Re: [PATCH v12 2/3] rust: add dma coherent allocator abstraction.

[Alice Ryhl]

On Tue, Feb 25, 2025 at 9:15 AM Abdiel Janulgue
<abdiel.janulgue@xxxxxxxxx> wrote:
>
> Hi,
>
> On 24/02/2025 22:07, Benno Lossin wrote:
> >> +    ///
> >> +    /// # Safety
> >> +    ///
> >> +    /// It is the callers responsibility to avoid separate read and write accesses to the region
> >> +    /// while the returned slice is live.
> > This safety requirement is worded quite differently compared to the one
> > on `as_slice`, why?
> >
> >> +    pub unsafe fn as_slice_mut(&self, offset: usize, count: usize) -> Result<&mut [T]> {
> >> +        let end = offset.checked_add(count).ok_or(EOVERFLOW)?;
> >> +        if end >= self.count {
> >> +            return Err(EINVAL);
> >> +        }
> >> +        // SAFETY:
> >> +        // - The pointer is valid due to type invariant on `CoherentAllocation`,
> >> +        // we've just checked that the range and index is within bounds. The immutability of the
> >> +        // of data is also guaranteed by the safety requirements of the function.
> >> +        // - `offset` can't overflow since it is smaller than `self.count` and we've checked
> >> +        // that `self.count` won't overflow early in the constructor.
> >> +        Ok(unsafe {core::slice::from_raw_parts_mut(self.cpu_addr.add(offset), count) })
> >> +    }
> >> +
> >> +    /// Writes data to the region starting from `offset`. `offset` is in units of `T`, not the
> >> +    /// number of bytes.
> >> +    ///
> >> +    /// # Examples
> >> +    ///
> >> +    /// ```
> >> +    /// # fn test(alloc: &mutkernel::dma::CoherentAllocation<u8>) -> Result {
> >> +    /// let somedata: [u8; 4] = [0xf; 4];
> >> +    /// let buf: &[u8] = &somedata;
> >> +    /// alloc.write(buf, 0)?;
> >> +    /// # Ok::<(), Error>(()) }
> >> +    /// ```
> >> +    pub fn write(&self, src: &[T], offset: usize) -> Result {
> >> +        let end = offset.checked_add(src.len()).ok_or(EOVERFLOW)?;
> >> +        if end >= self.count {
> >> +            return Err(EINVAL);
> >> +        }
> >> +        // SAFETY:
> >> +        // - The pointer is valid due to type invariant on `CoherentAllocation`
> >> +        // and we've just checked that the range and index is within bounds.
> >> +        // - `offset` can't overflow since it is smaller than `self.count` and we've checked
> >> +        // that `self.count` won't overflow early in the constructor.
> >> +        unsafe {
> >> +core::ptr::copy_nonoverlapping(src.as_ptr(), self.cpu_addr.add(offset), src.len())
> > Why are there no concurrent write or read operations on `cpu_addr`?
>
> Thanks for the feedback! I noticed an additional safety requirement in
> slice::from_raw_parts_mut:
>
> "The memory referenced by the returned slice must not be accessed
> through any other pointer (not derived from the return value) for the
> duration of lifetime 'a. Both read and write accesses are forbidden."
>
> I can see now though why both as_slice and as_slice_mut docs needs more
> clarity. i.e., they could be worded similarly and add the additional
> safety requirement of slice::from_raw_parts_mut of having no other r/w
> access while the slice is live?

You can use the same wordins as `Page::read_raw` and `Page::write_raw`
for your as_slice[_mut] methods. See rust/kernel/page.rs.

Alice