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

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

On Sun, Aug 10, 2025 at 10:06:21PM +0200, Benno Lossin wrote:
> On Sun Aug 10, 2025 at 4:46 PM CEST, Sidong Yang wrote:
> > On Sun, Aug 10, 2025 at 11:27:12AM -0300, Daniel Almeida wrote:
> >> > On 10 Aug 2025, at 10:50, Sidong Yang <sidong.yang@furiosa.ai> wrote:
> >> > 
> >> > On Sat, Aug 09, 2025 at 10:22:06PM +0200, Benno Lossin wrote:
> >> >> On Sat Aug 9, 2025 at 2:51 PM CEST, Sidong Yang wrote:
> >> >>> On Sat, Aug 09, 2025 at 12:18:49PM +0200, Benno Lossin wrote:
> >> >>>> We'd need to ensure that `borrow_pdu` can only be called if `store_pdu`
> >> >>>> has been called before. Is there any way we can just ensure that pdu is
> >> >>>> always initialized? Like a callback that's called once, before the value
> >> >>>> is used at all?
> >> >>> 
> >> >>> I've thought about this. As Celab said, returning `&mut MaybeUninit<[u8;32]> is
> >> >>> simple and best. Only driver knows it's initialized. There is no way to
> >> >>> check whether it's initialized with reading the pdu. The best way is to return
> >> >>> `&mut MaybeUninit<[u8;32]>` and driver initializes it in first time. After 
> >> >>> init, driver knows it's guranteed that it's initialized so it could call 
> >> >>> `assume_init_mut()`. And casting to other struct is another problem. The driver
> >> >>> is responsible for determining how to interpret the PDU, whether by using it
> >> >>> directly as a byte array or by performing an unsafe cast to another struct.
> >> >> 
> >> >> But then drivers will have to use `unsafe` & possibly cast the slice to
> >> >> a struct? I think that's bad design since we try to avoid unsafe code in
> >> >> drivers as much as possible. Couldn't we try to ensure from the
> >> >> abstraction side that any time you create such an object, the driver
> >> >> needs to provide the pdu data? Or we could make it implement `Default`
> >> >> and then set it to that before handing it to the driver.
> >> > 
> >> > pdu data is [u8; 32] memory space that driver can borrow. this has two kind of
> >> > issues. The one is that the array is not initialized and another one is it's
> >> > array type that driver should cast it to private data structure unsafely.
> >> > The first one could be resolved with returning `&mut MaybeUninit<>`. And the
> >> > second one, casting issue, is remaining. 
> >> > 
> >> > It seems that we need new unsafe trait like below:
> >> > 
> >> > /// Pdu should be... repr C or transparent, sizeof <= 20
> >> > unsafe trait Pdu: Sized {}
> >> > 
> >> > /// Returning to casted Pdu type T
> >> > pub fn pdu<T: Pdu>(&mut self) -> &mut MaybeUninit<T>
> >> 
> >> Wait, you receive an uninitialized array, and you´re supposed to cast it to
> >> T, is that correct? Because that does not fit the signature above.
> >
> > Sorry if my intent wasn´t clear. More example below:
> >
> > // in rust/kernel/io_uring.rs
> > unsafe trait Pdu: Sized {}
> > pub fn pdu<T: Pdu>(&mut self) -> &mut MaybeUninit<T> {
> >     let inner = unsafe { &mut *self.inner.get() };
> >     let ptr = &raw mut inner.pdu as *mut MaybeUninit<T>; // the cast here
> >     unsafe { &mut *ptr }
> > }
> >
> > // in driver code
> > #[repr(C)] struct MyPdu { value: u64 }
> > unsafe impl Pdu for MyPdu {}
> >
> > // initialize
> > ioucmd.pdu().write(MyPdu { value: 1 });
> >
> > // read or modify
> > let mypdu = unsafe { ioucmd.pdu().assume_init_mut() };
> 
> This is the kind of code I'd like to avoid, since it plans to use
> `unsafe` in driver code (the `unsafe impl` above is also a problem, but
> we can solve that with a derive macro).
> 
> Where are the entrypoints for `IoUringCmd` for driver code? I imagine
> that there is some kind of a driver callback (like `probe`, `open` etc)
> that contains an `Pin<&mut IoUringCmd>` as an argument, right? When is
> it created, can we control that & just write some default value to the
> pdu field?

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.

Thanks,
Sidong
> 
> ---
> Cheers,
> Benno