On 06/04/2018 03:02 PM, Toke Høiland-Jørgensen wrote: > Daniel Borkmann <dan...@iogearbox.net> writes: >> On 06/02/2018 06:22 AM, Daniel Borkmann wrote: >>> On 05/31/2018 11:44 AM, Toke Høiland-Jørgensen wrote: >>>> Song Liu <liu.song....@gmail.com> writes: >>>>> On Wed, May 30, 2018 at 9:45 AM, Toke Høiland-Jørgensen <t...@toke.dk> >>>>> wrote: >>>>>> This adds an example program showing how to sample packets from XDP using >>>>>> the perf event buffer. The example userspace program just prints the >>>>>> ethernet header for every packet sampled. >>>>>> >>>>>> Most of the userspace code is borrowed from other examples, most notably >>>>>> trace_output. >>>>>> >>>>>> Note that the example only works when everything runs on CPU0; so >>>>>> suitable smp_affinity needs to be set on the device. Some drivers seem >>>>>> to reset smp_affinity when loading an XDP program, so it may be >>>>>> necessary to change it after starting the example userspace program. >>>>> >>>>> Why does this only works when everything runs on CPU0? Is this >>>>> something we can improve? >>>> >>>> Yeah, good question. Basically, the call from XDP to >>>> bpf_perf_event_output() will fail with -EOPNOTSUPP. I tracked this down >>>> to this if statement in __bpf_perf_event_output() in bpf_trace.c: >>>> >>>>> if (unlikely(event->oncpu != cpu)) >>>>> return -EOPNOTSUPP; >>>> >>>> I *think* that the way to fix this is for the userspace program to open >>>> a perf file descriptor for each CPU in the system and poll all of them, >>>> in which case the XDP program can pass the BPF_F_CURRENT_CPU flag to >>>> access the right one. >>> That is correct, you need one perf fd per cpu, and map them accordingly >>> into the map slots when you use BPF_F_CURRENT_CPU. >> >> Given this is a sample that users are likely to copy from, I think it would >> be great if you could fix this up so you can just pass in BPF_F_CURRENT_CPU >> eventually. Thanks for working on this, Toke! > > You're welcome! And yup, I was planning to. I'll need to add a new > function to the trace helpers that can poll more than one fd; just > haven't gotten around to it yet. :)
Ok, great, looking forward! Cheers, Daniel
