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 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() };

Thanks,
Sidong
> 
> > 
> > I think it is like bytemuck::Pod trait. Pod meaning plain old data.
> > 
> > Thanks,
> > Sidong
> > 
> > 
> >> 
> >> ---
> >> Cheers,
> >> Benno
> 
> 
> I'm not really sure how this solves the transmute/cast problem. Is the trait
> you're referring to supposed to have any member functions? Or is it just a
> marker trait?
> 
> I wonder if we can fit the existing "kernel::FromBytes" for this problem.
> 
> - Daniel
>