[PATCH v8 27/31] Kbuild: add Rust support

[Björn Roy Baron]

On Wednesday, August 17th, 2022 at 17:13, Miguel Ojeda <miguel.ojeda.sandonis at gmail.com> wrote:

> Hi Arnd,
> 
> On Wed, Aug 17, 2022 at 4:40 PM Arnd Bergmann arnd at arndb.de wrote:
> 
> > Hi Miguel,
> > 
> > I tried enabling rust support in the gcc builds I provide at
> > https://mirrors.edge.kernel.org/pub/tools/crosstool/files/bin/arm64/12.1.0/
> 
> 
> Thanks for giving it a go!
> 
> > to make this more accessible, but it appears that the command line
> > options here are not portable:
> > 
> > /home/arnd/cross/x86_64/gcc-12.1.0+rust-nolibc/x86_64-linux/bin/x86_64-linux-gccrs
> 
> 
> So you mean with GCC Rust, right? (i.e. we have "GCC builds" working,
> via compiling the Rust side with LLVM and linking with the GCC C side,
> but it is not intended for production or to be supported, even if we
> cover it in our CI, test it boots and loads modules etc.).
> 
> Indeed, `gccrs` does not support `rustc` flags yet. I am not sure if
> the GCC Rust team will eventually provide a driver for those like
> clang does for e.g. `cl` -- I would hope they do, since many projects
> would benefit from it, but maybe they plan to start simply by
> modifying Cargo to call them as they need instead.

There is already a prototype of such a driver. It can be found at https://github.com/Rust-GCC/cargo-gccrs. Unlike what the name suggests it is not cargo specific. It consists of two binaries. The first calls cargo, but tells it to use the second binary instead of a real rustc. This second part then translates all arguments to what gccrs expects. It is possible to directly invoke this second binary. For now it probably won't work for rust-for-linux though as it doesn't have all arguments that are used by rust-for-linux implemented.

> 
> If they don't support it, we will have to map the flags on our side --
> it should not be a big problem. However, see below...
> 
> > I guess nobody has tried this so far. Would you think that fixing this is only
> > a matter for fixing the build system to pass the correct flags depending on the
> > compiler, or is this broken in a more fundamental way?
> 
> 
> If you meant GCC Rust, then it is a bit too early for the compiler. As
> far as I now, they are working on compiling the `core` crate and
> supporting more stable language features. They are also researching
> the integration of the borrow checker, though we wouldn't need that
> for "only" compiling the kernel.
> 
> Now, if they decided to focus on supporting Rust for Linux early on
> (which would be great), they would still need to work on the delta
> between what what they target now and what we use (which includes both
> stable and some unstable features), plus I assume infrastructure bits
> like the platform (target spec) support, the flags / `rustc` driver
> (though I would be happy to do as much as possible on our side to
> help), etc.
> 
> (We privately talked about possible timelines for all that if they
> were to focus on Rust for Linux etc., but I let them comment or not on
> that... Cc'ing them! :)
> 
> Cheers,
> Miguel

As alternative to GCC Rust there is also github.com/rust-lang/rustc_codegen_gcc/ which uses libgccjit as backend for the official rust compiler rather than writing a full Rust frontend for GCC from scratch. With a bit of patching to force it to be used, I was able to compile all Rust samples with GCC using rustc_codegen_gcc. However it gives warnings of the following kind:

    ld.lld: warning: rust/built-in.a(core.o):(.data.rel.local) is being placed in '.data.rel.local'

And hangs very early in the boot process. If I enable early logging, it prints up to "Booting the kernel." and then does nothing. This is probably because support for setting a different relocation model is not yet implemented. I opened https://github.com/rust-lang/rustc_codegen_gcc/issues/205 for this.

There may be other issues, but rustc_codegen_gcc is probably going to be the easiest route towards a LLVM free rust-for-linux build. By the way note that rust-bindgen which we use for generating rust bindings from C headers depends on LLVM. See https://github.com/rust-lang/rust-bindgen/issues/1949.

Cheers,
Bjorn