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

[Namhyung Kim]

Hi Song,

On Tue, Aug 23, 2022 at 3:19 PM Song Liu <songliubraving@xxxxxx> wrote:
>
>
>
> > On Aug 23, 2022, at 2:03 PM, Namhyung Kim <namhyung@xxxxxxxxxx> 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@xxxxxxxxxx>
> > ---
> > 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