linux-kernel - Re: [PATCH v4 05/39] x86/asm: Avoid emitting DWARF CFI for non-VDSO

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Message-ID: <20250124164752.hmwq6mctc6nf7uni@jpoimboe>
Date: Fri, 24 Jan 2025 08:47:52 -0800
From: Josh Poimboeuf <jpoimboe@...nel.org>
To: Jens Remus <jremus@...ux.ibm.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, Indu Bhagat <indu.bhagat@...cle.com>,
	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>,
	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>, Heiko Carstens <hca@...ux.ibm.com>,
	Vasily Gorbik <gor@...ux.ibm.com>
Subject: Re: [PATCH v4 05/39] x86/asm: Avoid emitting DWARF CFI for non-VDSO

On Fri, Jan 24, 2025 at 05:08:57PM +0100, Jens Remus wrote:
> On 22.01.2025 03:30, Josh Poimboeuf wrote:
> > -#ifndef BUILD_VDSO
> > -	/*
> > -	 * Emit CFI data in .debug_frame sections, not .eh_frame sections.
> > -	 * The latter we currently just discard since we don't do DWARF
> > -	 * unwinding at runtime.  So only the offline DWARF information is
> > -	 * useful to anyone.  Note we should not use this directive if we
> > -	 * ever decide to enable DWARF unwinding at runtime.
> > -	 */
> > -	.cfi_sections .debug_frame
> > -#else
> > -	 /*
> > -	  * For the vDSO, emit both runtime unwind information and debug
> > -	  * symbols for the .dbg file.
> > -	  */
> > -	.cfi_sections .eh_frame, .debug_frame
> > -#endif
> > +#else /* !BUILD_VDSO */
> > +
> 
> Did you remove ".cfi_sections .debug_frame" on purpose from the
> !BUILD_VDSO path compared to V3?

Yes, since non-VDSO assembly files won't be emitting any .cfi, there's
no .debug_frame to output anyway.

> Presumably to not only not emit DWARF CFI from assembler, but any
> source?

This only impacts assembly files, notice the __ASSEMBLY__ check at the
top of the file.

-- 
Josh