Re: [RFC PATCH 2/2] kbuild: rust: provide an option to inline C helpers into Rust

[Boqun Feng]

On Wed, May 29, 2024 at 09:28:15PM +0100, Gary Guo wrote:
[...]
> +$(obj)/helpers.bc: $(obj)/helpers.c FORCE
> +	+$(call if_changed_dep,rust_helper)
> +
>  quiet_cmd_exports = EXPORTS $@
>        cmd_exports = \
>  	$(NM) -p --defined-only $< \
> @@ -396,11 +413,13 @@ 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) $(patsubst %.o,%.bc,$@) $(obj)/helpers.bc -o $(patsubst %.o,%.m.bc,$@); \
> +		$(CC) $(KBUILD_CFLAGS) -mllvm=--ignore-tti-inline-compatible -c $(patsubst %.o,%.m.bc,$@) -o $@) \

I hit some errors when I tried to cross-compile arm64 on x86. Adding
$(CLANG_FLAGS) here resolves my issue, it's really because the target
selection for clang is done in $(CLANG_FLAGS) and carried into
$(KBUILD_CPPFLAGS), but seems you cannot use $(KBUILD_CPPFLAGS) here
because you only use clang as a linker. Without either, clang will just
use the target of host.

diff --git a/rust/Makefile b/rust/Makefile
index ad0797467102..84d698c09c65 100644
--- a/rust/Makefile
+++ b/rust/Makefile
@@ -419,7 +419,7 @@ quiet_cmd_rustc_library = $(if $(skip_clippy),RUSTC,$(RUSTC_OR_CLIPPY_QUIET)) L
                --crate-name $(patsubst %.o,%,$(notdir $@)) $< \
                --sysroot=/dev/null \
        $(if $(link_helper),;$(LLVM_LINK) $(patsubst %.o,%.bc,$@) $(obj)/helpers.bc -o $(patsubst %.o,%.m.bc,$@); \
-               $(CC) $(KBUILD_CFLAGS) -mllvm=--ignore-tti-inline-compatible -c $(patsubst %.o,%.m.bc,$@) -o $@) \
+               $(CC) $(CLANG_FLAGS) $(KBUILD_CFLAGS) -mllvm=--ignore-tti-inline-compatible -c $(patsubst %.o,%.m.bc,$@) -o $@) \
        $(if $(rustc_objcopy),;$(OBJCOPY) $(rustc_objcopy) $@)

 rust-analyzer:

>  	$(if $(rustc_objcopy),;$(OBJCOPY) $(rustc_objcopy) $@)
>  
>  rust-analyzer:
> @@ -463,4 +482,11 @@ $(obj)/kernel.o: $(src)/kernel/lib.rs $(obj)/alloc.o $(obj)/build_error.o \
>      $(obj)/libmacros.so $(obj)/bindings.o $(obj)/uapi.o FORCE
>  	+$(call if_changed_dep,rustc_library)
>  
> +ifdef CONFIG_RUST_LTO_HELPERS
> +
> +$(obj)/kernel.o: private link_helper = 1
> +$(obj)/kernel.o: $(obj)/helpers.bc
> +
> +endif
> +
>  endif # CONFIG_RUST
[...]
> diff --git a/scripts/Makefile.build b/scripts/Makefile.build
> index efacca63c897..201e7dc5ae5d 100644
> --- a/scripts/Makefile.build
> +++ b/scripts/Makefile.build
> @@ -288,7 +288,10 @@ 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_rustc_o_rs = $(rust_common_cmd) --emit=$(if $(CONFIG_RUST_LTO_HELPERS),llvm-bc=$(patsubst %.o,%.bc,$@),obj=$@) $< \
> +	$(if $(CONFIG_RUST_LTO_HELPERS),;$(LLVM_LINK) $(patsubst %.o,%.bc,$@) $(objtree)/rust/helpers.bc -o $(patsubst %.o,%.m.bc,$@); \
> +		$(CC) $(KBUILD_CFLAGS) -mllvm=--ignore-tti-inline-compatible -c $(patsubst %.o,%.m.bc,$@) -o $@)
> +
>  

Ditto.

Regards,
Boqun

>  $(obj)/%.o: $(obj)/%.rs FORCE
>  	+$(call if_changed_dep,rustc_o_rs)
> -- 
> 2.42.0
>