Message-ID: <CAH5fLgjys9wkSy=YSnw6QoOw_rbfgkLr5nNSGhh_+CSOUcaWkg@mail.gmail.com>
Date: Mon, 27 Jan 2025 20:05:46 +0100
From: Alice Ryhl <aliceryhl@...gle.com>
To: Boqun Feng <boqun.feng@...il.com>
Cc: Danilo Krummrich <dakr@...nel.org>, Abdiel Janulgue <abdiel.janulgue@...il.com>, 
	rust-for-linux@...r.kernel.org, daniel.almeida@...labora.com, 
	robin.murphy@....com, Miguel Ojeda <ojeda@...nel.org>, 
	Alex Gaynor <alex.gaynor@...il.com>, Gary Guo <gary@...yguo.net>, 
	Björn Roy Baron <bjorn3_gh@...tonmail.com>, 
	Benno Lossin <benno.lossin@...ton.me>, Andreas Hindborg <a.hindborg@...nel.org>, 
	Trevor Gross <tmgross@...ch.edu>, Valentin Obst <kernel@...entinobst.de>, 
	open list <linux-kernel@...r.kernel.org>, Christoph Hellwig <hch@....de>, 
	Marek Szyprowski <m.szyprowski@...sung.com>, airlied@...hat.com, 
	"open list:DMA MAPPING HELPERS" <iommu@...ts.linux.dev>
Subject: Re: [PATCH v11 2/3] rust: add dma coherent allocator abstraction.

On Mon, Jan 27, 2025 at 8:01 PM Boqun Feng <boqun.feng@...il.com> wrote:
>
> On Mon, Jan 27, 2025 at 07:46:07PM +0100, Alice Ryhl wrote:
> > On Mon, Jan 27, 2025 at 7:38 PM Boqun Feng <boqun.feng@...il.com> wrote:
> > >
> > > On Mon, Jan 27, 2025 at 07:32:29PM +0100, Alice Ryhl wrote:
> > > > On Mon, Jan 27, 2025 at 5:59 PM Boqun Feng <boqun.feng@...il.com> wrote:
> > > > >
> > > > > On Mon, Jan 27, 2025 at 11:43:39AM +0100, Alice Ryhl wrote:
> > > > > > On Mon, Jan 27, 2025 at 11:37 AM Danilo Krummrich <dakr@...nel.org> wrote:
> > > > > > >
> > > > > > > On Fri, Jan 24, 2025 at 08:27:36AM +0100, Alice Ryhl wrote:
> > > > > > > > On Thu, Jan 23, 2025 at 11:43 AM Abdiel Janulgue
> > > > > > > > <abdiel.janulgue@...il.com> wrote:
> > > > > > > > > +    /// Reads data from the region starting from `offset` as a slice.
> > > > > > > > > +    /// `offset` and `count` are in units of `T`, not the number of bytes.
> > > > > > > > > +    ///
> > > > > > > > > +    /// Due to the safety requirements of slice, the data returned should be regarded by the
> > > > > > > > > +    /// caller as a snapshot of the region when this function is called, as the region could
> > > > > > > > > +    /// be modified by the device at anytime. For ringbuffer type of r/w access or use-cases
> > > > > > > > > +    /// where the pointer to the live data is needed, `start_ptr()` or `start_ptr_mut()`
> > > > > > > > > +    /// could be used instead.
> > > > > > > > > +    ///
> > > > > > > > > +    /// # Safety
> > > > > > > > > +    ///
> > > > > > > > > +    /// Callers must ensure that no hardware operations that involve the buffer are currently
> > > > > > > > > +    /// taking place while the returned slice is live.
> > > > > > > > > +    pub unsafe fn as_slice(&self, offset: usize, count: usize) -> Result<&[T]> {
> > > > > > > >
> > > > > > > > You were asked to rename this function because it returns a slice, but
> > > > > > > > I wonder if it's better to take an `&mut [T]` argument and to have
> > > > > > > > this function copy data into that argument. That way, we could make
> > > > > > > > the function itself safe. Perhaps the actual copy needs to be
> > > > > > > > volatile?
> > > > > > >
> > > > > > > Why do we consider the existing one unsafe?
> > > > > > >
> > > > > > > Surely, it's not desirable that the contents of the buffer are modified by the
> > > > > > > HW unexpectedly, but is this a concern in terms of Rust safety requirements?
> > > > > > >
> > > > > > > And if so, how does this go away with the proposed approach?
> > > > > >
> > > > > > In Rust, it is undefined behavior if the value behind an immutable
> > > > > > reference changes (unless the type uses UnsafeCell / Opaque or
> > > > > > similar). That is, any two consecutive reads of the same immutable
> > > > > > reference must return the same byte value no matter what happened in
> > > > > > between those reads.
> > > > > >
> > > > > > If we manually perform the read as a volatile read, then it is
> > > > > > arguably allowed for the value to be modified by the hardware while we
> > > > > > read the value.
> > > > > >
> > > > >
> > > > > Do you also assume that volatile read/write provide some sort of
> > > > > atomicity? Because otherwise even though the read/write may not be
> > > > > considered as UB, then results can be load/store teared.
> > > > >
> > > > > I asked because I think in case that we need atomicity, we should just
> > > > > use atomic APIs.
> > > >
> > > > No, I'm not assuming that. I think it's like uaccess. Under normal
> > > > cases, it's not going to be concurrently modified, but it shouldn't
> > > > trigger UB if it is.
> > > >
> > >
> > > Let's say my_alloc[7].foo is a (u64, u64), would
> > >
> > >         dma_read!(my_alloc[7].foo)
> > >
> > > and
> > >
> > >         dma_write!(my_alloc[7].foo, (1u64, 2u64))
> > >
> > > trigger any UB when they are concurrent? (Of course, the example here is
> > > a bit inpropriate because it's DMA buff, but still the question is more
> > > on whatever atomic expectation we want from read_volatile() and
> > > write_volatile()).
> >
> > I imagine that it would be most convenient for it to not be UB, but I
> > also don't think people would have an expectation for that to not
> > involve tearing.
> >
>
> Depending on the granularity that tearing can happen, if .foo is an enum
> (or any other type that not all bit combinations are valid) and tearing
> can happen at byte levels, then a racing dma_read() may read invalid
> data.

T: FromBytes + ToBytes is already required for these types. You can't
use these operations with such an enum.

> I think it's fine to expect read_volatile() and write_volatile()
> themselves don't trigger UB, but we will need to be careful about the
> atomic granularity that we can expect on them. It would be more clear if
> we use the atomic API here (and implementation can be read_volatile()
> and write_volatile()), and it can avoid coding based on tribal knowledge
> such as "in kernel, read_volatile() and write_volatile() imply atomic".

Why should we use atomics for operations that don't need to be atomic?
Most of the time, dma memory is not *actually* changed while you read
it.

Alice

Powered by blists - more mailing lists