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

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

On Tue, Aug 12, 2025 at 09:43:56AM -0300, Daniel Almeida wrote:
> 
> 
> > On 12 Aug 2025, at 09:19, Sidong Yang <sidong.yang@furiosa.ai> wrote:
> > 
> > On Tue, Aug 12, 2025 at 10:34:56AM +0200, Benno Lossin wrote:
> >> 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.
> > 
> > `uring_cmd` could be called multiple times. So we can't initialize
> > in that time. I don't understand that how can we cast [u8; 32] to
> > `IoUringPdu` safely. It seems that casting can't help to use unsafe.
> > I think best way is that just return zerod `&mut [u8; 32]` and
> > each driver implements safe serde logic for its private data. 
> > 
> 
> Again, can´t we use FromBytes for this?

Agreed, we need FromBytes for read_pdu and AsBytes for write_pdu. I'll reference
dma code for next version.

Thanks,
Sidong
> 
>