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From:   Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>
Date:   Tue, 11 Jul 2023 16:39:27 +0200
Message-ID: <CANiq72k6um58AAydgkzhkmAdd8t1quzeGaPsR7-pS_ZXYf0-YQ@mail.gmail.com>
Subject: Re: [PATCH] btf, scripts: rust: drop is_rust_module.sh
To:     Andrea Righi <andrea.righi@canonical.com>
Cc:     Miguel Ojeda <ojeda@kernel.org>,
        Alex Gaynor <alex.gaynor@gmail.com>,
        Wedson Almeida Filho <wedsonaf@gmail.com>,
        Boqun Feng <boqun.feng@gmail.com>, Gary Guo <gary@garyguo.net>,
        =?UTF-8?Q?Bj=C3=B6rn_Roy_Baron?= <bjorn3_gh@protonmail.com>,
        Benno Lossin <benno.lossin@proton.me>,
        Masahiro Yamada <masahiroy@kernel.org>,
        Nathan Chancellor <nathan@kernel.org>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Nicolas Schier <nicolas@fjasle.eu>, Tom Rix <trix@redhat.com>,
        linux-kernel@vger.kernel.org, rust-for-linux@vger.kernel.org,
        linux-kbuild@vger.kernel.org, llvm@lists.linux.dev,
        bpf <bpf@vger.kernel.org>,
        Martin KaFai Lau <martin.lau@linux.dev>,
        Arnaldo Carvalho de Melo <acme@kernel.org>
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On Tue, Jul 4, 2023 at 7:21=E2=80=AFAM Andrea Righi <andrea.righi@canonical=
.com> wrote:
>
> With commit c1177979af9c ("btf, scripts: Exclude Rust CUs with pahole")
> we are now able to use pahole directly to identify Rust compilation
> units (CUs) and exclude them from generating BTF debugging information
> (when DEBUG_INFO_BTF is enabled).
>
> And if pahole doesn't support the --lang-exclude flag, we can't enable
> both RUST and DEBUG_INFO_BTF at the same time.
>
> So, in any case, the script is_rust_module.sh is just redundant and we
> can drop it.
>
> NOTE: we may also be able to drop the "Rust loadable module" mark
> inside Rust modules, but it seems safer to keep it for now to make sure
> we are not breaking any external tool that may potentially rely on it.

Just to recall the history of these changes:

  - The script got added in order to skip the BTF generation in the
`BTF [M]` step (under `DEBUG_INFO_BTF_MODULES`, which depends on
`DEBUG_INFO_BTF`).

  - A few months later, it was noticed that C modules couldn't be
loaded if Rust was enabled, due to the base BTF info in `vmlinux`.
That triggered the eventual addition of `--lang_exclude=3D` to `pahole`,
but meanwhile, we made `DEBUG_INFO_BTF` and `RUST` exclusive.

  - Now, this patch removes the script because having a newer `pahole`
also correctly skips the Rust CUs in the `BTF [M]` steps (i.e. and not
just the `vmlinux` one), since we pass `--lang_exclude=3D` to both cases
(`link-vmlinux.sh` and `Makefile.modfinal`), if I understand correctly
(the script could, in principle, have been removed even before
`pahole` got the new feature, given the exclusivity of the options).

If this is all correct, then the patch looks good to me. I am Cc'ing
Arnaldo, Martin and the BPF list.

If this goes through the Rust tree, I will also pick the older `Reviewed-by=
`s.

Thanks!

Cheers,
Miguel