linux-kernel - Re: [PATCH v3 1/2] kbuild: rust: add `CONFIG_RUSTC_CLANG_LLVM

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Message-ID: <CAJ-ks9kCTZZ-kq+VL3-4hxxz-q07vc4k6A6L9c6b6UYc5b5f3w@mail.gmail.com>
Date: Wed, 19 Mar 2025 17:13:13 -0400
From: Tamir Duberstein <tamird@...il.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>, 
	Alice Ryhl <aliceryhl@...gle.com>, Trevor Gross <tmgross@...ch.edu>, 
	Danilo Krummrich <dakr@...nel.org>, Nathan Chancellor <nathan@...nel.org>, 
	Nick Desaulniers <nick.desaulniers+lkml@...il.com>, Bill Wendling <morbo@...gle.com>, 
	Justin Stitt <justinstitt@...gle.com>, Masahiro Yamada <masahiroy@...nel.org>, Tejun Heo <tj@...nel.org>, 
	Andrew Morton <akpm@...ux-foundation.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, 
	Johannes Weiner <hannes@...xchg.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 1/2] kbuild: rust: add `CONFIG_RUSTC_CLANG_LLVM_COMPATIBLE`

On Wed, Mar 19, 2025 at 4:57 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]
> Reviewed-by: Andreas Hindborg <a.hindborg@...nel.org>
> 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 d0d021b3fa3b..e76e4ad7d4b9 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

s/requires/require/

> +         compatibility it is recommended that the exact same LLVM is used.

nit: s/LLVM/LLVM version/ unless you literally mean the same dylib.

> +
>  config CC_CAN_LINK
>         bool
>         default $(success,$(srctree)/scripts/cc-can-link.sh $(CC) $(CLANG_FLAGS) $(USERCFLAGS) $(USERLDFLAGS) $(m64-flag)) if 64BIT
> --
> 2.47.2
>
>

Reviewed-by: Tamir Duberstein <tamird@...il.com>