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Re: Package for development of out-of-tree kernel modules written in Rust

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Hi Miguel,

Sorry for the delay, I wanted to do some tests before answering.


On 06/04/25 23:21, Miguel Ojeda wrote:
> So you want to build a kernel for architecture X in a host of
> architecture Y, and then build an out-of-tree module for architecture
> X in a host of architecture Z (possibly X)?

This is my understanding based on what the kernel maintainers wrote in 
this thread. At least in terms of enablement (i.e. wanting to support 
this scenario, not sure they actually need to do such builds).

> As far as I have been told, the kernel generally requires that the
> exact same toolchain is used to build out-of-tree modules as the main
> kernel  -- so the same host/target pair should be used for both the
> kernel build as the out-of-tree builds. Of course, things may happen
> to work otherwise.

This is what I wanted to test. Using a different architecture's 
toolchain doesn't seem to work, even if the toolchain versions are the 
same. I built two twin kernels for arm64, first on an x86_64 host and 
then on an arm64 host. The toolchains were the exact same version-wise, 
but of course not the same in practice, being for two different hosts. 
Cross-compilation per se seems to work fine: I successfully 
cross-compiled the reference out-of-tree module using the build 
artifacts for the cross-compiled kernel and it loaded correctly on the 
target machine (after installing the cross-compiled kernel).

On the other hand, I was not able to compile the module on arm64 using 
the build files from the cross-compiled kernel. Since libmacros.so was 
compiled for x86_64, I tried using both the libmacros.so from the twin 
native build and a third libmacros.so which I cross-compiled using the 
same toolchain as the cross-compiled kernel. Neither of these worked, 
the build system failed to recognize the build artifacts:


```error[E0463]: can't find crate for `kernel`
  --> /home/noisycoil/Tmp/rust-out-of-tree-module/rust_out_of_tree.rs:5:5
   |
5 | use kernel::prelude::*;
   |     ^^^^^^ can't find crate

error: cannot find macro `pr_info` in this scope
   --> /home/noisycoil/Tmp/rust-out-of-tree-module/rust_out_of_tree.rs:35:9
    |
35 |         pr_info!("Rust out-of-tree sample (exit)\n");
```


Similarly, I was unable to cross-compile the module on x86_64 using the 
build files from the natively compiled kernel (again except for 
libmacros.so, which I took from the cross-build). In doing so I got the 
very same errors as above.

So I can confirm that builds in fact do not seem to work either way 
currently, the toolchain must be the exact same, as expected. In 
particular, it seems that the arch/config-(in)dependence of libmacros.so 
is not the main blocker here.

> Is the reason that there is a build farm on architecture Y that builds
> all the kernels for all X, and then you want users to be able to use
> their architecture X to build out-of-tree modules for their kernel on
> X? i.e. the X == Z case.

In Debian proper we build everything natively (32-bit builds usually 
being done on corresponding 64-bit machines). I cannot speak for the 
kernel maintainers, but my guess would be this discussion is in part 
motivated by simply wanting to support the cross-compilation scenario, 
in part by wanting to adapt to the current packaging scheme (some 
packages are for the host machine, others are for the target machine, 
libmacros.so technically is for the host machine so it should go in the 
corresponding package which is config-independent, etc.).

> Cheers,
> Miguel

Cheers!

