linux-kernel - Re: [PATCH 1/2] nova-core: Add a library for bitfields in Rust structs

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Message-Id: <DCJOUO214EXC.32MFBN80VJW3K@nvidia.com>
Date: Thu, 04 Sep 2025 12:16:40 +0900
From: "Alexandre Courbot" <acourbot@...dia.com>
To: "Joel Fernandes" <joelagnelf@...dia.com>,
 <linux-kernel@...r.kernel.org>, "Danilo Krummrich" <dakr@...nel.org>,
 "David Airlie" <airlied@...il.com>, "Simona Vetter" <simona@...ll.ch>,
 "Miguel Ojeda" <ojeda@...nel.org>, "Alex Gaynor" <alex.gaynor@...il.com>,
 "Boqun Feng" <boqun.feng@...il.com>, "Gary Guo" <gary@...yguo.net>,
 Björn Roy Baron <bjorn3_gh@...tonmail.com>, "Benno Lossin"
 <lossin@...nel.org>, "Andreas Hindborg" <a.hindborg@...nel.org>, "Alice
 Ryhl" <aliceryhl@...gle.com>, "Trevor Gross" <tmgross@...ch.edu>
Cc: "John Hubbard" <jhubbard@...dia.com>, "Alistair Popple"
 <apopple@...dia.com>, <nouveau@...ts.freedesktop.org>,
 <dri-devel@...ts.freedesktop.org>, <rust-for-linux@...r.kernel.org>
Subject: Re: [PATCH 1/2] nova-core: Add a library for bitfields in Rust
 structs

On Thu Sep 4, 2025 at 12:15 AM JST, Joel Fernandes wrote:
<snip>
>>> +use kernel::prelude::*;
>>> +
>>> +/// Macro for defining bitfield-packed structures in Rust.
>>> +/// The size of the underlying storage type is specified with #[repr(TYPE)].
>>> +///
>>> +/// # Example (just for illustration)
>>> +/// ```rust
>>> +/// bitstruct! {
>>> +///     #[repr(u64)]
>>> +///     pub struct PageTableEntry {
>>> +///         0:0       present     as bool,
>>> +///         1:1       writable    as bool,
>>> +///         11:9      available   as u8,
>>> +///         51:12     pfn         as u64,
>>> +///         62:52     available2  as u16,
>>> +///         63:63     nx          as bool,
>> 
>> A note on syntax: for nova-core, we may want to use the `H:L` notation,
>> as this is what OpenRM uses, but in the larger kernel we might want to
>> use inclusive ranges (`L..=H`) as it will look more natural in Rust
>> code (and is the notation the `bits` module already uses).
>
> Perhaps future add-on enhancement to have both syntax? I'd like to initially
> keep H:L and stabilize the code first, what do you think?

Let's have the discussion with the other stakeholders (Daniel?). I think
in Nova we want to keep the `H:L` syntax, as it matches what the OpenRM
headers do (so Nova would have its own `register` macro that calls into
the common one, tweaking things as it needs). But in the kernel crate we
should use something intuitive for everyone.