Re: [PATCH 0/4] Inline helpers into Rust without full LTO

[Alice Ryhl]

On Wed, Jan 7, 2026 at 2:40 PM Andreas Hindborg <a.hindborg@xxxxxxxxxx> wrote:
>
> "Peter Zijlstra" <peterz@xxxxxxxxxxxxx> writes:
>
> > On Wed, Jan 07, 2026 at 02:12:10PM +0100, Andreas Hindborg wrote:
> >> "Peter Zijlstra" <peterz@xxxxxxxxxxxxx> writes:
> >>
> >> > On Wed, Jan 07, 2026 at 01:23:38PM +0100, Andreas Hindborg wrote:
> >> >> Hi Alice,
> >> >>
> >> >> Alice Ryhl <aliceryhl@xxxxxxxxxx> writes:
> >> >>
> >> >> > Currently the only way for Rust code to call a static inline function is
> >> >> > to go through a helper in rust/helpers/. This introduces performance
> >> >> > costs due to additional function calls and also clutters backtraces and
> >> >> > flame graphs with helper symbols.
> >> >> >
> >> >> > To get rid of these helper symbols, provide functionality to inline
> >> >> > helpers into Rust using llvm-link. This option complements full LTO, by
> >> >> > being much cheaper and avoiding incompatibility with BTF.
> >> >> >
> >> >> > I ran a microbenchmark showing the benefit of this. All the benchmark
> >> >> > does is call refcount_inc() in a loop. This was chosen since refcounting
> >> >> > is quite hot in Binder. The results are that Rust spends 6.35 ns per
> >> >> > call vs 5.73 ns per call in C. When enabling this option, the two
> >> >> > languages become equally fast, and disassembly confirms the exact same
> >> >> > machine code is used (in particular there is no call to
> >> >> > rust_helper_refcount_inc). Benchmarking Binder also results in an
> >> >> > improvement from this change.
> >> >> >
> >> >> > This patch is complementary to:
> >> >> > https://lore.kernel.org/all/20251202-define-rust-helper-v1-0-a2e13cbc17a6@xxxxxxxxxx/
> >> >> >
> >> >> > Signed-off-by: Alice Ryhl <aliceryhl@xxxxxxxxxx>
> >> >>
> >> >> I get the following modpost errors when building with this applied on top
> >> >> of v6.19-rc4:
> >> >>
> >> >> ERROR: modpost: "__SCK__WARN_trap" [drivers/gpu/drm/nova/nova.ko] undefined!
> >> >> ERROR: modpost: "__SCK__WARN_trap" [drivers/gpu/nova-core/nova_core.ko] undefined!
> >> >> ERROR: modpost: "__SCK__WARN_trap" [drivers/block/rnull/rnull_mod.ko] undefined!
> >> >> ERROR: modpost: "__SCK__WARN_trap" [samples/rust/rust_minimal.ko] undefined!
> >> >> ERROR: modpost: "__SCK__WARN_trap" [samples/rust/rust_misc_device.ko] undefined!
> >> >> ERROR: modpost: "__SCK__WARN_trap" [samples/rust/rust_print.ko] undefined!
> >> >> ERROR: modpost: "__SCK__WARN_trap" [samples/rust/rust_dma.ko] undefined!
> >> >> ERROR: modpost: "__SCK__WARN_trap" [samples/rust/rust_driver_pci.ko] undefined!
> >> >> ERROR: modpost: "__SCK__WARN_trap" [samples/rust/rust_driver_platform.ko] undefined!
> >> >> ERROR: modpost: "__SCK__WARN_trap" [samples/rust/rust_driver_faux.ko] undefined!
> >> >
> >> > Looks like it used the wrong static_call_mod() version, was MODULE
> >> > defined?
> >>
> >> CONFIG_MODULES=y, yes
> >>
> >> I built without the patch first, then applied the series, enabled the
> >> option via menuconfig and ran the build. I thought maybe some dependency
> >> check is messed up so I retried the build from a clean state. Same
> >> result.
> >
> > No, I mean -DMODULE. Note how the quiet_cmd_bindgen target has -DMODULE,
> > but the new quiet_cmd_rust_helper target does not.
>
> Ah, that did the trick, thanks!
>
>   diff --git a/rust/Makefile b/rust/Makefile
>   index 5365d53b6cf96..08d3dc1038cf5 100644
>   --- a/rust/Makefile
>   +++ b/rust/Makefile
>   @@ -483,7 +483,7 @@ $(obj)/bindings/bindings_helpers_generated.rs: $(src)/helpers/helpers.c FORCE
>
>   quiet_cmd_rust_helper = HELPER  $@
>         cmd_rust_helper = \
>   -     $(CC) $(filter-out $(CFLAGS_REMOVE_helpers/helpers.o), $(c_flags)) -c -g0 $< -emit-llvm -o $@
>   +     $(CC) $(filter-out $(CFLAGS_REMOVE_helpers/helpers.o), $(c_flags)) -DMODULE -c -g0 $< -emit-llvm -o $@
>
>   $(obj)/helpers/helpers.bc: $(obj)/helpers/helpers.c FORCE
>     +$(call if_changed_dep,rust_helper)
>
> Is -DMOUDLE always appropriate to pass to the helpers? The helpers are
> also inlined into non module code.

With this option, helpers.bc gets linked into every Rust object file
separately (and symbols are internalized). We might want a separate
.bc file depending on whether the Rust object files is part of the
core kernel or a .ko file.

Alice