linux-kernel - Re: [PATCH 2/2] kallsyms: build faster by using .incbin

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Date: Wed, 21 Feb 2024 21:29:52 +0100
From: Jann Horn <jannh@...gle.com>
To: Andrew Morton <akpm@...ux-foundation.org>, Masahiro Yamada <masahiroy@...nel.org>
Cc: Nick Desaulniers <ndesaulniers@...gle.com>, Miguel Ojeda <ojeda@...nel.org>, 
	Zhen Lei <thunder.leizhen@...wei.com>, Arnd Bergmann <arnd@...db.de>, linux-kernel@...r.kernel.org, 
	linux-kbuild@...r.kernel.org
Subject: Re: [PATCH 2/2] kallsyms: build faster by using .incbin

On Wed, Feb 21, 2024 at 9:27 PM Jann Horn <jannh@...gle.com> wrote:
> Currently, kallsyms builds a big assembly file (~19M with a normal
> kernel config), and then the assembler has to turn that big assembly
> file back into binary data, which takes around a second per kallsyms
> invocation. (Normally there are two kallsyms invocations per build.)
>
> It is much faster to instead directly output binary data, which can
> be imported in an assembly file using ".incbin". This is also the
> approach taken by arch/x86/boot/compressed/mkpiggy.c.
> So this patch switches kallsyms to that approach.
>
> A complication with this is that the endianness of numbers between
> host and target might not match (for example, when cross-compiling);
> and there seems to be no kconfig symbol that tells us what endianness
> the target has.
> So pass the path to the intermediate vmlinux ELF file to the kallsyms
> tool, and let it parse the ELF header to figure out the target's
> endianness.
>
> I have verified that running kallsyms without these changes and
> kallsyms with these changes on the same input System.map results
> in identical object files.
>
> This change reduces the time for an incremental kernel rebuild
> (touch fs/ioctl.c, then re-run make) from 27.7s to 24.1s (medians
> over 16 runs each) on my machine - saving around 3.6 seconds.

Ah, I found no maintainer for this file in MAINTAINERS, but now that
I'm looking at the git history, it looks like fixes have come in
through Masahiro Yamada's kbuild tree? So I'm not entirely sure
whether the maintainer for this is Masahiro Yamada or akpm.