linux-kernel - Re: [PATCH v2 2/3] perf/ibs: Fix interface via core pmu events

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Date:   Mon, 13 Mar 2023 15:21:56 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Ravi Bangoria <ravi.bangoria@....com>
Cc:     namhyung@...nel.org, eranian@...gle.com, acme@...nel.org,
        mark.rutland@....com, jolsa@...nel.org, irogers@...gle.com,
        bp@...en8.de, x86@...nel.org, linux-perf-users@...r.kernel.org,
        linux-kernel@...r.kernel.org, sandipan.das@....com,
        ananth.narayan@....com, santosh.shukla@....com
Subject: Re: [PATCH v2 2/3] perf/ibs: Fix interface via core pmu events

On Mon, Mar 13, 2023 at 05:59:46PM +0530, Ravi Bangoria wrote:

> > Now, we already have a gruesome hack in there, and I'm thikning you
> > should use that instead of adding yet another one. Note:
> > 
> > 		if (ret == -ENOENT && event->attr.type != type && !extended_type) {
> > 			type = event->attr.type;
> > 			goto again;
> > 
> > So if you have amd_pmu_hw_config() do:
> > 
> > 	event->attr.type = ibs_pmu.type;
> > 	return -ENOENT;
> > 
> > it should all just work no?
> 
> IBS driver needs to convert RAW pmu config to IBS config, which it does
> based on original event->attr.type. See perf_ibs_precise_event(). This
> logic will fail with event->attr.type overwrite.

amd_pmu_hw_config() could also rewrite event->attr.config I suppose.

I don't think we actually use/expose these event->attr fields again
after all this, do wel?

The closest to that is perf_event_modify_attr(), but that is limited to
TYPE_BREAKPOINT for the time being (also, could this be used to cure
your in-kernel IBS usage woes?).