netdev - Re: [PATCH 2/6] bpf: Add bpf_perf_event_output

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Date:   Wed, 22 Jan 2020 08:51:24 +0100
From:   Jiri Olsa <jolsa@...hat.com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     Jiri Olsa <jolsa@...nel.org>, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>, netdev@...r.kernel.org,
        bpf@...r.kernel.org, Andrii Nakryiko <andriin@...com>,
        Yonghong Song <yhs@...com>, Martin KaFai Lau <kafai@...com>,
        Jakub Kicinski <jakub.kicinski@...ronome.com>,
        David Miller <davem@...hat.com>,
        Björn Töpel <bjorn.topel@...el.com>,
        John Fastabend <john.fastabend@...il.com>
Subject: Re: [PATCH 2/6] bpf: Add bpf_perf_event_output_kfunc

On Tue, Jan 21, 2020 at 04:03:23PM -0800, Alexei Starovoitov wrote:
> On Tue, Jan 21, 2020 at 01:05:08PM +0100, Jiri Olsa wrote:
> > Adding support to use perf_event_output in
> > BPF_TRACE_FENTRY/BPF_TRACE_FEXIT programs.
> > 
> > Using nesting regs array from raw tracepoint helpers.
> > 
> > Signed-off-by: Jiri Olsa <jolsa@...nel.org>
> > ---
> >  kernel/trace/bpf_trace.c | 41 ++++++++++++++++++++++++++++++++++++++++
> >  1 file changed, 41 insertions(+)
> > 
> > diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
> > index 19e793aa441a..6a18e2ae6e30 100644
> > --- a/kernel/trace/bpf_trace.c
> > +++ b/kernel/trace/bpf_trace.c
> > @@ -1172,6 +1172,43 @@ raw_tp_prog_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
> >  	}
> >  }
> >  
> > +BPF_CALL_5(bpf_perf_event_output_kfunc, void *, ctx, struct bpf_map *, map,
> > +	   u64, flags, void *, data, u64, size)
> > +{
> > +	struct pt_regs *regs = get_bpf_raw_tp_regs();
> > +	int ret;
> > +
> > +	if (IS_ERR(regs))
> > +		return PTR_ERR(regs);
> > +
> > +	perf_fetch_caller_regs(regs);
> > +	ret = ____bpf_perf_event_output(regs, map, flags, data, size);
> > +	put_bpf_raw_tp_regs();
> > +	return ret;
> > +}
> 
> I'm not sure why copy paste bpf_perf_event_output_raw_tp() into new function.
> 
> > @@ -1181,6 +1218,10 @@ tracing_prog_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
> >  		return &bpf_skb_output_proto;
> >  #endif
> >  	default:
> > +		if (prog->expected_attach_type == BPF_TRACE_FENTRY ||
> > +		    prog->expected_attach_type == BPF_TRACE_FEXIT)
> > +			return kfunc_prog_func_proto(func_id, prog);
> > +
> >  		return raw_tp_prog_func_proto(func_id, prog);
> 
> Are you saying bpf_perf_event_output_raw_tp() for some reason
> didn't work for fentry/fexit?
> But above is exact copy-paste and it somehow worked?
> 
> Ditto for patches 3,4.

ugh right.. did not realize that after switching to the rawtp
regs nest arrays it's identical and we don't need that

jirka