Re: [PATCH v2 1/3] kbuild: rust: add `CONFIG_RUSTC_CLANG_LLVM_COMPATIBLE`

[Alice Ryhl <aliceryhl@google.com>]

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