Re: [RFC PATCH v2 2/4] rust: io_uring: introduce rust abstraction for io-uring cmd

["Benno Lossin" <lossin@kernel.org>]

On Mon Aug 11, 2025 at 4:50 PM CEST, Sidong Yang wrote:
> On Mon, Aug 11, 2025 at 09:44:22AM -0300, Daniel Almeida wrote:
>> > There is `uring_cmd` callback in `file_operation` at c side. `Pin<&mut IoUringCmd>`
>> > would be create in the callback function. But the callback function could be
>> > called repeatedly with same `io_uring_cmd` instance as far as I know.
>> > 
>> > But in c side, there is initialization step `io_uring_cmd_prep()`.
>> > How about fill zero pdu in `io_uring_cmd_prep()`? And we could assign a byte
>> > as flag in pdu for checking initialized also we should provide 31 bytes except
>> > a byte for the flag.
>> > 
>> 
>> That was a follow-up question of mine. Can´t we enforce zero-initialization
>> in C to get rid of this MaybeUninit? Uninitialized data is just bad in general.
>> 
>> Hopefully this can be done as you've described above, but I don't want to over
>> extend my opinion on something I know nothing about.
>
> I need to add a commit that initialize pdu in prep step in next version. 
> I'd like to get a comment from io_uring maintainer Jens. Thanks.
>
> If we could initialize (filling zero) in prep step, How about casting issue?
> Driver still needs to cast array to its private struct in unsafe?

We still would have the casting issue.

Can't we do the following:

* Add a new associated type to `MiscDevice` called `IoUringPdu` that
  has to implement `Default` and have a size of at most 32 bytes.
* make `IoUringCmd` generic
* make `MiscDevice::uring_cmd` take `Pin<&mut IoUringCmd<Self::IoUringPdu>>`
* initialize the private data to be `IoUringPdu::default()` when we
  create the `IoUringCmd` object.
* provide a `fn pdu(&mut self) -> &mut Pdu` on `IoUringPdu<Pdu>`.

Any thoughts? If we don't want to add a new associated type to
`MiscDevice` (because not everyone has to declare the `IoUringCmd`
data), I have a small trait dance that we can do to avoid that:

    pub trait IoUringMiscDevice: MiscDevice {
        type IoUringPdu: Default; // missing the 32 byte constraint
    }

and then in MiscDevice we still add this function:

        fn uring_cmd(
            _device: <Self::Ptr as ForeignOwnable>::Borrowed<'_>,
            _io_uring_cmd: Pin<&mut IoUringCmd<Self::IoUringPdu>>,
            _issue_flags: u32,
        ) -> Result<i32>
        where
            Self: IoUringMiscDevice,
        {
            build_error!(VTABLE_DEFAULT_ERROR)
        }

It can only be called when the user also implements `IoUringMiscDevice`.

---
Cheers,
Benno