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Thread

Package for development of out-of-tree kernel modules written in Rust NoisyCoil <noisycoil@disroot.org> - 2025-02-26 01:30 +0100
  Re: Package for development of out-of-tree kernel modules written in  Rust Bastian Blank <waldi@debian.org> - 2025-02-26 12:50 +0100
    Re: Package for development of out-of-tree kernel modules written in  Rust NoisyCoil <noisycoil@disroot.org> - 2025-02-26 13:20 +0100
      Re: Package for development of out-of-tree kernel modules written in Rust Miguel Ojeda <miguel.ojeda.sandonis@gmail.com> - 2025-02-26 14:00 +0100
        Re: Package for development of out-of-tree kernel modules written in  Rust NoisyCoil <noisycoil@disroot.org> - 2025-02-26 15:20 +0100
          Re: Package for development of out-of-tree kernel modules written in Rust Miguel Ojeda <miguel.ojeda.sandonis@gmail.com> - 2025-02-26 19:30 +0100
            Re: Package for development of out-of-tree kernel modules written in  Rust NoisyCoil <noisycoil@disroot.org> - 2025-02-26 22:00 +0100
              Re: Package for development of out-of-tree kernel modules written in Rust Miguel Ojeda <miguel.ojeda.sandonis@gmail.com> - 2025-02-27 18:40 +0100
        Re: Package for development of out-of-tree kernel modules written  in Rust Ben Hutchings <ben@decadent.org.uk> - 2025-02-26 19:20 +0100
          Re: Package for development of out-of-tree kernel modules written in  Rust NoisyCoil <noisycoil@disroot.org> - 2025-02-26 19:30 +0100
            Re: Package for development of out-of-tree kernel modules written in Rust Miguel Ojeda <miguel.ojeda.sandonis@gmail.com> - 2025-02-26 20:10 +0100
              Re: Package for development of out-of-tree kernel modules written in  Rust NoisyCoil <noisycoil@disroot.org> - 2025-02-26 20:20 +0100
                Re: Package for development of out-of-tree kernel modules written in Rust Miguel Ojeda <miguel.ojeda.sandonis@gmail.com> - 2025-02-27 18:40 +0100
              Re: Package for development of out-of-tree kernel modules written in  Rust Bastian Blank <waldi@debian.org> - 2025-04-02 23:00 +0200
                Re: Package for development of out-of-tree kernel modules written in  Rust Bastian Blank <waldi@debian.org> - 2025-04-02 23:30 +0200
                Re: Package for development of out-of-tree kernel modules written in  Rust NoisyCoil <noisycoil@disroot.org> - 2025-04-02 23:40 +0200
                Re: Package for development of out-of-tree kernel modules written in  Rust NoisyCoil <noisycoil@disroot.org> - 2025-04-03 00:50 +0200
                Re: Package for development of out-of-tree kernel modules written in  Rust Bastian Blank <waldi@debian.org> - 2025-04-03 16:50 +0200
                Re: Package for development of out-of-tree kernel modules written in  Rust NoisyCoil <noisycoil@disroot.org> - 2025-04-03 19:30 +0200
                Re: Package for development of out-of-tree kernel modules written in  Rust Bastian Blank <waldi@debian.org> - 2025-04-03 21:00 +0200
                Re: Package for development of out-of-tree kernel modules written in Rust Miguel Ojeda <miguel.ojeda.sandonis@gmail.com> - 2025-04-03 23:30 +0200
                Re: Package for development of out-of-tree kernel modules written in  Rust NoisyCoil <noisycoil@disroot.org> - 2025-04-03 23:40 +0200
                Re: Package for development of out-of-tree kernel modules written in Rust Miguel Ojeda <miguel.ojeda.sandonis@gmail.com> - 2025-04-06 23:40 +0200
                Re: Package for development of out-of-tree kernel modules written in  Rust NoisyCoil <noisycoil@disroot.org> - 2025-04-12 18:50 +0200
                Re: Package for development of out-of-tree kernel modules written in  Rust Bastian Blank <waldi@debian.org> - 2025-04-13 13:40 +0200
                Re: Package for development of out-of-tree kernel modules written in  Rust NoisyCoil <noisycoil@disroot.org> - 2025-04-13 17:40 +0200
                Re: Package for development of out-of-tree kernel modules written in Rust Miguel Ojeda <miguel.ojeda.sandonis@gmail.com> - 2025-04-13 18:40 +0200
                Re: Package for development of out-of-tree kernel modules written in  Rust NoisyCoil <noisycoil@disroot.org> - 2025-04-13 20:30 +0200
                Re: Package for development of out-of-tree kernel modules written  in Rust Ben Hutchings <ben@decadent.org.uk> - 2025-04-16 17:40 +0200
                Re: Package for development of out-of-tree kernel modules written in Rust Miguel Ojeda <miguel.ojeda.sandonis@gmail.com> - 2025-05-03 17:10 +0200
                Re: Package for development of out-of-tree kernel modules written in  Rust Bastian Blank <waldi@debian.org> - 2025-04-04 11:20 +0200
                Re: Package for development of out-of-tree kernel modules written in Rust Miguel Ojeda <miguel.ojeda.sandonis@gmail.com> - 2025-04-06 23:40 +0200
          Re: Package for development of out-of-tree kernel modules written in Rust Miguel Ojeda <miguel.ojeda.sandonis@gmail.com> - 2025-02-26 19:40 +0100
      Re: Package for development of out-of-tree kernel modules written in  Rust NoisyCoil <noisycoil@disroot.org> - 2025-02-26 14:40 +0100
      Re: Package for development of out-of-tree kernel modules written in  Rust Bastian Blank <waldi@debian.org> - 2025-02-26 14:40 +0100

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