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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Mon, 19 Sep 2022 17:15:13 -0700
Message-ID: <CAHk-=wj6sDFk8ZXSEKUMj-J9zfrMSSO3jhBEaveVaJSUpr=O=w@mail.gmail.com>
Subject: Re: [PATCH v9 12/27] rust: add `kernel` crate
To:     Alex Gaynor <alex.gaynor@gmail.com>
Cc:     Wedson Almeida Filho <wedsonaf@gmail.com>,
        Matthew Wilcox <willy@infradead.org>,
        Kees Cook <keescook@chromium.org>,
        Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>,
        Konstantin Shelekhin <k.shelekhin@yadro.com>, ojeda@kernel.org,
        ark.email@gmail.com, bjorn3_gh@protonmail.com, bobo1239@web.de,
        bonifaido@gmail.com, boqun.feng@gmail.com, davidgow@google.com,
        dev@niklasmohrin.de, dsosnowski@dsosnowski.pl, foxhlchen@gmail.com,
        gary@garyguo.net, geofft@ldpreload.com, gregkh@linuxfoundation.org,
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        mjmouse9999@gmail.com, patches@lists.linux.dev,
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        viktor@v-gar.de, Andreas Hindborg <andreas.hindborg@wdc.com>
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On Mon, Sep 19, 2022 at 4:58 PM Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> This is not some kind of "a few special things".
>
> This is things like absolutely _anything_ that allocates memory, or
> takes a lock, or does a number of other things.

Examples of "number of other things" ends up being things like
"accessing user memory", which depending on what you are doing may be
very common too.

And btw, it's not only about the (multiple kinds of) atomic regions.

We have other context rules in the kernel too, like "does floating
point or vector unit calculations". Which you can actually do, but
only in a kernel_fpu_begin/kernel_fpu_end region.

Now, the floating point thing is rare enough that it's probably fine
to just say "no floating point at all in Rust code".  It tends to be
very special code, so you'd write it in C or inline assembly, because
you're doing special things like using the vector unit for crypto
hashes using special CPU instructions.

                  Linus