linux-kernel - Re: [PATCH v3] kbuild: distributed build support for Clang ThinLTO

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Message-ID: <CAK7LNAS4Ys-ekzjrRdfwKh5tEU=FKe1tE2-orj6LTs7EknZCZg@mail.gmail.com>
Date: Mon, 26 May 2025 22:11:51 +0900
From: Masahiro Yamada <masahiroy@...nel.org>
To: xur@...gle.com
Cc: Nathan Chancellor <nathan@...nel.org>, Eric Naim <dnaim@...hyos.org>, 
	Nicolas Schier <nicolas.schier@...ux.dev>, 
	Nick Desaulniers <nick.desaulniers+lkml@...il.com>, Bill Wendling <morbo@...gle.com>, 
	Justin Stitt <justinstitt@...gle.com>, Miguel Ojeda <ojeda@...nel.org>, 
	Thomas Gleixner <tglx@...utronix.de>, Alice Ryhl <aliceryhl@...gle.com>, 
	Sami Tolvanen <samitolvanen@...gle.com>, "Mike Rapoport (Microsoft)" <rppt@...nel.org>, 
	Rafael Aquini <aquini@...hat.com>, Michael Ellerman <mpe@...erman.id.au>, 
	Stafford Horne <shorne@...il.com>, Christophe Leroy <christophe.leroy@...roup.eu>, 
	Teresa Johnson <tejohnson@...gle.com>, linux-kernel@...r.kernel.org, 
	linux-kbuild@...r.kernel.org, llvm@...ts.linux.dev
Subject: Re: [PATCH v3] kbuild: distributed build support for Clang ThinLTO

On Thu, May 22, 2025 at 6:35 AM <xur@...gle.com> wrote:
>
> From: Rong Xu <xur@...gle.com>
>
> Add distributed ThinLTO build support for the Linux kernel.
> This new mode offers several advantages: (1) Increased
> flexibility in handling user-specified build options.
> (2) Improved user-friendliness for developers. (3) Greater
> convenience for integrating with objtool and livepatch.
>
> Note that "distributed" in this context refers to a term
> that differentiates in-process ThinLTO builds by invoking
> backend compilation through the linker, not necessarily
> building in distributed environments.
>
> Distributed ThinLTO is enabled via the
> `CONFIG_LTO_CLANG_THIN_DIST` Kconfig option. For example:
>  > make LLVM=1 defconfig
>  > scripts/config -e LTO_CLANG_THIN_DIST
>  > make LLVM=1 oldconfig
>  > make LLVM=1 vmlinux -j <..>
>
> The implementation changes the top-level Makefile with a
> macro for generating `vmlinux.o` for distributed ThinLTO
> builds. It uses the existing Kbuild infrastructure to
> perform two recursive passes through the subdirectories.
> The first pass generates LLVM IR object files, similar to
> in-process ThinLTO. Following the thin-link stage, a second
> pass compiles these IR files into the final native object
> files. The build rules and actions for this two-pass process
> are primarily implemented in `scripts/Makefile.build`.
>
> Currently, this patch focuses on building the main kernel
> image (`vmlinux`) only. Support for building kernel modules
> using this method is planned for a subsequent patch.
>
> Tested on the following arch: x86, arm64, loongarch, and
> riscv.
>
> Some implementation details can be found here:
> https://discourse.llvm.org/t/rfc-distributed-thinlto-build-for-kernel/85934
>
> Signed-off-by: Rong Xu <xur@...gle.com>
> ---
> Changelog since v1:
> - Updated the description in arch/Kconfig based on feedback
>   from Nathan Chancellor
> - Revised file suffixes: .final_o -> .o.thinlto.native, and
>   .final_a -> .a.thinlto.native
> - Updated list of ignored files in .gitignore
>
> Changelog since v2:
> - Changed file suffixes: .o.thinlto.native -> .o_thinlto_native,
>   and .a.thinlto.native -> .a_thinlto_native so that basename
>   works as intended.
> - Tested the patch with AutoFDO and Propeller.
> ---
>  .gitignore                        |  3 ++
>  MAINTAINERS                       |  5 +++
>  Makefile                          | 40 ++++++++++++++++++++---
>  arch/Kconfig                      | 19 +++++++++++
>  scripts/Makefile.build            | 52 +++++++++++++++++++++++++++---
>  scripts/Makefile.lib              |  7 +++-
>  scripts/Makefile.vmlinux_o        | 16 +++++++---
>  scripts/Makefile.vmlinux_thinlink | 53 +++++++++++++++++++++++++++++++
>  scripts/head-object-list.txt      |  1 +
>  9 files changed, 181 insertions(+), 15 deletions(-)
>  create mode 100644 scripts/Makefile.vmlinux_thinlink

I re-implemented the Makefiles to avoid
the second recursion and hacky ifdefs.
Attached.

The topic branch is available in

git://git.kernel.org/pub/scm/linux/kernel/git/masahiroy/linux-kbuild.git
thinlto-dist-refactor

I only compile and boot tested on x86 QEMU.


-- 
Best Regards
Masahiro Yamada

View attachment "0001-kbuild-move-vmlinux.a-build-rule-to-scripts-Makefile.patch" of type "text/x-patch" (3639 bytes)

View attachment "0002-kbuild-distributed-build-support-for-Clang-ThinLTO.patch" of type "text/x-patch" (9388 bytes)