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Message-ID: <CAGSQo01=7YDfYzbjipNtsK8AzXciSpY+v53kAhh+oZ_QisAh4Q@mail.gmail.com>
Date: Tue, 2 Dec 2025 16:40:41 -0800
From: Matthew Maurer <mmaurer@...gle.com>
To: Alice Ryhl <aliceryhl@...gle.com>
Cc: Miguel Ojeda <ojeda@...nel.org>, Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>,
Björn Roy Baron <bjorn3_gh@...tonmail.com>,
Benno Lossin <lossin@...nel.org>, Andreas Hindborg <a.hindborg@...nel.org>,
Trevor Gross <tmgross@...ch.edu>, Danilo Krummrich <dakr@...nel.org>,
Alexandre Courbot <acourbot@...dia.com>, Will Deacon <will@...nel.org>,
Peter Zijlstra <peterz@...radead.org>, Mark Rutland <mark.rutland@....com>,
Nathan Chancellor <nathan@...nel.org>, Nick Desaulniers <nick.desaulniers+lkml@...il.com>,
Bill Wendling <morbo@...gle.com>, Justin Stitt <justinstitt@...gle.com>,
Nicolas Schier <nicolas.schier@...ux.dev>, Andrew Morton <akpm@...ux-foundation.org>,
Uladzislau Rezki <urezki@...il.com>, rust-for-linux@...r.kernel.org,
linux-kernel@...r.kernel.org, llvm@...ts.linux.dev,
linux-kbuild@...r.kernel.org, linux-mm@...ck.org,
nouveau@...ts.freedesktop.org
Subject: Re: [PATCH 4/4] build: rust: provide an option to inline C helpers
into Rust
On Tue, Dec 2, 2025 at 12:28 PM Alice Ryhl <aliceryhl@...gle.com> wrote:
>
> From: Gary Guo <gary@...yguo.net>
>
> A new experimental Kconfig option, `RUST_INLINE_HELPERS` is added to
> allow C helpers (which were created to allow Rust to call into
> inline/macro C functions without having to re-implement the logic in
> Rust) to be inlined into Rust crates without performing global LTO.
>
> If the option is enabled, the following is performed:
> * For helpers, instead of compiling them to an object file to be linked
> into vmlinux, they're compiled to LLVM IR.
> * The LLVM IR is compiled to bitcode (This is step is not necessary, but
> is a performance optimisation to prevent LLVM from always have to
> reparse the same IR).
> * When a Rust crate is compiled, instead of generating an object file, we
> ask LLVM bitcode to be generated.
> * llvm-link is invoked with --internalize to combine the helper bitcode
> with the crate bitcode. This step is similar to LTO, but this is much
> faster since it only needs to inline the helpers.
> * clang is invoked to turn the combined bitcode into a final object file.
>
> The --internalize flag tells llvm-link to treat all symbols in
> helpers.bc using `internal` linkage. This matches the behavior of
> `clang` on `static inline` functions, and avoids exporting the symbol
> from the object file.
I've filed a PR with LLVM [1] to clarify that this is the intended
operation of the tool.
[1]: https://github.com/llvm/llvm-project/pull/170397
>
> To ensure that RUST_INLINE_HELPERS is not incompatible with BTF, we pass
> the -g0 flag when building helpers. See commit 5daa0c35a1f0 ("rust:
> Disallow BTF generation with Rust + LTO") for details.
>
> We have an intended triple mismatch of `aarch64-unknown-none` vs
> `aarch64-unknown-linux-gnu`, so we suppress the warning.
