Re: [PATCH v2 1/3] kbuild: rust: add `CONFIG_RUSTC_CLANG_LLVM_COMPATIBLE`

[Will Deacon <will@kernel.org>]

On Tue, Feb 03, 2026 at 11:34:08AM +0000, Alice Ryhl wrote:
> From: Gary Guo <gary@garyguo.net>
> 
> This config detects if Rust and Clang have matching LLVM major version.
> All IR or bitcode operations (e.g. LTO) rely on LLVM major version to be
> matching, otherwise it may generate errors, or worse, miscompile
> silently due to change of IR semantics.
> 
> It's usually suggested to use the exact same LLVM version, but this can
> be difficult to guarantee. Rust's suggestion [1] is also major-version
> only, so I think this check is sufficient for the kernel.
> 
> Link: https://doc.rust-lang.org/rustc/linker-plugin-lto.html [1]
> Reviewed-by: Andreas Hindborg <a.hindborg@kernel.org>
> Signed-off-by: Gary Guo <gary@garyguo.net>
> Signed-off-by: Matthew Maurer <mmaurer@google.com>
> Signed-off-by: Alice Ryhl <aliceryhl@google.com>
> ---
>  init/Kconfig | 15 +++++++++++++++
>  1 file changed, 15 insertions(+)
> 
> diff --git a/init/Kconfig b/init/Kconfig
> index e95d43457851862afc8313389777e4dd9348c178..0e900d3d8be7874a33e0f44754a8d038e68d7e65 100644
> --- a/init/Kconfig
> +++ b/init/Kconfig
> @@ -82,6 +82,21 @@ config RUSTC_LLVM_VERSION
>  	int
>  	default $(rustc-llvm-version)
>  
> +config RUSTC_LLVM_MAJOR_VERSION
> +	int
> +	default $(shell,expr $(rustc-llvm-version) / 10000)
> +
> +config RUSTC_CLANG_LLVM_COMPATIBLE
> +	bool
> +	default y if CC_IS_CLANG && RUSTC_LLVM_MAJOR_VERSION = $(shell,expr $(cc-version) / 10000)
> +	help
> +	  This indicates whether Rust and Clang use LLVM of the same major
> +	  version.
> +
> +	  Operations involving handling LLVM IR or bitcode (e.g. cross-language
> +	  LTO) requires the same LLVM major version to work properly. For best
> +	  compatibility it is recommended that the exact same LLVM is used.

Is cross-language LTO something we're actually looking at doing for the
kernel, or is this just stale help text given what you're using it for
in this series?

Will