netdev - Re: [PATCH] samples/bpf: Add xdp_sample

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Date:   Sat, 2 Jun 2018 06:22:50 +0200
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Toke Høiland-Jørgensen <toke@...e.dk>,
        Song Liu <liu.song.a23@...il.com>
Cc:     Networking <netdev@...r.kernel.org>
Subject: Re: [PATCH] samples/bpf: Add xdp_sample_pkts example

On 05/31/2018 11:44 AM, Toke Høiland-Jørgensen wrote:
> Song Liu <liu.song.a23@...il.com> writes:
> 
>> On Wed, May 30, 2018 at 9:45 AM, Toke Høiland-Jørgensen <toke@...e.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.