linux-kernel - Re: [PATCH v2] rust: macros: improve `#[vtable]` documentation

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Date:   Mon, 23 Oct 2023 10:30:06 +0200
From:   "Andreas Hindborg (Samsung)" <nmi@...aspace.dk>
To:     Benno Lossin <benno.lossin@...ton.me>
Cc:     Miguel Ojeda <ojeda@...nel.org>,
        Wedson Almeida Filho <wedsonaf@...il.com>,
        Alex Gaynor <alex.gaynor@...il.com>,
        Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>,
        Björn Roy Baron <bjorn3_gh@...tonmail.com>,
        Alice Ryhl <aliceryhl@...gle.com>,
        rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] rust: macros: improve `#[vtable]` documentation


Benno Lossin <benno.lossin@...ton.me> writes:

> On 20.10.23 11:06, Andreas Hindborg (Samsung) wrote:
>> Benno Lossin <benno.lossin@...ton.me> writes:
>>> +/// Error message for calling a default function of a [`#[vtable]`](macros::vtable) trait.
>>> +pub const VTABLE_DEFAULT_ERROR: &str =
>>> +    "This function must not be called, see the #[vtable] documentation.";
>>> diff --git a/rust/macros/lib.rs b/rust/macros/lib.rs
>>> index c42105c2ff96..daf1ef8baa62 100644
>>> --- a/rust/macros/lib.rs
>>> +++ b/rust/macros/lib.rs
>>> @@ -87,27 +87,41 @@ pub fn module(ts: TokenStream) -> TokenStream {
>>>   /// implementation could just return `Error::EINVAL`); Linux typically use C
>>>   /// `NULL` pointers to represent these functions.
>>>   ///
>>> -/// This attribute is intended to close the gap. Traits can be declared and
>>> -/// implemented with the `#[vtable]` attribute, and a `HAS_*` associated constant
>>> -/// will be generated for each method in the trait, indicating if the implementor
>>> -/// has overridden a method.
>>> +/// This attribute closes that gap. A trait can be annotated with the `#[vtable]` attribute.
>>> +/// Implementers of the trait will then also have to annotate the trait with `#[vtable]`. This
>>> +/// attribute generates a `HAS_*` associated constant bool for each method in the trait that is set
>>> +/// to true if the implementer has overridden the associated method.
>>> +///
>>> +/// For a function to be optional, it must have a default implementation. But this default
>>> +/// implementation will never be executed, since these functions are exclusively called from
>>> +/// callbacks from the C side. This is because the vtable will have a `NULL` entry and the C side
>>> +/// will execute the default behavior. Since it is not maintainable to replicate the default
>>> +/// behavior in Rust, the default implementation should be:
>> 
>> How about this?:
>> 
>> For a Rust trait method to be optional, it must have a default
>> implementation. For a trait marked with `#[vtable]`, the default
>> implementation will not be executed, as the only way the trait methods
>> should be called is through function pointers installed in C side
>> vtables. When a trait implementation marked with `#[vtable]` is missing
>> a method, a `NULL` pointer will be installed in the corresponding C side
>> vtable, and thus the Rust default implementation can not be called. The
>> default implementation should be:
>> 
>> Not sure if it is more clear 🤷
>
> I think it misses the following important point: why is it not
> possible to just replicate the default behavior?
>
> What do you think of this?:
>
> For a trait method to be optional, it must have a default implementation.
> This is also the case for traits annotated with `#[vtable]`, but in this
> case the default implementation will never be executed. The reason for this
> is that the functions will be called through function pointers installed in
> C side vtables. When an optional method is not implemented on a `#[vtable]`
> trait, a NULL entry is installed in the vtable. Thus the default
> implementation is never called. Since these traits are not designed to be
> used on the Rust side, it should not be possible to call the default
> implementation.

> It is not possible to replicate the default behavior from C
> in Rust, since that is not maintainable.

I don't feel that this bit should be included. It's not a matter of
maintainability. Why would we reimplement something that is already
present in a subsystem? The functionality is already present, so we use
it.

> The default implementaiton should
> therefore call `kernel::build_error`, thus preventing calls to this
> function at compile time:

Otherwise I think it is good 👍

BR Andreas