linux-kernel - Re: [PATCH v4 17/39] unwind_user/sframe: Add support for reading .sframe headers

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Message-ID: <9a726893-4973-40f0-a2b5-63b2036e2e4e@linux.ibm.com>
Date: Wed, 5 Feb 2025 14:56:36 +0100
From: Jens Remus <jremus@...ux.ibm.com>
To: Josh Poimboeuf <jpoimboe@...nel.org>,
        Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc: Indu Bhagat <indu.bhagat@...cle.com>, 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,
        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 17/39] unwind_user/sframe: Add support for reading
 .sframe headers

On 29.01.2025 03:02, Josh Poimboeuf wrote:

> Note FDEs aren't even needed here as the unwinder doesn't need to know
> when a function begins/ends.  The only info needed by the unwinder is
> just the fre_data struct.  So a simple binary search of fres[] is all
> that's really needed.

In SFrame V2 FDEs specify ranges bound by function start address and
length.  FREs in contrast specify open ranges bounded by start address.
Their effect ends either with the next FRE becoming into effect or when
their FDE range ends.
This concept enables holes in the .text section which do not have any
valid FDE/FRE information associated.

Your proposal lacks some sort of mechanism to replicate those holes.
It could be FDEs with a flag (or no offsets?) that specifies their
range has no valid information.

Regards,
Jens
-- 
Jens Remus
Linux on Z Development (D3303)
+49-7031-16-1128 Office
jremus@...ibm.com

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