides, but I am more curious about what does each program do in real use cases.

Thanks,
Song