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From: Marco Elver <elver@google.com>
To: ojeda@kernel.org
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	rust-for-linux@vger.kernel.org, linux-kbuild@vger.kernel.org,
	linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org,
	Alex Gaynor <alex.gaynor@gmail.com>,
	Geoffrey Thomas <geofft@ldpreload.com>,
	Finn Behrens <me@kloenk.de>,
	Adam Bratschi-Kaye <ark.email@gmail.com>,
	Wedson Almeida Filho <wedsonaf@google.com>,
	Boqun Feng <boqun.feng@gmail.com>,
	Sumera Priyadarsini <sylphrenadin@gmail.com>,
	Michael Ellerman <mpe@ellerman.id.au>,
	Sven Van Asbroeck <thesven73@gmail.com>,
	Gary Guo <gary@garyguo.net>,
	Boris-Chengbiao Zhou <bobo1239@web.de>,
	Fox Chen <foxhlchen@gmail.com>,
	Ayaan Zaidi <zaidi.ayaan@gmail.com>,
	Douglas Su <d0u9.su@outlook.com>, Yuki Okushi <jtitor@2k36.org>,
	clang-built-linux@googlegroups.com
Subject: Re: [PATCH 05/17] rust: add C helpers
Date: Fri, 9 Jul 2021 12:31:41 +0200	[thread overview]
Message-ID: <CANpmjNMOou5DccZvGAcy4U7iqoLk6NRuTFkd-6JzTmoSFR+YYg@mail.gmail.com> (raw)
In-Reply-To: <YOV/oj0rjHhzluH2@elver.google.com>

On Wed, 7 Jul 2021 at 12:19, Marco Elver <elver@google.com> wrote:
> On Sun, Jul 04, 2021 at 10:27PM +0200, ojeda@kernel.org wrote:
> > From: Miguel Ojeda <ojeda@kernel.org>
> >
> > This source file contains forwarders to C macros and inlined
> > functions.
>
> 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).

I'll answer my own question: it looks like Linux Rust code currently
does _not_ generate LLVM-LTO compatible object files, but only native
object files (which still link fine if LTO is enabled, but doesn't
permit the optimizations below we'd want).

rustc already supports playing nicely with LLVM LTO via `-C
linker-plugin-lto`:
https://doc.rust-lang.org/rustc/linker-plugin-lto.html

So, hopefully it should only require kernel work to make it play
nicely with CONFIG_LTO_CLANG.

> 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.
>
> I would almost go so far and suggest that CONFIG_RUST be modified as
> follows:
>
> --- a/init/Kconfig
> +++ b/init/Kconfig
> @@ -2028,6 +2028,7 @@ config RUST
>         depends on HAS_RUST
>         depends on !COMPILE_TEST
>         depends on !MODVERSIONS
> +       depends on LTO || EXPERT
>         default n
>         help
>           Enables Rust support in the kernel.
>
> [ I'm sure there are configs that don't yet work with LTO, but could be
>   useful to enable for debugging or testing purposes, and therefore would
>   make it conditional on CONFIG_EXPERT as well. ]
>
> [...]
> > +unsigned long rust_helper_copy_from_user(void *to, const void __user *from, unsigned long n)
> > +{
> > +     return copy_from_user(to, from, n);
> > +}
> > +
> [...]
>
> From some local tests, it looks like simply attaching
> __attribute__((always_inline)) will do what one would expect when
> compiling with Clang LTO (I checked -flto=thin).
>
> If you confirm this also works across C and Rust TUs when enabling LTO,
> I would then suggested adding __attribute__((always_inline)) to all
> these helpers.
>
> Thanks,
> -- Marco

  reply	other threads:[~2021-07-09 10:31 UTC|newest]

