Re: [PATCH] perf: Ignore format attributes with an unknown perf_event_attr field

[Rob Herring <robh@kernel.org>]

On Tue, Sep 6, 2022 at 1:16 PM Namhyung Kim <namhyung@kernel.org> wrote:
>
> On Fri, Sep 2, 2022 at 8:25 AM Rob Herring <robh@kernel.org> wrote:
> >
> > On Fri, Sep 2, 2022 at 1:53 AM Namhyung Kim <namhyung@kernel.org> wrote:
> > >
> > > Hello,
> > >
> > > On Thu, Sep 1, 2022 at 11:55 AM Rob Herring <robh@kernel.org> wrote:
> > > >
> > > > If the kernel exposes a new perf_event_attr field in a format attr, perf
> > > > will return an error stating the specified PMU can't be found. For
> > > > example, a format attr with 'config3:0-63' causes an error if config3 is
> > > > unknown to perf. This causes a compatibility issue between a newer
> > > > kernel and an older perf tool.
> > > >
> > > > The addition here makes any attr string up to the ':' ignored, but
> > > > still checks the 'bits' portion.
> > > >
> > > > Signed-off-by: Rob Herring <robh@kernel.org>
> > > > ---
> > > > This is the YACC mud I threw and seems to stick. Maybe there's a better
> > > > way to handle this. It doesn't seem like there's a way to do wildcards
> > > > (i.e. config.*) in YACC.
> > > >
> > > > This is needed for this series[1]. Unfortunately the best we do to avoid
> > > > the issue is applying this to stable. I think there's some time before
> > > > v8.7 h/w is deployed, too.
> > >
> > > Maybe you could change the format_term rule to take an identifier instead
> > > of PP_CONFIG* directly and pass it to perf_pmu__new_format().  Then
> > > it could check the string and create an appropriate PERF_PMU_FORMAT_VALUE_*
> > > or ignore it according to the PERF_ATTR_SIZE_VER*.
> >
> > That only moves parsing of configN from YACC to strcmp in C. In doing
> > so, we'd be left with just the 'error' token case which seems a bit
> > odd (if there's another way to do it, I don't know. yacc is not my
> > thing). Is that really better?
>
> I thought we could do more flexible handling and detailed error reporting
> in the C code.  But it could be done in the lex/yacc as well..
>
> I think the general idea is that we want to run a more recent version of
> perf tools than the kernel.  So if it detects the tool is older, it can show
> a warning message like:
>
> "config3 is not in the perf_event_attr.. skipping.
>  Maybe you're running on a newer kernel. Please upgrade the perf tool."

I figured out how to simplify the yacc code and add a warning.
However, one thing to note is that we'll always get the warning if any
PMU has an unsupported format attr because all the PMUs are scanned.
For example, just this gives a warning even though the SPE PMU is not
used:

perf record -e cycles -- true

So the warning might be misleading. On the flip side, new additions are rare.

Rob