Re: [PATCH v2] kbuild: distributed build support for Clang ThinLTO

[Eric Naim <dnaim@cachyos.org>]

Hi Rong Xu,

On 5/8/25 04:55, xur@google.com wrote:
> From: Rong Xu <xur@google.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@google.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
> ---
>  .gitignore                        |  2 ++
>  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, 180 insertions(+), 15 deletions(-)
>  create mode 100644 scripts/Makefile.vmlinux_thinlink
> 

I noticed that both Makefile.autofdo and Makefile.propeller add extra linker flags when building with ThinLTO. Did you miss updating that or is the omission there intentional? 

-- 
Regards,
  Eric