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

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Message-ID: <CAH5fLgjjWW5Jrt+k76wJUhmLKTKUW5zThwUoxXuwBmk2-gyMxQ@mail.gmail.com>
Date: Mon, 6 Jan 2025 10:02:41 +0100
From: Alice Ryhl <aliceryhl@...gle.com>
To: Gary Guo <gary@...yguo.net>
Cc: Miguel Ojeda <ojeda@...nel.org>, Alex Gaynor <alex.gaynor@...il.com>, 
	Boqun Feng <boqun.feng@...il.com>, Björn Roy Baron <bjorn3_gh@...tonmail.com>, 
	Benno Lossin <benno.lossin@...ton.me>, Andreas Hindborg <a.hindborg@...nel.org>, 
	Trevor Gross <tmgross@...ch.edu>, Nathan Chancellor <nathan@...nel.org>, 
	Nick Desaulniers <ndesaulniers@...gle.com>, Bill Wendling <morbo@...gle.com>, 
	Justin Stitt <justinstitt@...gle.com>, Masahiro Yamada <masahiroy@...nel.org>, 
	Andrew Morton <akpm@...ux-foundation.org>, Petr Mladek <pmladek@...e.com>, Tejun Heo <tj@...nel.org>, 
	Yoann Congal <yoann.congal@...le.fr>, Roman Gushchin <roman.gushchin@...ux.dev>, 
	Jens Axboe <axboe@...nel.dk>, Chen Ridong <chenridong@...wei.com>, Jann Horn <jannh@...gle.com>, 
	Mark Rutland <mark.rutland@....com>, Vincent Guittot <vincent.guittot@...aro.org>, 
	rust-for-linux@...r.kernel.org, llvm@...ts.linux.dev, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/3] kbuild: rust: add `CONFIG_RUSTC_CLANG_LLVM_COMPATIBLE`

On Sun, Jan 5, 2025 at 9:02 PM Gary Guo <gary@...yguo.net> wrote:
>
> 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]
> Signed-off-by: Gary Guo <gary@...yguo.net>
> ---
>  init/Kconfig | 15 +++++++++++++++
>  1 file changed, 15 insertions(+)
>
> diff --git a/init/Kconfig b/init/Kconfig
> index a20e6efd3f0f..cbe4b2922ce2 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
> +         compatible it is recommended that the exact same LLVM is used.

typo: compatibility

Alice