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

[Christian Schrefl <chrisi.schrefl@gmail.com>]

On 31.01.25 7:58 PM, 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. Only `__aeabi_uldivmod` is still
> required for `parse_u64_into` since [0] allows disabling float formatting.

Seems like Rust 1.78 still needs at least __aeabi_fcmpeq __aeabi_fcmpun
__aeabi_dcmpun and __aeabi_uldivmod.

> 
> 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