Re: [PATCH v7 2/2] rust: add dma coherent allocator abstraction.

[Abdiel Janulgue]

On 13/12/2024 21:08, Daniel Almeida wrote:
> Hi Robin,
>
> > On 13 Dec 2024, at 12:28, Robin Murphy <robin.murphy@xxxxxxx> wrote:
> >
> > On 13/12/2024 2:47 pm, Daniel Almeida wrote:
> > [...]
> > > > > +    /// Returns the CPU-addressable region as a slice.
> > > > > +    pub fn cpu_buf(&self) -> &[T]
> > > > > +    {
> > > > > +        // SAFETY: The pointer is valid due to type invariant on `CoherentAllocation` and
> > > > > +        // is valid for reads for `self.count * size_of::<T>` bytes.
> > > > > +        unsafe { core::slice::from_raw_parts(self.cpu_addr, self.count) }
> > > >
> > > > Immutable slices require that the data does not change while the
> > > > reference is live. Is that the case? If so, your safety comment should
> > > > explain that.
> > > >
> > > > > +    }
> > > > > +
> > > > > +    /// Performs the same functionality as `cpu_buf`, except that a mutable slice is returned.
> > > > > +    pub fn cpu_buf_mut(&mut self) -> &mut [T]
> > > > > +    {
> > > > > +        // SAFETY: The pointer is valid due to type invariant on `CoherentAllocation` and
> > > > > +        // is valid for reads for `self.count * size_of::<T>` bytes.
> > > > > +        unsafe { core::slice::from_raw_parts_mut(self.cpu_addr, self.count) }
> > > >
> > > > Mutable slices require that the data is not written to *or read* by
> > > > anybody else while the reference is live. Is that the case? If so,
> > > > your safety comment should explain that.
> > > The buffer will probably be shared between the CPU and some hardware device, since this is the
> > > point of the dma mapping API.
> > > It’s up to the caller to ensure that no hardware operations that involve the buffer are currently taking
> > > place while the slices above are alive.
> >
> > Hmm, that sounds troublesome... the nature of coherent allocations is that both CPU and device may access them at any time, and you can definitely expect ringbuffer-style usage models where a CPU is writing to part of the buffer while the device is reading/writing another part, but also cases where a CPU needs to poll for a device write to a particular location.
>
> Ok, I had based my answer on some other drivers I’ve worked on in the past where the approach I cited would work.
>
> I can see it not working for what you described, though.
>
> This is a bit unfortunate, because it means we are back to square one, i.e.: back to read() and write() functions and
> to the bound on `Copy`. That’s because, no matter how you try to dress this, there is no way to give safe and direct access
> to the underlying memory if you can’t avoid situations where both the CPU and the device will be accessing the memory
> at the same time.

This is unfortunate indeed. Thanks Alice for pointing out the 
limitations of slice.

Btw, do we have any other concerns in going back to plain old raw 
pointers instead? i.e.,

    pub fn read(&self, index: usize) -> Result<T> {
        if index >= self.count {
            return Err(EINVAL);
        }

        let ptr = self.cpu_addr.wrapping_add(index);
        // SAFETY: We just checked that the index is within bounds.
        Ok(unsafe { ptr.read() })
    }

    pub fn write(&self, index: usize, value: &T) -> Result
    where
        T: Copy,
    {
        if index >= self.count {
            return Err(EINVAL);
        }

        let ptr = self.cpu_addr.wrapping_add(index);
        // SAFETY: We just checked that the index is within bounds.
        unsafe { ptr.write(*value) };
        Ok(())
    }

> I guess the only improvement that could be made over the approach used for v2 is to at least use copy_nonoverlapping
> instead, 

You mean introduce something like read_raw(dst: *mut u8,...) and 
write_raw(&self, src: *const u8,...)?

Regards,
Abdiel