John Fastabend <john.fastabend@xxxxxxxxx> writes: > Toke Høiland-Jørgensen 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://linuxplumbersconf.org/event/4/contributions/460/ >> > > Can we add RFC to the title if we are just iterating through > idea-space here. I don't view this as "just iterating through idea-space". I'll admit that I may have overestimated the extent to which we were all on the same page on this after LPC, but I do view these submissions as serious proposals that are making progress... :) >> # HIGH-LEVEL IDEA >> >> Since Alexei pointed out some issues with trying to rewrite the eBPF byte code, >> let's try a third approach: We add the ability to chain call programs into the >> eBPF execution core itself, but without rewriting the eBPF byte code. >> >> As in the previous version, the bpf() syscall gets a couple of new commands >> which takes a pair of BPF program fds and a return code. It will then attach the >> second program to the first one in a structured keyed by return code. When a >> program chain is thus established, the former program will tail call to the >> latter at the end of its execution. >> >> The actual tail calling is achieved by adding a new flag to struct bpf_prog and >> having BPF_PROG_RUN run the chain call logic if that flag is set. This means >> that if the feature is *not* used, the overhead is a single conditional branch >> (which means I couldn't measure a performance difference, as can be seen in the >> results below). >> > > I still believe user space should be able to link these multiple > programs together as Ed and I were suggesting in the last series. I expect that userspace probably could (I mean, after all, eBPF is within spitting distance of a full almost-Turing-complete executing environment so userspace can conceivably do pretty much anything). However, I don't believe that doing it in userspace is the best solution. I view it as a tradeoff: How much complexity do we have to add to the kernel to make things easier for userspace. And given that we can do this without negatively impacting performance, and with a reasonable cost in terms of complexity (both of which I think this series demonstrates that yes, we can), I think this makes sense to put in the kernel. > Also by doing it by linking your control program can be arbitrary > complex. For example not just taking the output of one program and > jumping to another but doing arbitrary more complex/interesting > things. Taking the input from multiple programs to pick next call for > example. I expect there will indeed be more complex use cases where combining multiple programs in arbitrary complex ways would make a lot of sense, and doing that by linking in userspace is probably a good fit for that. But for the simple use case of running multiple programs after one another, I think it is overkill, and something that is better to do in the kernel. -Toke
