Re: [PATCH v2 08/15] kernel-yocto.bbclass: Disable ccache when rust-kernel is enabled

[alban.moizan@smile.fr]

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Excuse me for the double mail, I was not yet subscribing to the mailing list, so my mail was hidden.

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Hi everyone,

I've been investigating the issue when enabling `ccache` for kernel builds that involve Rust.

First of all, as documented in the `ccache` issue tracker [1], `ccache` is not directly compatible with Rust.
Therefore the current goal is to keep `ccache` enabled for the C portions of the kernel while bypassing it for Rust.
While an alternative like `sccache` [2], compatible with both C and Rust exists, you will see below that the underlying issue is the same, so just replacing `ccache` by `sccache` will change nothing here.

The issue (commit 235e6d49e5888ad04416219e10b6df91a738661a + this patch):

When `ccache` is enabled, `${CC}` to `${CCACHE}{CC}`, ex: `gcc` to `ccache gcc`

Then, when compiling the kernel, `scripts/Makefile.host` will give it to `rustc` with `-Clinker=$(HOSTCC)` [3], but the shell expands `$(HOSTCC)` to two words, therefore `rustc` takes `gcc` as another parameters, and it errors out. 

I attempted to resolve it by modifying the incriminated Makefile in the following ways:

Trial 1: Quoting HOSTCC in Kernel Makefile
Modify `scripts/Makefile.host` to wrap `HOSTCC` in quotes to ensure it is passed as a single string parameter:
Replace `-Clinker=$(HOSTCC)` to `-Clinker='$(HOSTCC)'`
Result: Failure, `rustc` looks for a binary path literally named `ccache gcc`, which does not exist, indeed, `-Clinker` argument value is treated by `rustc` as a real path, and not a command [4].

Trial 2: Splitting HOSTCC into Linker and Pre-link Args
Modified `scripts/Makefile.host` to force the linker to the first word of `HOSTCC` and pass subsequent words to `-Zpre-link-args`:
Replace `-Clinker=$(HOSTCC)` by `-Clinker=$(firstword $(HOSTCC)) -Zpre-link-args='$(wordlist 2,$(words $(HOSTCC)),$(HOSTCC))'`
Result: Failure, `rustc` executes the command as: "ccache" "-m64" "gcc" ....
This is invalid because in `rustc`, the target-specific pre-link arguments (from `compiler/rustc_target/src/spec/targets/x86_64_unknown_linux_gnu.rs`) are added before our custom args, placing `-m64` before the `gcc` executable [5] .

Proposed Solutions
1. Kernel + Rust Patch (A): Patch the kernel Makefile (like the Trial 1) and patch `rustc` to accept a command string (with arguments) for `-Clinker` instead of a strict Path.
2. Kernel + Rust Patch (B): Patch the kernel Makefile (like the Trial 2) and patch `rustc` to use `post_link_args` instead of `pre_link_args` for target-specific values, ensuring the executable stays at the end of the command.
3. Shell Wrappers: Implement shell wrappers for CC, CXX, etc. This would allow `rustc` to call a single wrapper script, hiding `ccache` from it.
4. Kernel only: Patch the kernel Makefile to filter out `ccache` from CC (works in practice but not generic for a possible other wrapper in the future).
5. Leave as-is: disable `ccache` when rust is enabled .

Risks and Considerations
- Upstreamability: Patching rust and the kernel at the same time could take time. 
- Maintainability: While the shell wrapper approach is more flexible and solves the issue for other tools with similar limitations, it might make debugging in Yocto more difficult and requires significant changes.

I'm leaning towards the 1st or the 3rd approach, but I'd appreciate the maintainers' thoughts on which path fits the OE architecture best.

[1] https://github.com/ccache/ccache/issues/364

[2] https://github.com/mozilla/sccache

[3] https://git.yoctoproject.org/linux-yocto/tree/scripts/Makefile.host?h=v6.18.1#n94

[4] https://github.com/rust-lang/rust/blob/4931e09e3ac3182d2a00f38cccfdf68e8e385e1c/compiler/rustc_codegen_ssa/src/back/link.rs#L1419

[5] https://github.com/rust-lang/rust/blob/4931e09e3ac3182d2a00f38cccfdf68e8e385e1c/compiler/rustc_codegen_ssa/src/back/link.rs#L1870

Best regards,

Alban Moizan


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