Re: [PATCH v7 3/3] kbuild: distributed build support for Clang ThinLTO

[Nicolas Schier <nsc@kernel.org>]

On Sat, Mar 28, 2026 at 01:19:27AM +0000, 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 build flow proceeds in four stages:
>   1. Perform FE compilation, mirroring the in-process ThinLTO mode.
>   2. Thin-link the generated IR files and object files.
>   3. Find all IR files and perform BE compilation, using the flags
>     stored in the .*.o.cmd files.
>   4. Link the BE results to generate the final vmlinux.o.
> 
> NOTE: This patch currently implements the build for the main kernel
> image (vmlinux) only. Kernel module support is planned for a
> subsequent patch.
> 
> Tested on the following arch: x86, arm64, loongarch, and
> riscv.
> 
> The earlier 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>
> Co-developed-by: Masahiro Yamada <masahiroy@kernel.org>
> Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
> ---
>  .gitignore                 |  2 ++
>  Makefile                   |  9 +++++----
>  arch/Kconfig               | 19 ++++++++++++++++++
>  scripts/Makefile.lib       |  7 +++++++
>  scripts/Makefile.thinlto   | 40 ++++++++++++++++++++++++++++++++++++++
>  scripts/Makefile.vmlinux_a | 37 +++++++++++++++++++++++++++++++++++
>  scripts/mod/modpost.c      | 15 +++++++++++---
>  7 files changed, 122 insertions(+), 7 deletions(-)
>  create mode 100644 scripts/Makefile.thinlto
> 
[...]
> diff --git a/Makefile b/Makefile
> index 69ccf9b8507d..d474b6f0f212 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -1047,11 +1047,11 @@ export CC_FLAGS_SCS
>  endif
>  
>  ifdef CONFIG_LTO_CLANG
> -ifdef CONFIG_LTO_CLANG_THIN
> +ifdef CONFIG_LTO_CLANG_FULL
> +CC_FLAGS_LTO	:= -flto
> +else
>  CC_FLAGS_LTO	:= -flto=thin -fsplit-lto-unit
>  KBUILD_LDFLAGS += $(call ld-option,--lto-whole-program-visibility -mllvm -always-rename-promoted-locals=false)
> -else
> -CC_FLAGS_LTO	:= -flto
>  endif
>  CC_FLAGS_LTO	+= -fvisibility=hidden
>  

This hunk is just reordering but does not change any functionality,
right?

Iff yes, I'd skip this one.


Nathan, Piotr: your tags got dropped from v6 to v7.  Do you want to
renew them?

Kind regards
Nicolas