Thread overview: 73+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-07-04 20:27 [PATCH 00/17] Rust support ojeda
2021-07-04 20:27 ` [PATCH 01/17] kallsyms: support big kernel symbols (2-byte lengths) ojeda
2021-07-04 20:52   ` Linus Torvalds
2021-07-04 21:15     ` Miguel Ojeda
2021-07-04 21:28       ` Linus Torvalds
2021-07-04 21:04   ` Matthew Wilcox
2021-07-04 21:17     ` Miguel Ojeda
2021-07-04 21:33       ` Matthew Wilcox
2021-07-04 21:49         ` Matthew Wilcox
2021-07-04 22:07           ` Miguel Ojeda
2021-07-04 22:20         ` Gary Guo
2021-07-04 22:42           ` Matthew Wilcox
2021-07-04 23:14             ` Gary Guo
2021-07-13 18:02             ` Kent Overstreet
2021-07-04 21:20     ` Gary Guo
2021-07-05  4:35       ` Willy Tarreau
2021-07-04 20:27 ` [PATCH 02/17] kallsyms: increase maximum kernel symbol length to 512 ojeda
2021-07-14 18:20   ` Nick Desaulniers
2021-07-14 19:04     ` Linus Torvalds
2021-07-14 20:09       ` Eric W. Biederman
2021-07-14 20:21         ` Linus Torvalds
2021-07-14 20:48           ` Eric W. Biederman
2021-07-14 22:40             ` Linus Torvalds
2021-07-14 20:19     ` Miguel Ojeda
2021-07-04 20:27 ` [PATCH 03/17] Makefile: generate `CLANG_FLAGS` even in GCC builds ojeda
2021-07-14 18:13   ` Nick Desaulniers
2021-07-14 18:16     ` Nick Desaulniers
2021-07-14 20:31     ` Miguel Ojeda
2021-07-04 20:27 ` [PATCH 04/17] vsprintf: add new `%pA` format specifier ojeda
2021-07-07 20:31   ` Nick Desaulniers
2021-07-07 22:33     ` Miguel Ojeda
2021-07-04 20:27 ` [PATCH 05/17] rust: add C helpers ojeda
2021-07-07 10:19   ` Marco Elver
2021-07-09 10:31     ` Marco Elver [this message]
2021-07-09 14:32     ` Miguel Ojeda
2021-07-04 20:27 ` [PATCH 06/17] rust: add `compiler_builtins` crate ojeda
2021-07-04 20:27 ` [PATCH 07/17] rust: add `alloc` crate ojeda
2021-07-04 20:27 ` [PATCH 08/17] rust: add `build_error` crate ojeda
2021-07-04 20:27 ` [PATCH 09/17] rust: add `macros` crate ojeda
2021-07-04 20:27 ` [PATCH 10/17] rust: add `kernel` crate ojeda
2021-07-04 20:27 ` [PATCH 11/17] rust: export generated symbols ojeda
2021-07-04 20:27 ` [PATCH 12/17] Kbuild: add Rust support ojeda
2021-07-04 20:27 ` [PATCH 13/17] docs: add Rust documentation ojeda
2021-07-05  5:02   ` Willy Tarreau
2021-07-06  0:06     ` Miguel Ojeda
2021-07-06  2:09       ` Willy Tarreau
2021-07-14 18:37   ` Nick Desaulniers
2021-07-14 19:55     ` Miguel Ojeda
2021-07-04 20:27 ` [PATCH 14/17] samples: add Rust examples ojeda
2021-07-04 20:27 ` [PATCH 15/17] scripts: add `generate_rust_analyzer.py` ojeda
2021-07-05  5:05   ` Willy Tarreau
2021-07-05 23:41     ` Miguel Ojeda
2021-07-04 20:27 ` [PATCH 16/17] MAINTAINERS: Rust ojeda
2021-07-06  0:26   ` Pedro Tammela
2021-07-06 22:14     ` Miguel Ojeda
2021-07-04 20:27 ` [RFC PATCH 17/17] Android: Binder IPC in Rust (WIP) ojeda
2021-07-04 23:11 ` [PATCH 00/17] Rust support Miguel Ojeda
2021-07-07  6:43 ` Christoph Hellwig
2021-07-07 12:33   ` Miguel Ojeda
2021-07-07 12:50     ` Greg Kroah-Hartman
2021-07-07 14:07       ` Wedson Almeida Filho
2021-07-07 14:12         ` Christoph Hellwig
2021-07-07 15:02         ` Greg Kroah-Hartman
2021-07-07 20:56           ` Finn Behrens
2021-07-08  0:58             ` Matthew Wilcox
2021-07-08  6:39               ` [PATCH 00/17] Rust support, NVMe Geert Stappers
2021-07-08  8:29                 ` Finn Behrens
2021-07-22 22:55               ` [PATCH 00/17] Rust support Wedson Almeida Filho
2021-07-23  1:17                 ` Matthew Wilcox
2021-07-23 11:33                   ` [PATCH 00/17] Rust support, GPIO driver Geert Stappers
2021-07-23 15:08                 ` [PATCH 00/17] Rust support Keith Busch
2021-07-07 10:51 ` Marco Elver
2021-07-07 11:28   ` Miguel Ojeda

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