linux-kernel - Re: [RFC PATCH 7/8] perf probe powerpc: Use DWARF info only if necessary

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Message-ID: <20141210114844.GE16045@naverao1-tp.in.ibm.com>
Date:	Wed, 10 Dec 2014 17:18:44 +0530
From:	"Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com>
To:	Michael Ellerman <mpe@...erman.id.au>
Cc:	linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org,
	acme@...nel.org, ananth@...ibm.com
Subject: Re: [RFC PATCH 7/8] perf probe powerpc: Use DWARF info only if
 necessary

On 2014/12/10 09:17PM, Michael Ellerman wrote:
> On Tue, 2014-12-09 at 23:04 +0530, Naveen N. Rao wrote:
> > Use symbol table lookups by default if DWARF is not necessary, since
> > powerpc ABIv2 encodes local entry points in the symbol table and the
> > function entry address in DWARF may not be appropriate for kprobes,
> > as described here:
> > https://sourceware.org/bugzilla/show_bug.cgi?id=17638
> 
> Needs a better changelog.

Ok. Will add, but to elaborate quickly: DWARF will only include the 
entire function in low_pc/entry_pc and high_pc. It can't indicate the 
local entry point. Hence, we need to use the symbol table instead of 
DWARF on ABIv2.

> 
> > diff --git a/tools/perf/util/probe-event.c b/tools/perf/util/probe-event.c
> > index 174c22e..adcdbd2 100644
> > --- a/tools/perf/util/probe-event.c
> > +++ b/tools/perf/util/probe-event.c
> > @@ -2382,6 +2382,14 @@ static int convert_to_probe_trace_events(struct perf_probe_event *pev,
> >  		}
> >  	}
> >  
> > +#if defined(__powerpc64__) && defined(_CALL_ELF) && _CALL_ELF == 2
> > +	if (!perf_probe_event_need_dwarf(pev)) {
> > +		ret = find_probe_trace_events_from_map(pev, tevs, max_tevs, target);
> > +		if (ret > 0)
> > +			return ret;	/* Found in symbol table */
> > +	}
> > +#endif
> 
> And should be in an arch helper, not a big powerpc wart dropped in the middle
> of the generic code.

Sure - will change.

Thanks for the review!
- Naveen

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