linux-kernel - Re: [PATCH 5/7] rust: file: add `Kuid` wrapper

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Date:   Thu, 7 Dec 2023 21:28:06 -0800
From:   comex <comexk@...il.com>
To:     Christian Brauner <brauner@...nel.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Alice Ryhl <aliceryhl@...gle.com>,
        Miguel Ojeda <ojeda@...nel.org>,
        Alex Gaynor <alex.gaynor@...il.com>,
        Wedson Almeida Filho <wedsonaf@...il.com>,
        Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>,
        Björn Roy Baron <bjorn3_gh@...tonmail.com>,
        Benno Lossin <benno.lossin@...ton.me>,
        Andreas Hindborg <a.hindborg@...sung.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Arve Hjønnevåg <arve@...roid.com>,
        Todd Kjos <tkjos@...roid.com>,
        Martijn Coenen <maco@...roid.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Carlos Llamas <cmllamas@...gle.com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Kees Cook <keescook@...omium.org>,
        Matthew Wilcox <willy@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Daniel Xu <dxu@...uu.xyz>, linux-kernel@...r.kernel.org,
        rust-for-linux@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH 5/7] rust: file: add `Kuid` wrapper

> On Nov 30, 2023, at 4:46 AM, Christian Brauner <brauner@...nel.org> wrote:
> 
> I wouldn't even
> complain if it they were somehow auto-generated but as you say that
> might be out of scope.

FYI, rust-bindgen got an experimental feature of this nature earlier this year:

https://github.com/rust-lang/rust-bindgen/pull/2335

Though apparently it has significant limitations meriting it the “experimental” title.

Regarding the issue of wrappers not being inlined, it's possible to get LLVM to optimize C and Rust code together into an object file, with the help of a compatible Clang and LLD:

@ rustc -O --emit llvm-bc a.rs                                         
@ clang --target=x86_64-unknown-linux-gnu -O2 -c -emit-llvm -o b.bc b.c
@ ld.lld -r -o c.o a.bc b.bc

Basically LTO but within the scope of a single object file.  This would be redundant in cases where kernel-wide LTO is enabled.

Using this approach might slow down compilation a bit due to needing to pass the LLVM bitcode between multiple commands, but probably not very much.

Just chiming in as someone not involved in Rust for Linux but familiar with these tools.  Perhaps this has been considered before and rejected for some reason; I wouldn’t know.