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

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Message-Id: <F3F2A1C5-AD63-4F6C-A60D-F7C5CE7E65A9@collabora.com>
Date: Tue, 12 Aug 2025 10:59:55 -0300
From: Daniel Almeida <daniel.almeida@...labora.com>
To: Sidong Yang <sidong.yang@...iosa.ai>
Cc: Benno Lossin <lossin@...nel.org>,
 Caleb Sander Mateos <csander@...estorage.com>,
 Miguel Ojeda <ojeda@...nel.org>,
 Arnd Bergmann <arnd@...db.de>,
 Jens Axboe <axboe@...nel.dk>,
 Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
 rust-for-linux@...r.kernel.org,
 linux-kernel@...r.kernel.org,
 io-uring@...r.kernel.org
Subject: Re: [RFC PATCH v2 2/4] rust: io_uring: introduce rust abstraction for
 io-uring cmd



> On 12 Aug 2025, at 10:56, Sidong Yang <sidong.yang@...iosa.ai> wrote:
> 
> On Tue, Aug 12, 2025 at 09:43:56AM -0300, Daniel Almeida wrote:
>> 
>> 
>>> On 12 Aug 2025, at 09:19, Sidong Yang <sidong.yang@...iosa.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

Also, I agree about zeroing out the array, let’s try to not have this
MaybeUninit thing here if possible.

>>> 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
>> 
>>