linux-kernel - Re: [PATCH 05/17] rust: add C helpers

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Date:   Fri, 9 Jul 2021 16:32:27 +0200
From:   Miguel Ojeda <miguel.ojeda.sandonis@...il.com>
To:     Marco Elver <elver@...gle.com>
Cc:     Miguel Ojeda <ojeda@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        rust-for-linux <rust-for-linux@...r.kernel.org>,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        Linux Doc Mailing List <linux-doc@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Alex Gaynor <alex.gaynor@...il.com>,
        Geoffrey Thomas <geofft@...reload.com>,
        Finn Behrens <me@...enk.de>,
        Adam Bratschi-Kaye <ark.email@...il.com>,
        Wedson Almeida Filho <wedsonaf@...gle.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Sumera Priyadarsini <sylphrenadin@...il.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Sven Van Asbroeck <thesven73@...il.com>,
        Gary Guo <gary@...yguo.net>,
        Boris-Chengbiao Zhou <bobo1239@....de>,
        Fox Chen <foxhlchen@...il.com>,
        Ayaan Zaidi <zaidi.ayaan@...il.com>,
        Douglas Su <d0u9.su@...look.com>,
        Yuki Okushi <jtitor@...6.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>
Subject: Re: [PATCH 05/17] rust: add C helpers

On Wed, Jul 7, 2021 at 12:19 PM 'Marco Elver' via Clang Built Linux
<clang-built-linux@...glegroups.com> wrote:
>
> What is the story with Rust and LTO? Intuitively, I would expect Rust
> code to only perform optimally if the kernel is built with LTO
> (currently only supported via Clang).

Cross-language LTO is possible and is used in other major projects.
For instance, Firefox seems to use it:

    https://bugzilla.mozilla.org/show_bug.cgi?id=1486042

However, we have not taken a look at it yet for the kernel.

> Because if calls to every one of these helpers are real calls, I would
> expect performance to be pretty poor. There's probably a reason these
> are macros or inlinable functions.

Maybe -- we need to measure.

> I would almost go so far and suggest that CONFIG_RUST be modified as
> follows:

Even if the performance was unacceptable, there are other ways than
LTO to solve this, such as implementing the equivalent macro/function
on Rust's side.

Furthermore, we need to consider GCC too.

Thus I would not be premature in making it depend on LTO.

Cheers,
Miguel