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

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Message-ID: <87pl7lh77v.fsf@t14s.mail-host-address-is-not-set>
Date: Wed, 07 Jan 2026 14:40:04 +0100
From: Andreas Hindborg <a.hindborg@...nel.org>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Alice Ryhl <aliceryhl@...gle.com>, 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>, Trevor
 Gross <tmgross@...ch.edu>, Danilo Krummrich <dakr@...nel.org>, Alexandre
 Courbot <acourbot@...dia.com>, Will Deacon <will@...nel.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, Matthew Maurer <mmaurer@...gle.com>
Subject: Re: [PATCH 0/4] Inline helpers into Rust without full LTO

"Peter Zijlstra" <peterz@...radead.org> writes:

> On Wed, Jan 07, 2026 at 02:12:10PM +0100, Andreas Hindborg wrote:
>> "Peter Zijlstra" <peterz@...radead.org> writes:
>>
>> > On Wed, Jan 07, 2026 at 01:23:38PM +0100, Andreas Hindborg wrote:
>> >> Hi Alice,
>> >>
>> >> Alice Ryhl <aliceryhl@...gle.com> 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@google.com/
>> >> >
>> >> > Signed-off-by: Alice Ryhl <aliceryhl@...gle.com>
>> >>
>> >> 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.

Best regards,
Andreas Hindborg