linux-kernel - Re: [PATCH RFC LKMM 5/7] docs/memory-barriers.txt: Enforce heavy ordering for port I/O accesses

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Message-ID: <CAK8P3a3bkQoVfkV-+pNniirLN5Z_Wv3eG2TXW35kdJixCOH9_Q@mail.gmail.com>
Date:   Mon, 11 Feb 2019 18:11:48 +0100
From:   Arnd Bergmann <arnd@...db.de>
To:     Will Deacon <will.deacon@....com>
Cc:     "Paul E. McKenney" <paulmck@...ux.ibm.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Alan Stern <stern@...land.harvard.edu>,
        Andrea Parri <parri.andrea@...il.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Boqun Feng <boqun.feng@...il.com>,
        Nicholas Piggin <npiggin@...il.com>,
        David Howells <dhowells@...hat.com>, j.alglave@....ac.uk,
        luc.maranget@...ia.fr, Matthew Wilcox <willy@...radead.org>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        David Laight <David.Laight@...lab.com>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>
Subject: Re: [PATCH RFC LKMM 5/7] docs/memory-barriers.txt: Enforce heavy
 ordering for port I/O accesses

On Mon, Feb 11, 2019 at 4:30 PM Will Deacon <will.deacon@....com> wrote:

> Given the lack of Intel response here, I went away to do some digging.
> As evidenced by the commit message, there is certainly an understanding
> amongst some developers that inX/outX() are strongly ordered on x86 and
> this was re-enforced by Linus in March last year:
>
> https://www.mail-archive.com/linuxppc-dev@lists.ozlabs.org/msg131212.html
>
> It was this information on which I based my patch. The Intel SDM is not
> quite as assertive in its claims.
>
> However, it has also occurred to me that this patch is actually missing
> the point. memory-barriers.txt should be documenting the *Linux* memory
> model, not the x86 one, and so the port accessors should be defined to
> have the same ordering semantics as the MMIO accessors. If this wasn't
> the case, then macros such as ioreadX() and iowriteX() would be unusable
> in portable driver code.

My interpretation of the ioreadX() and iowriteX() semantics is that they
only guarantee readl()/writel() barrier semantics, even though they
may in fact provide stronger barriers for PIO on architectures that use
CONFIG_GENERIC_IOMAP (which falls back to inX()/outX()).

> The inX/outX implementation in asm-generic would
> also be bogus, despite being widely used.

They likely are. The asm-generic files tend to provide a generic
abstraction as much as that is possible, but without having access
to the architecture specific semantics, they raditionally don't know
what should be done here. We now have __io_pbw()/__io_paw()/
__io_pbr()/__io_par() to let architectures get it right, but that is
a fairly recent addition, so nothing other than riscv defines them
today.
To make things worse, a lot of machines are unable to provide
__io_paw(), e.g. when all bus writes are posted.

      Arnd