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

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Message-Id: <DCJTY0OQFG83.1AX49CQARXCEX@kernel.org>
Date: Thu, 04 Sep 2025 09:16:08 +0200
From: "Danilo Krummrich" <dakr@...nel.org>
To: "Alexandre Courbot" <acourbot@...dia.com>
Cc: "Joel Fernandes" <joelagnelf@...dia.com>,
 <linux-kernel@...r.kernel.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>, "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 5:16 AM CEST, Alexandre Courbot wrote:
> 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.

I don't care too much about whether it's gonna be H:L or L:H [1], but I do care
about being consistent throughout the kernel. Let's not start the practice of
twisting kernel APIs to NV_* specific APIs that differ from what people are used
to work with in the kernel.

[1] If it's gonna be H:L, I think we should also list things in reverse order,
    i.e.:

	pub struct PageTableEntry {
	    63:63     nx          as bool,
	    62:52     available2  as u16,
	    51:12     pfn         as u64,
	    11:9      available   as u8,
	    1:1       writable    as bool,
	    0:0       present     as bool,
	}

This is also what would be my preferred style for the kernel in general.

- Danilo