linux-kernel - Re: [PATCH v4 19/39] unwind_user/sframe: Add support for reading .sframe contents

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Message-ID: <20250204183847.mr2dnge55simbmkl@jpoimboe>
Date: Tue, 4 Feb 2025 10:38:47 -0800
From: Josh Poimboeuf <jpoimboe@...nel.org>
To: Indu Bhagat <indu.bhagat@...cle.com>
Cc: x86@...nel.org, Peter Zijlstra <peterz@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ingo Molnar <mingo@...nel.org>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	linux-kernel@...r.kernel.org, Mark Rutland <mark.rutland@....com>,
	Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
	Jiri Olsa <jolsa@...nel.org>, Namhyung Kim <namhyung@...nel.org>,
	Ian Rogers <irogers@...gle.com>,
	Adrian Hunter <adrian.hunter@...el.com>,
	linux-perf-users@...r.kernel.org, Mark Brown <broonie@...nel.org>,
	linux-toolchains@...r.kernel.org, Jordan Rome <jordalgo@...a.com>,
	Sam James <sam@...too.org>, linux-trace-kernel@...r.kernel.org,
	Andrii Nakryiko <andrii.nakryiko@...il.com>,
	Jens Remus <jremus@...ux.ibm.com>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Florian Weimer <fweimer@...hat.com>,
	Andy Lutomirski <luto@...nel.org>,
	Masami Hiramatsu <mhiramat@...nel.org>,
	Weinan Liu <wnliu@...gle.com>
Subject: Re: [PATCH v4 19/39] unwind_user/sframe: Add support for reading
 .sframe contents

On Thu, Jan 30, 2025 at 07:07:32AM -0800, Indu Bhagat wrote:
> On 1/21/25 6:31 PM, Josh Poimboeuf wrote:
> > +	for (i = 0; i < fde->fres_num; i++) {
> > +		int ret;
> > +
> > +		/*
> > +		 * Alternate between the two fre_addr[] entries for 'fre' and
> > +		 * 'prev_fre'.
> > +		 */
> > +		fre = which ? fres : fres + 1;
> > +		which = !which;
> > +
> > +		ret = __read_fre(sec, fde, fre_addr, fre);
> > +		if (ret)
> > +			return ret;
> > +
> 
> It should be possible to only read the ip_off and info from FRE and defer
> the reading of offsets (as done in __read_fre) until later when you do need
> the offsets. See below.
> 
> We can find the relevant FRE with the following pieces of information:
>   - ip_off
>   - fre_size (this will mean we need to read the uin8_t info in the FRE)

Indeed, I'll skip reading the offsets until after the loop.

-- 
Josh