linux-kernel - Re: [PATCH 04/13] Kbuild: Rust support

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Message-ID: <CANiq72=5pMzSS5V7h-QcQvYgyZUwdE=T705KtBWrNYZPjMYK3Q@mail.gmail.com>
Date:   Fri, 16 Apr 2021 23:39:00 +0200
From:   Miguel Ojeda <miguel.ojeda.sandonis@...il.com>
To:     Willy Tarreau <w@....eu>
Cc:     Connor Kuehl <ckuehl@...hat.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Miguel Ojeda <ojeda@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        rust-for-linux@...r.kernel.org,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
        Linux Kernel Mailing List <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>,
        Michael Ellerman <mpe@...erman.id.au>
Subject: Re: [PATCH 04/13] Kbuild: Rust support

On Fri, Apr 16, 2021 at 10:58 PM Willy Tarreau <w@....eu> wrote:
>
> No, two:
>   - ok in %rax (seems like it's "!ok" technically speaking since it
>     returns 1 on !ok and 0 on ok)
>   - foo_or_err in %rdx

Yes, but that is the implementation -- conceptually you only have one
or the other, and Rust won't allow you to use the wrong one.

> However then I'm bothered because Miguel's example showed that regardless
> of OK, EINVAL was always returned in foo_or_err, so maybe it's just
> because his example was not well chosen but it wasn't very visible from
> the source:

That is the optimizer being fancy since the error can be put
unconditionally in `rdx`.

If you compile:

    pub fn it_is_ok() -> KernelResult<Bar> {
        Ok(Bar)
    }

you will see it just clears `rax`.

Cheers,
Miguel