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

[Sidong Yang <sidong.yang@furiosa.ai>]

On Fri, Aug 08, 2025 at 10:49:14AM +0200, Benno Lossin wrote:
> 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.

Most driver uses `io_uring_cmd_to_pdu` macro that casts address of pdu to
private data type. It seems that all driver use this macro has it's own
struct type. How about make 2 function for pdu? like store_pdu(), borrow_pdu().

> 
> ---
> Cheers,
> Benno