>
> Co-developed-by: Boqun Feng <boqun.feng@...il.com>
> Signed-off-by: Boqun Feng <boqun.feng@...il.com>
> Co-developed-by: Matthew Maurer <mmaurer@...gle.com>
> Signed-off-by: Matthew Maurer <mmaurer@...gle.com>
> Signed-off-by: Gary Guo <gary@...yguo.net>
> Co-developed-by: Alice Ryhl <aliceryhl@...gle.com>
> Signed-off-by: Alice Ryhl <aliceryhl@...gle.com>
> ---
> Makefile | 4 +++-
> lib/Kconfig.debug | 15 +++++++++++++++
> rust/Makefile | 26 ++++++++++++++++++++++----
> rust/exports.c | 5 ++++-
> scripts/Makefile.build | 5 ++++-
> 5 files changed, 48 insertions(+), 7 deletions(-)
>
> diff --git a/Makefile b/Makefile
> index 96ddbaae7e12de71bcfabef4639de3a13a6e4815..5834bfd568548d1bee34b328dccce5d60f85526f 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -517,6 +517,8 @@ OBJCOPY = $(LLVM_PREFIX)llvm-objcopy$(LLVM_SUFFIX)
> OBJDUMP = $(LLVM_PREFIX)llvm-objdump$(LLVM_SUFFIX)
> READELF = $(LLVM_PREFIX)llvm-readelf$(LLVM_SUFFIX)
> STRIP = $(LLVM_PREFIX)llvm-strip$(LLVM_SUFFIX)
> +LLVM_LINK = $(LLVM_PREFIX)llvm-link$(LLVM_SUFFIX)
> +LLVM_AS = $(LLVM_PREFIX)llvm-as$(LLVM_SUFFIX)
> else
> CC = $(CROSS_COMPILE)gcc
> LD = $(CROSS_COMPILE)ld
> @@ -625,7 +627,7 @@ export RUSTC_BOOTSTRAP := 1
> export CLIPPY_CONF_DIR := $(srctree)
>
> export ARCH SRCARCH CONFIG_SHELL BASH HOSTCC KBUILD_HOSTCFLAGS CROSS_COMPILE LD CC HOSTPKG_CONFIG
> -export RUSTC RUSTDOC RUSTFMT RUSTC_OR_CLIPPY_QUIET RUSTC_OR_CLIPPY BINDGEN
> +export RUSTC RUSTDOC RUSTFMT RUSTC_OR_CLIPPY_QUIET RUSTC_OR_CLIPPY BINDGEN LLVM_LINK LLVM_AS
> export HOSTRUSTC KBUILD_HOSTRUSTFLAGS
> export CPP AR NM STRIP OBJCOPY OBJDUMP READELF PAHOLE RESOLVE_BTFIDS LEX YACC AWK INSTALLKERNEL
> export PERL PYTHON3 CHECK CHECKFLAGS MAKE UTS_MACHINE HOSTCXX
> diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
> index 3034e294d50df55c4003c5fa5df442f59e711bd8..e63c5eb57b049aff988419ccd12dfd99d59f5080 100644
> --- a/lib/Kconfig.debug
> +++ b/lib/Kconfig.debug
> @@ -3427,6 +3427,21 @@ config RUST_KERNEL_DOCTESTS
>
> If unsure, say N.
>
> +config RUST_INLINE_HELPERS
> + bool "Inline C helpers into Rust crates (EXPERIMENTAL)"
> + depends on RUST && RUSTC_CLANG_LLVM_COMPATIBLE
> + depends on EXPERT
> + help
> + Links C helpers into Rust crates through LLVM IR.
> +
> + If this option is enabled, instead of generating object files directly,
> + rustc is asked to produce LLVM IR, which is then linked together with
> + the LLVM IR of C helpers, before object file is generated.
> +
> + This requires a matching LLVM version for Clang and rustc.
> +
> + If unsure, say N.
> +
> endmenu # "Rust"
>
> endmenu # Kernel hacking
> diff --git a/rust/Makefile b/rust/Makefile
> index d7d19c21b671dea10242b1772a8bcf0bf5dcc1cd..2344e2662ce29280582215954132c09f63cd8c9d 100644
> --- a/rust/Makefile
> +++ b/rust/Makefile
> @@ -6,15 +6,19 @@ rustdoc_output := $(objtree)/Documentation/output/rust/rustdoc
> obj-$(CONFIG_RUST) += core.o compiler_builtins.o ffi.o
> always-$(CONFIG_RUST) += exports_core_generated.h
>
> +ifdef CONFIG_RUST_INLINE_HELPERS
> +always-$(CONFIG_RUST) += helpers/helpers.bc
> +else
> +obj-$(CONFIG_RUST) += helpers/helpers.o
> +always-$(CONFIG_RUST) += exports_helpers_generated.h
> +endif
> # Missing prototypes are expected in the helpers since these are exported
> # for Rust only, thus there is no header nor prototypes.
