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

[Sidong Yang]

On Fri, Aug 08, 2025 at 09:55:22AM -0400, Caleb Sander Mateos wrote:
> On Fri, Aug 8, 2025 at 2:56 AM Sidong Yang <sidong.yang@xxxxxxxxxx> 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@xxxxxxxxxx> 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@xxxxxxxxxx> 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.
> 
> MaybeUninit is correct. The io_uring/uring_cmd layer doesn't
> initialize the pdu field. struct io_uring_cmd is overlaid with struct
> io_kiocb's cmd field. struct io_kiocb's are allocated using
> kmem_cache_alloc(_bulk)() in __io_alloc_req_refill(). io_preinit_req()
> is called to initialize each struct io_kiocb but doesn't initialize
> the cmd field. As Sidong said, it's uninitialized memory for the
> ->uring_cmd() implementation to use however it wants for the duration
> of the command.

Agreed. Thanks.

Thanks,
Sidong
> 
> Best,
> Caleb