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Date:   Fri, 13 Oct 2023 12:00:06 -0700
From:   "H. Peter Anvin" <hpa@zytor.com>
To:     Linus Torvalds <torvalds@linux-foundation.org>,
        Ramon de C Valle <rcvalle@google.com>
CC:     Peter Zijlstra <peterz@infradead.org>,
        Kees Cook <keescook@chromium.org>,
        Sami Tolvanen <samitolvanen@google.com>,
        Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>,
        Miguel Ojeda <ojeda@kernel.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        rust-for-linux@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-fsdevel@vger.kernel.org, patches@lists.linux.dev,
        Jarkko Sakkinen <jarkko@kernel.org>,
        Alex Gaynor <alex.gaynor@gmail.com>,
        Wedson Almeida Filho <wedsonaf@google.com>,
        David Gow <davidgow@google.com>,
        Boqun Feng <boqun.feng@gmail.com>, Gary Guo <gary@garyguo.net>,
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        Jonathan Corbet <corbet@lwn.net>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Dave Hansen <dave.hansen@linux.intel.com>, x86@kernel.org,
        linux-doc@vger.kernel.org
Subject: Re: [PATCH v10 25/27] x86: enable initial Rust support
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On October 13, 2023 11:54:46 AM PDT, Linus Torvalds <torvalds@linux-foundat=
ion=2Eorg> wrote:
>On Fri, 13 Oct 2023 at 05:18, Ramon de C Valle <rcvalle@google=2Ecom> wro=
te:
>>
>> Both C and repr(C) Rust structs have this encoding, but I understand
>> the problems with doing this in C since it doesn't have
>> repr(transparent) structs so there would be a lot of casting back and
>> forth=2E Maybe there is an alternative or this could be done for less
>> used function pairs?
>
>We actually have some C variations of what I think people want to use
>"repr(transparent) struct" for in Rust=2E
>
>Of course, that is depending on what kind of context you want to use
>it for, and I might have lost some background=2E But I'm assuming you're
>talking about the situation where you want to treat two or more types
>as being "compatible" within certain contexts=2E
>
>There's the actual standard C "_Generic()" alternative, which allows
>you to make macros etc that use different types transparently=2E
>
>It's not very widely used in the kernel, because we only fairly
>recently moved to require recent enough compiler versions, but we do
>use it now in a couple of places=2E
>
>And there's the much more traditional gcc extension in the form of the
>__attribute__((__transparent_union__)) thing=2E In the kernel, that one
>is even less used, and that one use is likely going away since the
>need for it is going away=2E
>
>But while it's not standard C, it's actually been supported by
>relevant compilers for much longer than "_Generic" has, and is
>designed exactly for the "I have a function that can take arguments of
>different types", either because the types are bitwise identical (even
>if _conceptually_ not the same), or simply because you have a
>different argument that describes the type (the traditional C union
>model)=2E
>
>I suspect, for example, that we *should* have used those transparent
>unions for the "this function can take either a folio or a page" case,
>instead of duplicating functions for the two uses=2E
>
>But probably because few people aren familiar with the syntax, that's
>not what happened=2E
>
>             Linus

Transparent unions have been standard C since C99=2E