> -obj-$(CONFIG_RUST) += helpers/helpers.o
> CFLAGS_REMOVE_helpers/helpers.o = -Wmissing-prototypes -Wmissing-declarations
>
> always-$(CONFIG_RUST) += bindings/bindings_generated.rs bindings/bindings_helpers_generated.rs
> obj-$(CONFIG_RUST) += bindings.o pin_init.o kernel.o
> -always-$(CONFIG_RUST) += exports_helpers_generated.h \
> - exports_bindings_generated.h exports_kernel_generated.h
> +always-$(CONFIG_RUST) += exports_bindings_generated.h exports_kernel_generated.h
>
> always-$(CONFIG_RUST) += uapi/uapi_generated.rs
> obj-$(CONFIG_RUST) += uapi.o
> @@ -468,6 +472,13 @@ $(obj)/bindings/bindings_helpers_generated.rs: private bindgen_target_extra = ;
> $(obj)/bindings/bindings_helpers_generated.rs: $(src)/helpers/helpers.c FORCE
> $(call if_changed_dep,bindgen)
>
> +quiet_cmd_rust_helper = HELPER $@
> + cmd_rust_helper = \
> + $(CC) $(filter-out $(CFLAGS_REMOVE_helpers/helpers.o), $(c_flags)) -c -g0 $< -emit-llvm -o $@
> +
> +$(obj)/helpers/helpers.bc: $(obj)/helpers/helpers.c FORCE
> + +$(call if_changed_dep,rust_helper)
> +
> rust_exports = $(NM) -p --defined-only $(1) | awk '$$2~/(T|R|D|B)/ && $$3!~/__(pfx|cfi|odr_asan)/ { printf $(2),$$3 }'
>
> quiet_cmd_exports = EXPORTS $@
> @@ -547,11 +558,13 @@ quiet_cmd_rustc_library = $(if $(skip_clippy),RUSTC,$(RUSTC_OR_CLIPPY_QUIET)) L
> OBJTREE=$(abspath $(objtree)) \
> $(if $(skip_clippy),$(RUSTC),$(RUSTC_OR_CLIPPY)) \
> $(filter-out $(skip_flags),$(rust_flags)) $(rustc_target_flags) \
> - --emit=dep-info=$(depfile) --emit=obj=$@ \
> + --emit=dep-info=$(depfile) --emit=$(if $(link_helper),llvm-bc=$(patsubst %.o,%.bc,$@),obj=$@) \
> --emit=metadata=$(dir $@)$(patsubst %.o,lib%.rmeta,$(notdir $@)) \
> --crate-type rlib -L$(objtree)/$(obj) \
> --crate-name $(patsubst %.o,%,$(notdir $@)) $< \
> --sysroot=/dev/null \
> + $(if $(link_helper),;$(LLVM_LINK) --internalize --suppress-warnings $(patsubst %.o,%.bc,$@) $(obj)/helpers/helpers.bc -o $(patsubst %.o,%.m.bc,$@); \
> + $(CC) $(CLANG_FLAGS) $(KBUILD_CFLAGS) -Wno-override-module -c $(patsubst %.o,%.m.bc,$@) -o $@) \
> $(if $(rustc_objcopy),;$(OBJCOPY) $(rustc_objcopy) $@) \
> $(cmd_objtool)
>
> @@ -678,4 +691,9 @@ $(obj)/kernel.o: $(obj)/kernel/generated_arch_warn_asm.rs $(obj)/kernel/generate
> endif
> endif
>
> +ifdef CONFIG_RUST_INLINE_HELPERS
> +$(obj)/kernel.o: private link_helper = 1
> +$(obj)/kernel.o: $(obj)/helpers/helpers.bc
> +endif
> +
> endif # CONFIG_RUST
> diff --git a/rust/exports.c b/rust/exports.c
> index 587f0e776aba52854080f15aa91094b55996c072..1b52460b0f4eeef6df9081abb9b7e054a28c3c21 100644
> --- a/rust/exports.c
> +++ b/rust/exports.c
> @@ -16,10 +16,13 @@
> #define EXPORT_SYMBOL_RUST_GPL(sym) extern int sym; EXPORT_SYMBOL_GPL(sym)
>
> #include "exports_core_generated.h"
> -#include "exports_helpers_generated.h"
> #include "exports_bindings_generated.h"
> #include "exports_kernel_generated.h"
>
> +#ifndef CONFIG_RUST_INLINE_HELPERS
> +#include "exports_helpers_generated.h"
> +#endif
> +
> // For modules using `rust/build_error.rs`.
> #ifdef CONFIG_RUST_BUILD_ASSERT_ALLOW
> EXPORT_SYMBOL_RUST_GPL(rust_build_error);
> diff --git a/scripts/Makefile.build b/scripts/Makefile.build
> index d0ee33a487be95f8ba9a5c964ebecfbebc6c4bf8..04eaf2b4fbca2245f904a6dc7875cb3275aa7df6 100644
> --- a/scripts/Makefile.build
> +++ b/scripts/Makefile.build
> @@ -343,7 +343,10 @@ rust_common_cmd = \
> # would not match each other.
>
> quiet_cmd_rustc_o_rs = $(RUSTC_OR_CLIPPY_QUIET) $(quiet_modtag) $@
> - cmd_rustc_o_rs = $(rust_common_cmd) --emit=obj=$@ $< $(cmd_objtool)
> + cmd_rustc_o_rs = $(rust_common_cmd) --emit=$(if $(CONFIG_RUST_INLINE_HELPERS),llvm-bc=$(patsubst %.o,%.bc,$@),obj=$@) $< \
> + $(if $(CONFIG_RUST_INLINE_HELPERS),;$(LLVM_LINK) --internalize --suppress-warnings $(patsubst %.o,%.bc,$@) $(objtree)/rust/helpers/helpers.bc -o $(patsubst %.o,%.m.bc,$@); \
> + $(CC) $(CLANG_FLAGS) $(KBUILD_CFLAGS) -Wno-override-module -c $(patsubst %.o,%.m.bc,$@) -o $@) \
> + $(cmd_objtool)
>
> define rule_rustc_o_rs
> $(call cmd_and_fixdep,rustc_o_rs)
>
> --
> 2.52.0.158.g65b55ccf14-goog
>
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