linux-kernel - Re: [PATCH v3] arm: rust: Enable Rust support for ARMv7

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Message-ID: <399cceb4-dcf5-4af8-a8b7-6741e9b7e8ae@gmail.com>
Date: Sat, 5 Apr 2025 22:05:19 +0200
From: Christian Schrefl <chrisi.schrefl@...il.com>
To: Linus Walleij <linus.walleij@...aro.org>
Cc: Miguel Ojeda <ojeda@...nel.org>, Alex Gaynor <alex.gaynor@...il.com>,
 Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>,
 Björn Roy Baron <bjorn3_gh@...tonmail.com>,
 Benno Lossin <benno.lossin@...ton.me>,
 Andreas Hindborg <a.hindborg@...nel.org>, Alice Ryhl <aliceryhl@...gle.com>,
 Trevor Gross <tmgross@...ch.edu>, Jonathan Corbet <corbet@....net>,
 Russell King <linux@...linux.org.uk>, Rudraksha Gupta <guptarud@...il.com>,
 Ard Biesheuvel <ardb@...nel.org>, Geert Stappers <stappers@...ppers.nl>,
 Andrew Lunn <andrew@...n.ch>, Jamie Cunliffe <Jamie.Cunliffe@....com>,
 Sven Van Asbroeck <thesven73@...il.com>, rust-for-linux@...r.kernel.org,
 linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
 linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v3] arm: rust: Enable Rust support for ARMv7

Hi Linus,

On 21.03.25 8:24 AM, Linus Walleij wrote:
> Hi Christian,
> 
> thanks for your patch!
> 
> Sorry for being late to the show. I missed this very nice patch
> that was actually on my personal TODO but I have to much
> to do and also I'm not smart with Rust, but I'm a big supporter.
> 
> On Thu, Jan 23, 2025 at 11:40 PM Christian Schrefl
> <chrisi.schrefl@...il.com> wrote:
> 
>> +       select HAVE_RUST if CPU_LITTLE_ENDIAN && CPU_32v7
> 
> Nothing in the patch series really explains this restriction, so it
> should be in the commit message. Arnd mentions some atomics
> etc, but we really need to know exactly why this is in the patch.

This should probably work with armv6 targets, for v5 or v4 Targets a 
different target would have to be specified in the KBUILD_RUSTFLAGS.

Rust also has some big endian arm targets.

> 
> I'm a bit surprised by this since the rustc LLVM backend nowadays
> support all old ARM ISAs. I would have expected:
> 
> select HAVE_RUST if AEABI
> 
> Ideally this should work on any ARM core, but it's fair to require
> EABI.
> 
> The big reason: I think we want to be able to use Rust in kernel
> core components sooner than ARMv5 goes away from the kernel.
> 
> If testing is the only issue, I can provide testing on ARMv4, v5,
> ARMv5 BE etc, just tell me how to test. (But I guess it's more than
> that...)

The main reason that I only did this for ARMv7 was that I'm not very
familiar with older arm versions and that I only have v7 hardware for
testing.

I currently don't have much time to work on this, but when/if it 
works in qemu I'll send you a message to test it on HW.

Cheers
Christian