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

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

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@gmail.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.
> 
> 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...)

I've done a quick test on armv5 with the `armv5te-none-eabi` rust 
target and it needs some atomics (`AtomicU64` type and for 
`AtomicBool` the `compare_exchange` and `compare_exchange` 
functions) to build:

```
  RUSTC L rust/kernel.o
error[E0432]: unresolved import `core::sync::atomic::AtomicU64`
  --> rust/kernel/block/mq/operations.rs:15:33
   |
15 | use core::{marker::PhantomData, sync::atomic::AtomicU64, sync::atomic::Ordering};
   |                                 ^^^^^^^^^^^^^^---------
   |                                 |             |
   |                                 |             help: a similar name exists in the module: `AtomicU8`
   |                                 no `AtomicU64` in `sync::atomic`

error[E0432]: unresolved import `core::sync::atomic::AtomicU64`
  --> rust/kernel/block/mq/request.rs:16:20
   |
16 |     sync::atomic::{AtomicU64, Ordering},
   |                    ^^^^^^^^^
   |                    |
   |                    no `AtomicU64` in `sync::atomic`
   |                    help: a similar name exists in the module: `AtomicU8`

error[E0599]: no method named `compare_exchange` found for struct `AtomicBool` in the current scope
   --> rust/kernel/list/arc.rs:518:14
    |
517 | /         self.inner
518 | |             .compare_exchange(false, true, Ordering::Acquire, Ordering::Relaxed)
    | |             -^^^^^^^^^^^^^^^^ method not found in `AtomicBool`
    | |_____________|
    |

error[E0599]: no method named `swap` found for struct `AtomicBool` in the current scope
   --> rust/kernel/revocable.rs:130:30
    |
130 |         if self.is_available.swap(false, Ordering::Relaxed) {
    |                              ^^^^ method not found in `AtomicBool`

error: aborting due to 4 previous errors

Some errors have detailed explanations: E0432, E0599.
For more information about an error, try `rustc --explain E0432`.
```

When gating all the modules that need these linking fails because
of missing __eabi__* intrinsics:

```
ld.lld: error: undefined symbol: __aeabi_memcpy
>>> referenced by kernel.2bb770ae1dba3d33-cgu.0
>>>               rust/kernel.o:(<kernel::str::RawFormatter as core::fmt::Write>::write_char) in archive vmlinux.a
>>> referenced by kernel.2bb770ae1dba3d33-cgu.0
>>>               rust/kernel.o:(<kernel::str::Formatter as core::fmt::Write>::write_char) in archive vmlinux.a
>>> referenced by kernel.2bb770ae1dba3d33-cgu.0
>>>               rust/kernel.o:(<kernel::page::Page>::read_raw) in archive vmlinux.a
>>> referenced 29 more times

ld.lld: error: undefined symbol: __aeabi_memclr8
>>> referenced by bindings.4cab29b7397d35cb-cgu.0
>>>               rust/bindings.o:(<bindings::bindings_raw::module as core::default::Default>::default) in archive vmlinux.a
>>> referenced by uapi.fe46408b8870a0f6-cgu.0
>>>               rust/uapi.o:(<uapi::module as core::default::Default>::default) in archive vmlinux.a
>>> referenced by uapi.fe46408b8870a0f6-cgu.0
>>>               rust/uapi.o:(<uapi::thread_info as core::default::Default>::default) in archive vmlinux.a
>>> referenced 86 more times

ld.lld: error: undefined symbol: __aeabi_memclr4
>>> referenced by bindings.4cab29b7397d35cb-cgu.0
>>>               rust/bindings.o:(<bindings::bindings_raw::fp_hard_struct as core::default::Default>::default) in archive vmlinux.a
>>> referenced by bindings.4cab29b7397d35cb-cgu.0
>>>               rust/bindings.o:(<bindings::bindings_raw::iwmmxt_struct as core::default::Default>::default) in archive vmlinux.a
>>> referenced by uapi.fe46408b8870a0f6-cgu.0
>>>               rust/uapi.o:(<uapi::fp_soft_struct as core::default::Default>::default) in archive vmlinux.a
>>> referenced 95 more times

ld.lld: error: undefined symbol: __aeabi_memset
>>> referenced by core.64aa0a46a0f8f964-cgu.0
>>>               rust/core.o:(<core::num::fmt::Part>::write) in archive vmlinux.a
>>> referenced by core.64aa0a46a0f8f964-cgu.0
>>>               rust/core.o:(core::fmt::num::fmt_u128) in archive vmlinux.a

ld.lld: error: undefined symbol: __aeabi_memcpy4
>>> referenced by kernel.2bb770ae1dba3d33-cgu.0
>>>               rust/kernel.o:(<kernel::of::DeviceId>::new) in archive vmlinux.a
>>> referenced by core.64aa0a46a0f8f964-cgu.0
>>>               rust/core.o:(<core::char::ToLowercase as core::fmt::Display>::fmt) in archive vmlinux.a

ld.lld: error: undefined symbol: __aeabi_memclr
>>> referenced by kernel.2bb770ae1dba3d33-cgu.0
>>>               rust/kernel.o:(<kernel::page::Page>::fill_zero_raw) in archive vmlinux.a
>>> referenced by uapi.fe46408b8870a0f6-cgu.0
>>>               rust/uapi.o:(<uapi::__kernel_sockaddr_storage__bindgen_ty_1__bindgen_ty_1 as core::default::Default>::default) in archive vmlinux.a
>>> referenced by uapi.fe46408b8870a0f6-cgu.0
>>>               rust/uapi.o:(<uapi::sockaddr__bindgen_ty_1 as core::default::Default>::default) in archive vmlinux.a
>>> referenced 16 more times

ld.lld: error: undefined symbol: __aeabi_memset4
>>> referenced by core.64aa0a46a0f8f964-cgu.0
>>>               rust/core.o:(core::fmt::num::fmt_u128) in archive vmlinux.a
make[2]: *** [scripts/Makefile.vmlinux:91: vmlinux] Error 1
make[1]: *** [/home/chrisi/Documents/code/rust-arm32/Makefile:1242: vmlinux] Error 2
make: *** [Makefile:248: __sub-make] Error 2
```

With define_panicking_intrinsics! for these an empty sample works
(not yet tested as module), but even printing a non formatted
string (with `pr_info!("Hi rust\n");`) fails because rust tries
to use __adddf3:

``
Hardware name: ARM-Versatile (Device Tree Support)
PC is at __rust__adddf3+0x0/0x4
LR is at _RNvXsg_NtCs3KHxpmQFgFb_6kernel3strNtB5_12RawFormatterNtNtCs8DPF7ip8WBQ_4core3fmt5Write9write_str+0x34/0x40
pc : [<c028950c>]    lr : [<c028b5ec>]    psr: 20000193
sp : c8811a78  ip : c8811ae8  fp : c058e44e
r10: 00000000  r9 : c058e450  r8 : c07ac118
r7 : 00000000  r6 : 00000000  r5 : c07ac120  r4 : c8811adc
r3 : c07ac120  r2 : 00000008  r1 : c0667f60  r0 : c07ac118
Flags: nzCv  IRQs off  FIQs on  Mode SVC_32  ISA ARM  Segment none
```

I'm not sure how to proceed from here and I most likely won't
find time to continue working on this anytime soon.

My very hacky changes are available on Github at:
https://github.com/onestacked/linux/commit/edddb388267ab1afe5255bea4d84f96cbdb37978

Cheers,
Christian