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Subject: Re: [PATCH 5/7] rust: file: add `Kuid` wrapper
From:   comex <comexk@gmail.com>
In-Reply-To: <CANiq72k4H2_NZuQcpeKANqyi_9W01fLC0WxXon5cx4z=WsgeXQ@mail.gmail.com>
Date:   Fri, 8 Dec 2023 23:24:35 -0800
Cc:     Christian Brauner <brauner@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Alice Ryhl <aliceryhl@google.com>,
        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>,
        Andreas Hindborg <a.hindborg@samsung.com>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        =?utf-8?Q?Arve_Hj=C3=B8nnev=C3=A5g?= <arve@android.com>,
        Todd Kjos <tkjos@android.com>,
        Martijn Coenen <maco@android.com>,
        Joel Fernandes <joel@joelfernandes.org>,
        Carlos Llamas <cmllamas@google.com>,
        Suren Baghdasaryan <surenb@google.com>,
        Dan Williams <dan.j.williams@intel.com>,
        Kees Cook <keescook@chromium.org>,
        Matthew Wilcox <willy@infradead.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Daniel Xu <dxu@dxuuu.xyz>, linux-kernel@vger.kernel.org,
        rust-for-linux@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        Nick Desaulniers <ndesaulniers@google.com>
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To:     Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>
Precedence: bulk

On Dec 8, 2023, at 8:19=E2=80=AFAM, Miguel Ojeda =
<miguel.ojeda.sandonis@gmail.com> wrote:
>=20
> If LLVM folks think LLVM-wise nothing will break, then we are happy to
> go ahead with that (since it also solves the performance side), but it
> would be nice to know if it will always be OK to build like that, i.e.
> I think Andreas actually tried it and it seemed to work and boot, but
> the worry is whether there is something subtle that could have bad
> codegen in the future.

One potential issue is incompatibility between the LLVM versions used by =
rustc, Clang, and LLD.  At minimum, whichever tool is reading bitcode =
(LLD in my example) should have an LLVM version >=3D that of the tools =
producing bitcode, since newer LLVM versions can read older bitcode but =
not vice versa.  But ideally the tools would all just be linked against =
the same copy of LLVM.

If you=E2=80=99re getting your tools from a distro, then that may =
already be true for you.  But if you=E2=80=99re using upstream rustc =
binaries, those are built against a custom branch of LLVM, which is =
based on upstream release versions but adds a handful of patches [1]; by =
policy, those patches can include cherry-picks of miscompilation fixes =
that are upstream but haven=E2=80=99t made it into a release yet [2].  =
Upstream rustc binaries are accompanied by a copy of LLD linked against =
the same LLVM library, named rust-lld, but there=E2=80=99s no =
corresponding copy of Clang [3].  I=E2=80=99d say that agreement between =
rustc and LLD is the most important thing, but it would be nice if =
they'd make a matching Clang available through rustup.

[1] =
https://github.com/llvm/llvm-project/compare/release/17.x...rust-lang:llvm=
-project:rustc/17.0-2023-09-19
[2] =
https://rustc-dev-guide.rust-lang.org/backend/updating-llvm.html#bugfix-up=
dates
[3] https://github.com/rust-lang/rust/issues/56371=