linux-kernel - Re: [PATCH bpf-next] bpf: Add bpf_read_raw

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Date:   Tue, 23 Aug 2022 22:32:23 -0700
From:   John Fastabend <john.fastabend@...il.com>
To:     Namhyung Kim <namhyung@...nel.org>,
        Song Liu <songliubraving@...com>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>, Martin Lau <kafai@...com>,
        Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>,
        Stanislav Fomichev <sdf@...gle.com>,
        Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        "bpf@...r.kernel.org" <bpf@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH bpf-next] bpf: Add bpf_read_raw_record() helper

Namhyung Kim wrote:
> Hi Song,
> 
> On Tue, Aug 23, 2022 at 3:19 PM Song Liu <songliubraving@...com> wrote:
> >
> >
> >
> > > On Aug 23, 2022, at 2:03 PM, Namhyung Kim <namhyung@...nel.org> wrote:
> > >
> > > The helper is for BPF programs attached to perf_event in order to read
> > > event-specific raw data.  I followed the convention of the
> > > bpf_read_branch_records() helper so that it can tell the size of
> > > record using BPF_F_GET_RAW_RECORD flag.
> > >
> > > The use case is to filter perf event samples based on the HW provided
> > > data which have more detailed information about the sample.
> > >
> > > Note that it only reads the first fragment of the raw record.  But it
> > > seems mostly ok since all the existing PMU raw data have only single
> > > fragment and the multi-fragment records are only for BPF output attached
> > > to sockets.  So unless it's used with such an extreme case, it'd work
> > > for most of tracing use cases.
> > >
> > > Signed-off-by: Namhyung Kim <namhyung@...nel.org>
> > > ---
> > > I don't know how to test this.  As the raw data is available on some
> > > hardware PMU only (e.g. AMD IBS).  I tried a tracepoint event but it was
> > > rejected by the verifier.  Actually it needs a bpf_perf_event_data
> > > context so that's not an option IIUC.
> >
> > Can we add a software event that generates raw data for testing?
> 
> Ok, now I think that I can use a bpf-output sw event.  It would need
> another BPF program to write data to the event and the test program
> can read it from BPF using this helper. :)
> 
> Thanks,
> Namhyung

Ah good idea. Feel free to carry my ACK to the v2 with the test.