netdev - Re: Allow bpf_perf_event_output to access packet data

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Date:   Tue, 11 Sep 2018 09:37:30 +0100
From:   Lorenz Bauer <lmb@...udflare.com>
To:     Jakub Kicinski <jakub.kicinski@...ronome.com>
Cc:     netdev@...r.kernel.org
Subject: Re: Allow bpf_perf_event_output to access packet data

On 10 September 2018 at 09:26, Jakub Kicinski
<jakub.kicinski@...ronome.com> wrote:
> The 0x20ffffffffULL will mean use the index in the map for current CPU
> (0xffffffff), and output 32 bytes of the context (0x20 << 32).  For
> networking programs context means the packet (slightly confusingly).
>
> These are the relevant defines from bpf.h:
>
> /* BPF_FUNC_perf_event_output, BPF_FUNC_perf_event_read and
>  * BPF_FUNC_perf_event_read_value flags.
>  */
> #define BPF_F_INDEX_MASK                0xffffffffULL
> #define BPF_F_CURRENT_CPU               BPF_F_INDEX_MASK
> /* BPF_FUNC_perf_event_output for sk_buff input context. */
> #define BPF_F_CTXLEN_MASK               (0xfffffULL << 32)
>
> Also check out:
>
> bpftool map event_pipe id $ID
>
> For simple way to dump the events in user space.

Thanks for pointing me in the right direction! I managed to find
samples/bpf/xdp_sample_pkts_kern.c as well, which was helpful.

My next gotcha is that perf_event_output seems to ignore the
sample_period parameter passed to perf_event_output. This is not a big
problem since I can just implement the sampling in BPF, but am I
missing something again?


-- 
Lorenz Bauer  |  Systems Engineer
25 Lavington St., London SE1 0NZ

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