Re: [PATCH v1 2/7] bpf: Add bpf_perf_event_aux_pause kfunc

[Leo Yan <leo.yan@arm.com>]

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?

Thanks,
Leo