Re: [PATCH 2/2] ARM: rust: Enable Rust support for ARMv5TE

[Karl Mehltretter <kmehltretter@gmail.com>]

On Sat, Oct 03, 2026 at 12:45:07PM +0100, Arnd Bergmann wrote:

Hi Arnd,

> > Kernels that also contain ARMv4 or ARMv4T CPUs are built for the
> > lowest architecture and stay excluded. CPU_32v5 also covers the ARMv5T
> > ARM1020. C code is already built with -march=armv5te there, so Rust
> > matches.
> 
> None of this makes sense to me: The target should not control
> the instruction set, that is what the -march= flag is needed for.
> Does that not get passed for Rust?

No. arch/arm/Makefile only passes --target=arm-unknown-linux-gnueabi,
no CPU or feature flags. That target has "+v6" built in, so Rust code
in an ARMv7 kernel is built as ARMv6 today.

I tried the generic target with CPU flags (libcore for versatile on
v7.3-rc1, rustc 1.99.0, CPU arch from the build attributes of core.o):

  --target=arm-unknown-linux-gnueabi
      ARMv6, uses uxtb/uxth/sxth/rev
  --target=arm-unknown-linux-gnueabi -Ctarget-cpu=arm926ej-s
      still ARMv6
  --target=arm-unknown-linux-gnueabi -Ctarget-cpu=arm926ej-s
  -Ctarget-feature=-v6
      ARMv5TE, but every rustc call warns
      "unstable feature specified for `-Ctarget-feature`: `v6`"
  --target=armv5te-unknown-linux-gnueabi
      ARMv5TE
  --target=armv4t-unknown-linux-gnueabi
      ARMv4T, no clz, no blx

So -Ctarget-cpu can add features but does not remove the +v6. The
generic target also declares 64-bit atomics, armv5te and armv4t only
32 bit. Below ARMv6 I only see the separate targets.

> 
> If an ARMv7 kernel includes ARMv6 instructions, that is broken
> on ARMv8 CPUs that are lacking the CP15 barriers and swp style
> atomics, so that needs to be fixed.

The Rust objects from the generic target have no swp and no CP15
access. The atomics go through the C helpers.

Building the Rust code of ARMv7 kernels as ARMv7 would be a separate
change. I can look at that after this series.

> 
> I don't see what part of rust would depend on ARMv5 instructions,
> it should just work on ARMv4T as well, though ARMv4 may be
> trickier because missing bx instructions etc.

Agreed. !CPU_32v4T only came from the armv5te target, which emits clz
and blx. With the armv4t target it can go. ARMv4 has no rustc target.

> 
> > ==============================================
> > -``arm``        Maintained        ARMv7 Little Endian only.
> > +``arm``        Maintained        ARMv5TE and ARMv7, Little Endian only.
> 
> Here you exclude ARMv6K and ARMv8-A-aarch32...
> [..] 
> but here you allow it, so I think one of them should change,
> 

Right. A v6K+v7 kernel has CPU_32v7 and gets HAVE_RUST already, a
v6K-only kernel does not. No technical reason. I have a patch for
v6K-only that I held back until I have a Pi 1, but I guess QEMU
suffices.

> This looks wrong, the choice between armv5 and armv7 should work
> the same way as the choice between armv6 and armv7/v8, if I read
> the rustc docs correctly, this should be using the target-cpu=
> argument on the generic arm-unknown-linux-gnueabi target.

There is no choice between ARMv6 and ARMv7 today. Both get the generic
target without flags and so ARMv6 code. And target-cpu= on the generic
target does not get below ARMv6, see the table above. That is why I
used a separate target for ARMv5.

For a v2 of this I would

- pick the rustc target next to the -march lines: armv4t for
  CPU_32v4T, armv5te for CPU_32v5, the generic one from ARMv6K upwards
- select HAVE_RUST for everything except CPU_32v4 and plain CPU_V6
- make arch-support.rst say the same

I have this running in QEMU on v7.3-rc1 with
- sx1 (OMAP310, ARM925T, ARMv4T), omap1_defconfig
- versatilepb (ARM926EJ-S), versatile_defconfig
- raspi0 (ARM1176), bcm2835_defconfig without ARCH_MULTI_V7

Would that be ok for you?

Thanks
Karl