linux-kernel - Re: [RFC PATCH] docs/memory-barriers.txt: Rewrite "KERNEL I/O BARRIER EFFECTS" section

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Date:   Wed, 13 Feb 2019 17:20:47 +0000
From:   Will Deacon <will.deacon@....com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     linux-arch <linux-arch@...r.kernel.org>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        "Paul E. McKenney" <paulmck@...ux.ibm.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Arnd Bergmann <arnd@...db.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Andrea Parri <andrea.parri@...rulasolutions.com>,
        Daniel Lustig <dlustig@...dia.com>,
        David Howells <dhowells@...hat.com>,
        Alan Stern <stern@...land.harvard.edu>, tony.luck@...el.com
Subject: Re: [RFC PATCH] docs/memory-barriers.txt: Rewrite "KERNEL I/O
 BARRIER EFFECTS" section

[+Tony]

On Mon, Feb 11, 2019 at 02:34:31PM -0800, Linus Torvalds wrote:
> On Mon, Feb 11, 2019 at 9:30 AM Will Deacon <will.deacon@....com> wrote:
> >
> > +
> > +     1. All readX() and writeX() accesses to the same peripheral are ordered
> > +        with respect to each other. For example, this ensures that MMIO register
> > +       writes by the CPU to a particular device will arrive in program order.
> 
> Hmm. I'd like more people look at strengthening this one wrt across
> CPUs and locking.
> 
> Right now we document mmiowb(), but that "documentation" is really
> just a fairy tale. Very *very* few drivers actually do mmiowb() on
> their own.
> 
> IOW, we should seriously just consider making the rule be that locking
> will order mmio too. Because that's practically the rule anyway.

I would /love/ to get rid of mmiowb() because I think it's both extremely
difficult to use and also pretty much never needed. It reminds me a lot of
smp_read_barrier_depends(), which we finally pushed into READ_ONCE for
Alpha.

> Powerpc already does it. IO within a locked region will serialize with the
> lock.

I thought ia64 was the hold out here? Did they actually have machines that
needed this in practice? If so, I think we can either:

  (a) Add an mmiowb() to their spin_unlock() code, or
  (b) Remove ia64 altogether if nobody complains

I know that Peter has been in favour of (b) for a while...

Will