linux-kernel - Re: [PATCH v6 18/18] Documentation/kbuild: Add DWARF module versioning

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Message-ID: <CABCJKudLD15R1YoR37NcKGZPqdwrwBVLcoCiH1m-HDZYUt+Gfw@mail.gmail.com>
Date: Mon, 16 Dec 2024 16:28:53 -0800
From: Sami Tolvanen <samitolvanen@...gle.com>
To: Masahiro Yamada <masahiroy@...nel.org>
Cc: Luis Chamberlain <mcgrof@...nel.org>, Miguel Ojeda <ojeda@...nel.org>, 
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>, Matthew Maurer <mmaurer@...gle.com>, 
	Alex Gaynor <alex.gaynor@...il.com>, Gary Guo <gary@...yguo.net>, 
	Petr Pavlu <petr.pavlu@...e.com>, Daniel Gomez <da.gomez@...sung.com>, Neal Gompa <neal@...pa.dev>, 
	Hector Martin <marcan@...can.st>, Janne Grunau <j@...nau.net>, Miroslav Benes <mbenes@...e.cz>, 
	Asahi Linux <asahi@...ts.linux.dev>, Sedat Dilek <sedat.dilek@...il.com>, 
	linux-kbuild@...r.kernel.org, linux-kernel@...r.kernel.org, 
	linux-modules@...r.kernel.org, rust-for-linux@...r.kernel.org
Subject: Re: [PATCH v6 18/18] Documentation/kbuild: Add DWARF module versioning

Hi,

On Sat, Dec 14, 2024 at 3:34 AM Masahiro Yamada <masahiroy@...nel.org> wrote:
>
> On Fri, Nov 22, 2024 at 5:43 AM Sami Tolvanen <samitolvanen@...gle.com> wrote:
> >
> > +When a symbol pointer is found in DWARF, gendwarfksyms can use its
> > +type for calculating symbol versions even if the symbol is defined
> > +elsewhere. The name of the symbol pointer is expected to start with
> > +`__gendwarfksyms_ptr_`, followed by the name of the exported symbol.
>
> I am interested in this sentence.
>
> __GENDWARFKSYMS_EXPORT() is primarily introduced to handle
> EXPORT_SYMBOL() in *.S files.
> In fact, .discard.gendwarfksyms is always output for EXPORT_SYMBOL()
> from *.c files.
>
> Can we make it faster by processing only symbol references in the
> .dscard.gendwarfksyms section, and skipping all other sections entirely?

I don't think we can, because the DWARF type information is still all
in the same .debug_info section no matter where we place the actual
symbol references. Also, locating exported symbols from the debugging
information is already pretty quick, the slow part is actually
recursing through the types, which would still be the same amount of
work even if we had separate debugging information only for
.discard.gendwarfksyms.

> > +4.3. Adding structure members
> > +=============================
> > +
> > +Perhaps the most common ABI compatible changeis adding a member to a
>
> changeis -> change is

Thanks, I'll fix this.

Sami