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.
+
 config ARCH_HAS_CC_CAN_LINK
 	bool
 

-- 
2.53.0.rc1.225.gd81095ad13-goog

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

On Tue, Feb 03, 2026 at 11:49:08AM +0000, Will Deacon wrote:
> 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?

I would like to see CONFIG_LTO actually perform cross-language inlining,
but it'd require doing something about both:

1. The issues mentioned in patch 2 of this series.
2. The issues mentioned in commit 5daa0c35a1f0 ("rust: Disallow BTF
   generation with Rust + LTO").

As for whether it's stale help text ... CONFIG_RUST_INLINE_HELPERS works
by performing cross-language LTO too. But I can see your point that it
would make sense to improve this wording given that we're not making
CONFIG_LTO work now.

Alice