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

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Message-ID: <5e8a99a5-f39c-46da-b54d-7b69307b1c66@gmail.com>
Date: Fri, 31 Jan 2025 19:58:23 +0100
From: Christian Schrefl <chrisi.schrefl@...il.com>
To: Andrew Lunn <andrew@...n.ch>, Miguel Ojeda <ojeda@...nel.org>
Cc: Arnd Bergmann <arnd@...db.de>, 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>,
 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

On 31.01.25 5:05 PM, Andrew Lunn wrote:
>> To fix this Rust would have to provide a way to build the core
>> library without float support. I don't know if there is a plan
>> already to allow this.
> 
> Floating point is banned within the kernel, except for in very narrow
> conditions, because the floating point registers are lazy saved on
> context switch. If the kernel uses the floating point registers, you
> can break user space in bad ways.
> 
> I expect this has been discussed, since it is well known kernel
> restriction. Maybe go see what happened to that discussion within RfL?

After checking again, it seems the float intrinsics are actually not
needed anymore at least for my config. Only `__aeabi_uldivmod` is still
required for `parse_u64_into` since [0] allows disabling float formatting.

Link error without the `__aeabi_uldivmod` symbol defined:

ld.lld: error: undefined symbol: __aeabi_uldivmod
>>> referenced by num.rs:580 (/home/chrisi/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/fmt/num.rs:580)
>>>               rust/core.o:(core::fmt::num::parse_u64_into::<39>) in archive vmlinux.a
>>> referenced by num.rs:589 (/home/chrisi/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/fmt/num.rs:589)
>>>               rust/core.o:(core::fmt::num::parse_u64_into::<39>) in archive vmlinux.a
>>> referenced by num.rs:589 (/home/chrisi/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/fmt/num.rs:589)
>>>               rust/core.o:(core::fmt::num::parse_u64_into::<39>) in archive vmlinux.a
>>> referenced 34 more times
>>> did you mean: __aeabi_uidivmod
>>> defined in: vmlinux.a(arch/arm/lib/lib1funcs.o)

Not sure if we should just implement `__aeabi_uldivmod`, keep the 
panicking intrinsic for it or somehow fix it in upstream Rust?

Miguel do you know how this is best handled?

Link: https://github.com/rust-lang/rust/pull/86048 [0]

Cheers
Christian