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

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Message-ID: <254593b3-613d-4536-b69b-74a41f0feb09@gmail.com>
Date: Sat, 1 Feb 2025 01:58:29 +0100
From: Christian Schrefl <chrisi.schrefl@...il.com>
To: Arnd Bergmann <arnd@...db.de>, Andrew Lunn <andrew@...n.ch>,
 Miguel Ojeda <ojeda@...nel.org>
Cc: 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 01.02.25 12:42 AM, Christian Schrefl wrote:
> On 31.01.25 8:37 PM, Arnd Bergmann wrote:
>> On Fri, Jan 31, 2025, at 19:58, Christian Schrefl wrote:
>>> 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.
>>
>> Ah, nice! If this is true for all architectures using the current
>> rust compiler, it would be great to remove the FP stubs entirely
>> and have link errors instead of panicking, to make it consistent
>> with C.
> 
> After a quick test it seems that (most?) intrinsics are not needed
> anymore on x86, but not sure if that's valid for all supported rust
> versions and config options.

On Rust 1.84.1 only __udivti3 is requied, with 1.78 most of the
intrinsics seem to still be required.

Cheers
Christian