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

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

On Fri Aug 8, 2025 at 8:56 AM CEST, Sidong Yang wrote:
> On Wed, Aug 06, 2025 at 03:38:24PM +0200, Benno Lossin wrote:
>> On Wed Aug 6, 2025 at 2:38 PM CEST, Daniel Almeida wrote:
>> > Hi Benno,
>> >
>> >> On 2 Aug 2025, at 07:52, Benno Lossin <lossin@kernel.org> wrote:
>> >> 
>> >> On Fri Aug 1, 2025 at 3:48 PM CEST, Daniel Almeida wrote:
>> >>>> On 27 Jul 2025, at 12:03, Sidong Yang <sidong.yang@furiosa.ai> wrote:
>> >>>> +    #[inline]
>> >>>> +    pub fn pdu(&mut self) -> &mut MaybeUninit<[u8; 32]> {
>> >>> 
>> >>> Why MaybeUninit? Also, this is a question for others, but I don´t think
>> >>> that `u8`s can ever be uninitialized as all byte values are valid for `u8`.
>> >> 
>> >> `u8` can be uninitialized. Uninitialized doesn't just mean "can take any
>> >> bit pattern", but also "is known to the compiler as being
>> >> uninitialized". The docs of `MaybeUninit` explain it like this:
>> >> 
>> >>    Moreover, uninitialized memory is special in that it does not have a
>> >>    fixed value ("fixed" meaning "it won´t change without being written
>> >>    to"). Reading the same uninitialized byte multiple times can give
>> >>    different results.
>> >> 
>> >> But the return type probably should be `&mut [MaybeUninit<u8>; 32]`
>> >> instead.
>> >
>> >
>> > Right, but I guess the question then is why would we ever need to use
>> > MaybeUninit here anyways.
>> >
>> > It's a reference to a C array. Just treat that as initialized.
>> 
>> AFAIK C uninitialized memory also is considered uninitialized in Rust.
>> So if this array is not properly initialized on the C side, this would
>> be the correct type. If it is initialized, then just use `&mut [u8; 32]`.
>
> pdu field is memory chunk for driver can use it freely. The driver usually
> saves a private data and read or modify it on the other context. using
> just `&mut [u8;32]` would be simple and easy to use.

Private data is usually handled using `ForeignOwnable` in Rust. What
kind of data would be stored there? If it's a pointer, then `&mut [u8;
32]` would not be the correct choice.

---
Cheers,
Benno