Re: [PATCH bpf-next] bpf: Add bpf_read_raw_record() helper

[John Fastabend <john.fastabend@gmail.com>]

Namhyung Kim wrote:
> Hi Song,
> 
> On Tue, Aug 23, 2022 at 3:19 PM Song Liu <songliubraving@fb.com> wrote:
> >
> >
> >
> > > On Aug 23, 2022, at 2:03 PM, Namhyung Kim <namhyung@kernel.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@kernel.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.