linux-kernel - Re: [PATCH v2 1/3] kbuild: rust: add `CONFIG_RUSTC_CLANG_LLVM

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Message-ID: <aYHgtJ_xLeo5W-lZ@willie-the-truck>
Date: Tue, 3 Feb 2026 11:49:08 +0000
From: Will Deacon <will@...nel.org>
To: Alice Ryhl <aliceryhl@...gle.com>
Cc: Miguel Ojeda <ojeda@...nel.org>, Boqun Feng <boqun.feng@...il.com>,
	Gary Guo <gary@...yguo.net>,
	Björn Roy Baron <bjorn3_gh@...tonmail.com>,
	Benno Lossin <lossin@...nel.org>,
	Andreas Hindborg <a.hindborg@...nel.org>,
	Trevor Gross <tmgross@...ch.edu>,
	Danilo Krummrich <dakr@...nel.org>,
	Alexandre Courbot <acourbot@...dia.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Mark Rutland <mark.rutland@....com>,
	Nathan Chancellor <nathan@...nel.org>,
	Nick Desaulniers <nick.desaulniers+lkml@...il.com>,
	Bill Wendling <morbo@...gle.com>,
	Justin Stitt <justinstitt@...gle.com>,
	Nicolas Schier <nicolas.schier@...ux.dev>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Uladzislau Rezki <urezki@...il.com>, rust-for-linux@...r.kernel.org,
	linux-kernel@...r.kernel.org, llvm@...ts.linux.dev,
	linux-kbuild@...r.kernel.org, linux-mm@...ck.org,
	Matthew Maurer <mmaurer@...gle.com>, elver@...gle.com
Subject: Re: [PATCH v2 1/3] kbuild: rust: add
 `CONFIG_RUSTC_CLANG_LLVM_COMPATIBLE`

On Tue, Feb 03, 2026 at 11:34:08AM +0000, Alice Ryhl wrote:
> From: Gary Guo <gary@...yguo.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@...nel.org>
> Signed-off-by: Gary Guo <gary@...yguo.net>
> Signed-off-by: Matthew Maurer <mmaurer@...gle.com>
> Signed-off-by: Alice Ryhl <aliceryhl@...gle.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