linux-kernel - [PATCH v2 0/3] Inline helpers into Rust without full LTO

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Message-ID: <20260203-inline-helpers-v2-0-beb8547a03c9@google.com>
Date: Tue, 03 Feb 2026 11:34:07 +0000
From: Alice Ryhl <aliceryhl@...gle.com>
To: Miguel Ojeda <ojeda@...nel.org>
Cc: 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, 
	Alice Ryhl <aliceryhl@...gle.com>, Matthew Maurer <mmaurer@...gle.com>
Subject: [PATCH v2 0/3] Inline helpers into Rust without full LTO

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>
---
Changes in v2:
- Dropped merged patches.
- Generate two copies of helpers.bc, one for built-in and one for modules.
- Remove unused llvm-as tool.
- Add $(cmd_ld_single) to avoid breaking CONFIG_LTO builds (note that
  LTO does not happen over the language boundary, except for helpers).
- Improve Kconfig help message.
- Link to v1: https://lore.kernel.org/r/20251202-inline-helpers-v1-0-879dae33a66a@google.com

---
Gary Guo (3):
      kbuild: rust: add `CONFIG_RUSTC_CLANG_LLVM_COMPATIBLE`
      rust: helpers: #define __rust_helper
      build: rust: provide an option to inline C helpers into Rust

 Makefile               |  3 ++-
 init/Kconfig           | 15 +++++++++++++++
 lib/Kconfig.debug      | 15 +++++++++++++++
 rust/Makefile          | 29 +++++++++++++++++++++++++----
 rust/exports.c         |  5 ++++-
 rust/helpers/helpers.c | 29 +++++++++++++++++++++++++++++
 scripts/Makefile.build |  7 ++++++-
 7 files changed, 96 insertions(+), 7 deletions(-)
---
base-commit: 33a647c659ffa5bdb94abc345c8c86768ff96215
change-id: 20251202-inline-helpers-996f4db65e18

Best regards,
-- 
Alice Ryhl <aliceryhl@...gle.com>