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Date: Fri, 9 Sep 2022 15:11:20 -0500 From: Rob Herring <robh@...nel.org> To: Namhyung Kim <namhyung@...nel.org> Cc: Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>, Arnaldo Carvalho de Melo <acme@...nel.org>, Mark Rutland <mark.rutland@....com>, Alexander Shishkin <alexander.shishkin@...ux.intel.com>, Jiri Olsa <jolsa@...nel.org>, James Clark <james.clark@....com>, linux-perf-users <linux-perf-users@...r.kernel.org>, linux-kernel <linux-kernel@...r.kernel.org> Subject: Re: [PATCH] perf: Ignore format attributes with an unknown perf_event_attr field On Tue, Sep 6, 2022 at 1:16 PM Namhyung Kim <namhyung@...nel.org> wrote: > > On Fri, Sep 2, 2022 at 8:25 AM Rob Herring <robh@...nel.org> wrote: > > > > On Fri, Sep 2, 2022 at 1:53 AM Namhyung Kim <namhyung@...nel.org> wrote: > > > > > > Hello, > > > > > > On Thu, Sep 1, 2022 at 11:55 AM Rob Herring <robh@...nel.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@...nel.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
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