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

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

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.
> 
>> 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?
> 
> The 64-bit division is particularly easy to introduce by accident
> on 32-bit architectures, so ending up in a panic here is clearly
> a problem. From the message above it appears that there is only 
> a single calling function (parse_u64_into()) in the rust library,
> so I wonder if it might be sufficient to split that out into
> another object file that then doesn't need to get linked into
> the kernel, or for the kernel to override it with an implementation
> that does not rely on __aeabi_uldivmod() but calls __do_div64()
> instead.
> 
> Since parse_u64_into seems to be a parsing function that is
> expected to be slow, it should be acceptable to call __do_div64()
> here, while we still prevent calling __aeabi_uldivmod() from
> kernel source code.
> 
> Note that on earlier ARMv7 (Cortex-A8, A9), even a 32-bit
> division is implemented through an expensive software loop.
> Later cores (Cortex-A7, A15, A17) have native 32-bit division
> instructions but still no 64-bit ones.

It seems be possible to implement `__aeabi_uldivmod` like this [0]:

#[naked]
#[cfg(target_arch = "arm")]
#[export_name = "__rust__aeabi_uldivmod"]
pub unsafe extern "C" fn __aeabi_uldivmod() {
    unsafe {
        core::arch::naked_asm!(
            "push {{r4, lr}}",
            "sub sp, sp, #16",
            "add r4, sp, #8",
            "str r4, [sp]",
            "bl __udivmoddi4",
            "ldr r2, [sp, #8]",
            "ldr r3, [sp, #12]",
            "add sp, sp, #16",
            "pop {{r4, pc}}",
        );
    }
}

However that requires the `naked_functions` unstable feature.
Or it should be possible to just implement in a asm file.

I think it would be very difficult to entirely build core 
without needing `__aeabi_uldivmod`. 

Link: https://github.com/rust-lang/compiler-builtins/blob/6f96bccc5d4aa3ba4c4cebdf23a3ccc3bc7fe77c/src/arm.rs#L64-L77 [0]

Christian