Re: LKMM: Read dependencies of writes ordered by dma_wmb()?

From: Marco Elver
Date: Tue Aug 17 2021 - 09:27:41 EST

On Tue, 17 Aug 2021 at 14:28, Will Deacon <will@xxxxxxxxxx> wrote:
> Just on this bit...
> On Mon, Aug 16, 2021 at 01:50:57PM -0700, Paul E. McKenney wrote:
> > 5. The dma_mb(), dma_rmb(), and dma_wmb() appear to be specific
> > to ARMv8.
> These are useful on other architectures too! IIRC, they were added by x86 in
> the first place. They're designed to be used with dma_alloc_coherent()
> allocations where you're sharing something like a ring buffer with a device
> and they guarantee accesses won't be reordered before they become visible
> to the device. They _also_ provide the same ordering to other CPUs.

Ah, good you pointed it out again. Re-reading memory-barriers.txt and
it does also say these provide order for other CPUs...

> I gave a talk at LPC about some of this, which might help (or might make
> things worse...):

Nice, thank you!

> Ignore the bits about mmiowb() as we got rid of that.
> Will