linux-kernel - Re: LKMM: Read dependencies of writes ordered by dma

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Date:   Tue, 17 Aug 2021 15:27:23 +0200
From:   Marco Elver <elver@...gle.com>
To:     Will Deacon <will@...nel.org>
Cc:     "Paul E. McKenney" <paulmck@...nel.org>,
        Alan Stern <stern@...land.harvard.edu>,
        Boqun Feng <boqun.feng@...il.com>,
        Andrea Parri <parri.andrea@...il.com>,
        Mark Rutland <mark.rutland@....com>,
        Dmitry Vyukov <dvyukov@...gle.com>, kasan-dev@...glegroups.com,
        linux-kernel@...r.kernel.org
Subject: Re: LKMM: Read dependencies of writes ordered by dma_wmb()?

On Tue, 17 Aug 2021 at 14:28, Will Deacon <will@...nel.org> 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...):
>
> https://www.youtube.com/watch?v=i6DayghhA8Q

Nice, thank you!

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