linux-kernel - Re: [PATCH 4/4] build: rust: provide an option to inline C helpers into Rust

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Message-ID: <20251204142742.GM2528459@noisy.programming.kicks-ass.net>
Date: Thu, 4 Dec 2025 15:27:42 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Alice Ryhl <aliceryhl@...gle.com>
Cc: Emilio Cobos Álvarez <emilio@...sal.io>,
	Miguel Ojeda <miguel.ojeda.sandonis@...il.com>,
	Antoni Boucher <bouanto@...o.com>,
	Arthur Cohen <arthur.cohen@...ecosm.com>,
	Gary Guo <gary@...yguo.net>, Josh Triplett <josh@...htriplett.org>,
	Miguel Ojeda <ojeda@...nel.org>, Boqun Feng <boqun.feng@...il.com>,
	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>, 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 4/4] build: rust: provide an option to inline C helpers
 into Rust

On Thu, Dec 04, 2025 at 01:15:34PM +0000, Alice Ryhl wrote:
> On Thu, Dec 04, 2025 at 01:49:28PM +0100, Emilio Cobos Álvarez wrote:
> > On 12/4/25 12:57 PM, Miguel Ojeda wrote:
> > > On Thu, Dec 4, 2025 at 12:11 PM Peter Zijlstra <peterz@...radead.org> wrote:
> > > > 
> > > > Right. Earlier I also proposed using libclang to parse the C header and
> > > > inject that. This might be a little simpler, in that..
> > > 
> > > Yeah, that would be closer to the `bindgen` route in that `libclang`
> > > gets already involved.
> > 
> > Yeah, so... there are existing tools (c2rust [0] being the actively
> > maintained one IIUC) that in theory could do something like that (translate
> > the bodies of the functions from C to Rust so that rustc could consume them
> > directly rather than via LLVM LTO).
> > 
> > I think the intended use case is more "translate a whole C project into
> > rust", but it could be interesting to test how well / poorly it performs
> > with the kernel helpers / with a single header translated to Rust.
> > 
> > I personally haven't tried it because for work I need to deal with C++,
> > which means that automatic translation to Rust is a lot harder / probably
> > impossible in general. So for Firefox we end up relying on bindgen +
> > cross-language LTO for this kind of thing, and it works well for us.
> > 
> > If I'm understanding correctly, it seems the kernel needs this extra bit of
> > help (__always_inline) to push LLVM to inline C functions into rust, which
> > is a bit unfortunate... But this approach seems sensible to me, for now at
> > least.
> > 
> > FWIW Bindgen recently gained an option to generate inline functions [1],
> > which could help avoid at least the bindgen ifdef in the patch series?
> > 
> > Anyways, it might be interesting to give c2rust a go on the kernel helpers
> > if nobody has done so, and see how well / poorly it works in practice? Of
> > course probably introducing a new dependency would be kind of a pain, but
> > could be a good data point for pushing into adding something like it built
> > into rustc...
> 
> I already tried c2rust as an alternative to this patch. It works okay
> for many functions, but it's missing support for some features such as
> asm goto, though this is fixable. But a larger issue is that some things
> simply do not translate to Rust right now. For example:
> 
> * Atomics use the Ir operand.
> * static_branch uses the i operand.
> 
> neither of which translate directly to Rust.

Right this. AFAIK Rust simply does not have feature parity with inline
asm. Them having picked a wildly different syntax for inline asm didn't
help either of course. But Rust is Rust, must have terrible syntax :-)