Re: [PATCH] samples/bpf: Add xdp_sample_pkts example

[Daniel Borkmann <daniel@iogearbox.net>]

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.a23@gmail.com> writes:
>>
>>> On Wed, May 30, 2018 at 9:45 AM, Toke Høiland-Jørgensen <toke@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!