Song Liu <songliubrav...@fb.com> writes: >> On Oct 2, 2019, at 12:23 PM, Toke Høiland-Jørgensen <t...@redhat.com> wrote: >> >> Song Liu <songliubrav...@fb.com> writes: >> >>>> On Oct 2, 2019, at 6:30 AM, Toke Høiland-Jørgensen <t...@redhat.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.
Yes, thinking some more about this I don't think it's actually a problem. I'll go prototype something...
