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

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Message-ID: <87y0m9har9.fsf@t14s.mail-host-address-is-not-set>
Date: Wed, 07 Jan 2026 13:23:38 +0100
From: Andreas Hindborg <a.hindborg@...nel.org>
To: Alice Ryhl <aliceryhl@...gle.com>, 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>,
 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,
 nouveau@...ts.freedesktop.org, Alice Ryhl <aliceryhl@...gle.com>, Matthew
 Maurer <mmaurer@...gle.com>
Subject: Re: [PATCH 0/4] Inline helpers into Rust without full LTO

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!

I also applied the series adding the inline macro [1].


Best regards,
Andreas Hindborg


[1] https://lore.kernel.org/rust-for-linux/20260105-define-rust-helper-v2-0-51da5f454a67@google.com/