linux-kernel - Re: [PATCH v1 2/7] bpf: Add bpf_perf_event_aux

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Message-ID: <ba5c04f4-a33d-4d7f-9272-eee4a4389def@linux.dev>
Date: Tue, 15 Jul 2025 10:12:02 -0700
From: Yonghong Song <yonghong.song@...ux.dev>
To: Leo Yan <leo.yan@....com>
Cc: Arnaldo Carvalho de Melo <acme@...nel.org>,
 Adrian Hunter <adrian.hunter@...el.com>, Namhyung Kim <namhyung@...nel.org>,
 Jiri Olsa <jolsa@...nel.org>, Ian Rogers <irogers@...gle.com>,
 James Clark <james.clark@...aro.org>, "Liang, Kan"
 <kan.liang@...ux.intel.com>, Alexei Starovoitov <ast@...nel.org>,
 Daniel Borkmann <daniel@...earbox.net>, Andrii Nakryiko <andrii@...nel.org>,
 Martin KaFai Lau <martin.lau@...ux.dev>, Eduard Zingerman
 <eddyz87@...il.com>, Song Liu <song@...nel.org>,
 John Fastabend <john.fastabend@...il.com>, KP Singh <kpsingh@...nel.org>,
 Stanislav Fomichev <sdf@...ichev.me>, Hao Luo <haoluo@...gle.com>,
 Matt Bobrowski <mattbobrowski@...gle.com>,
 Steven Rostedt <rostedt@...dmis.org>, Masami Hiramatsu
 <mhiramat@...nel.org>, Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
 linux-kernel@...r.kernel.org, linux-perf-users@...r.kernel.org,
 bpf@...r.kernel.org, linux-trace-kernel@...r.kernel.org,
 Suzuki K Poulose <suzuki.poulose@....com>, Mike Leach <mike.leach@...aro.org>
Subject: Re: [PATCH v1 2/7] bpf: Add bpf_perf_event_aux_pause kfunc



On 7/14/25 10:45 AM, Leo Yan wrote:
> Hi Yonghong,
>
> Really sorry for the long delay. Now I am restarting this work.
>
> On Mon, Dec 16, 2024 at 09:21:15AM -0800, Yonghong Song wrote:
>> On 12/15/24 11:34 AM, Leo Yan wrote:
>>> The bpf_perf_event_aux_pause kfunc will be used to control the Perf AUX
>>> area to pause or resume.
>>>
>>> An example use-case is attaching eBPF to Ftrace tracepoints.  When a
>>> tracepoint is hit, the associated eBPF program will be executed.  The
>>> eBPF program can invoke bpf_perf_event_aux_pause() to pause or resume
>>> AUX trace.  This is useful for fine-grained tracing by combining
>>> Perf and eBPF.
>>>
>>> This commit implements the bpf_perf_event_aux_pause kfunc, and make it
>>> pass the eBPF verifier.
>> The subject and commit message mentions to implement a kfunc,
>> but actually you implemented a uapi helper. Please implement a kfunc
>> instead (searching __bpf_kfunc in kernel/bpf directory).
> After some research, my understanding is that kfunc is flexible for
> exposing APIs via BTF, whereas BPF_CALL is typically used for core BPF
> features - such as accessing BPF maps.
>
> Coming back to this patch: it exposes a function with the following
> definition:
>
>    int bpf_perf_event_aux_pause(struct bpf_map *map, u64 flags, u32 pause);
>
> I'm not certain whether using __bpf_kfunc is appropriate here, or if I
> should stick to BPF_CALL to ensure support for accessing bpf_map
> pointers?

Using helpers (BPF_CALL) is not an option as the whole bpf ecosystem
moves to kfunc mechanism. You can certainly use kfunc with 'struct bpf_map *'
as the argument. For example the following kfunc:
   __bpf_kfunc s64 bpf_map_sum_elem_count(const struct bpf_map *map)
in kernel/bpf/map_iter.c
   

>
> Thanks,
> Leo