Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
MIME-Version: 1.0
References: <20210414184604.23473-1-ojeda@kernel.org> <YHiMyE4E1ViDcVPi@hirez.programming.kicks-ass.net>
 <YHj02M3jMSweoP4l@google.com> <CACRpkdat8bny=D2mAsUXcDQvFJ=9jSZSccMMZzH=10dHQ_bXrQ@mail.gmail.com>
 <CANiq72niCj9SfPhfQBMtxF+jth--cXdPQtUo5jhDDJgL6DTXZQ@mail.gmail.com>
In-Reply-To: <CANiq72niCj9SfPhfQBMtxF+jth--cXdPQtUo5jhDDJgL6DTXZQ@mail.gmail.com>
From:   Linus Walleij <linus.walleij@linaro.org>
Date:   Mon, 26 Apr 2021 02:31:26 +0200
Message-ID: <CACRpkdarfkA1P0ERCXHSA=6VTBn6FXgOxB8haneQtN_4-tyQ0w@mail.gmail.com>
Subject: Re: [PATCH 00/13] [RFC] Rust support
To:     Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>
Cc:     Wedson Almeida Filho <wedsonaf@google.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Miguel Ojeda <ojeda@kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        rust-for-linux <rust-for-linux@vger.kernel.org>,
        linux-kbuild <linux-kbuild@vger.kernel.org>,
        Linux Doc Mailing List <linux-doc@vger.kernel.org>,
        linux-kernel <linux-kernel@vger.kernel.org>
Content-Type: text/plain; charset="UTF-8"
Precedence: bulk

On Thu, Apr 22, 2021 at 11:29 PM Miguel Ojeda
<miguel.ojeda.sandonis@gmail.com> wrote:

> > But as subsystem maintainer I'd like a clear picture of this wrapper
> > overhead, what does it usually entail? A typical kernel API has
> > vtable and a few variables, not much more than that.
>
> If you mean runtime-overhead, i.e. performance, it should be very
> small or even zero. It should be possible to perform LTO across
> languages too.
>
> If you mean source code overhead, or cognitive overhead, then it is
> quite a bit, yes. Please see below.

Yeah that is what I mean :)

> I hear you! I do not think it will take decades for kernel developers
> to get up to speed, but I agree that having some help/backup is a very
> good idea in the beginning.
>
> Our hope is that, if Rust advantages prove themselves, then it will
> the subsystem maintainers the ones that will want to create and
> maintain the wrappers so that drivers in their tree are easier to
> maintain and less prone to mistakes ;-)

I am not really convinced that (leaf) drivers is where Rust will
help most.

As I mentioned in my mail to Wedson that I think things like network
protocols that deal with abstract entities will have more "pure code"
(not deal with machine registers, just RAM memory).
File systems would be another example.

I think the Rust proponents should be open to the fact that their
work will eventually depend on themselves or someone else
fixing a working compiler for the maintained architectures in
the Linux kernel one way or the other, so they will be able to
work with Rust project anywhere in the kernel.

For example m68k is not going away. Avoiding this question
of compiler support, just waiting and hoping that these old
architectures will disappear is the wrong idea. The right idea
is to recognize that LLVM and/or GCC Rust needs to
support all these architectures so they can all use Rust.
Someone needs to put in the effort.

After all fixing that compiler support is an insignificant amount
of work compared to what Rust in the core kernel will be.

Yours,
Linus Walleij