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

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Message-ID: <20250319205141.3528424-2-gary@garyguo.net>
Date: Wed, 19 Mar 2025 20:50:38 +0000
From: Gary Guo <gary@...yguo.net>
To: Miguel Ojeda <ojeda@...nel.org>,
	Alex Gaynor <alex.gaynor@...il.com>,
	Boqun Feng <boqun.feng@...il.com>,
	Gary Guo <gary@...yguo.net>,
	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>
Cc: rust-for-linux@...r.kernel.org,
	llvm@...ts.linux.dev,
	Johannes Weiner <hannes@...xchg.org>,
	linux-kernel@...r.kernel.org
Subject: [PATCH v3 1/2] kbuild: rust: add `CONFIG_RUSTC_CLANG_LLVM_COMPATIBLE`

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