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

[Daniel Almeida <daniel.almeida@collabora.com>]

> 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?