Re: [PATCH v2 3/3] build: rust: provide an option to inline C helpers into Rust

[Nathan Chancellor <nathan@kernel.org>]

On Sat, Mar 14, 2026 at 11:22:17AM +0000, Alice Ryhl wrote:
> On Fri, Mar 13, 2026 at 05:40:12PM -0700, Nathan Chancellor wrote:
> > On Fri, Mar 06, 2026 at 05:32:33PM +0000, Alice Ryhl wrote:
> > > On Tue, Feb 03, 2026 at 11:34:10AM +0000, Alice Ryhl wrote:
> > > >  $(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 $< $(if $(filter %_module.bc,$@),-DMODULE) -emit-llvm -o $@
> > > > +
> > > > +$(obj)/helpers/helpers.bc $(obj)/helpers/helpers_module.bc: $(src)/helpers/helpers.c FORCE
> > > > +	+$(call if_changed_dep,rust_helper)
> > > 
> > > I think this needs:
> > > 
> > > $(obj)/helpers/helpers.bc: private part-of-builtin := y
> > > $(obj)/helpers/helpers_module.bc: private part-of-module := y
> > > 
> > > at least it seems to make a difference for AutoFDO.
> > 
> > Would
> > 
> >   $(obj)/helpers/helpers.bc $(obj)/helpers/helpers_module.bc: private is-kernel-object := y
> > 
> > work as well? Presumably, this is because c_flags only has the AutoFDO
> > flags when is-kernel-object is set, which is only true when
> > helpers/helpers.o is a part of obj-y.
> 
> I assume so, but isn't it more correct to set part-of-builtin and
> part-of-module? I don't see any make rules making use of those directly
> (instead of is-kernel-object) right now, but if someone adds such uses,
> then this could become wrong, right?

Hmmm, yeah, I suppose so. Actually, adding those would allow you to drop
the manual inclusion of -DMODULE in cmd_rust_helper because it will be
included via modkern_cflags.

Cheers,
Nathan