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Date:   Wed, 7 Jul 2021 12:19:14 +0200
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
Message-ID: <YOV/oj0rjHhzluH2@elver.google.com>
References: <20210704202756.29107-1-ojeda@kernel.org>
 <20210704202756.29107-6-ojeda@kernel.org>
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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).

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