From: Arnd Bergmann <arnd@arndb.de>
To: Will Deacon <will.deacon@arm.com>
Cc: "Paul E. McKenney" <paulmck@linux.ibm.com>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
linux-arch <linux-arch@vger.kernel.org>,
Ingo Molnar <mingo@kernel.org>,
Alan Stern <stern@rowland.harvard.edu>,
Andrea Parri <parri.andrea@gmail.com>,
Peter Zijlstra <peterz@infradead.org>,
Boqun Feng <boqun.feng@gmail.com>,
Nicholas Piggin <npiggin@gmail.com>,
David Howells <dhowells@redhat.com>,
j.alglave@ucl.ac.uk, luc.maranget@inria.fr,
Matthew Wilcox <willy@infradead.org>,
Benjamin Herrenschmidt <benh@kernel.crashing.org>,
David Laight <David.Laight@aculab.com>,
"open list:DOCUMENTATION" <linux-doc@vger.kernel.org>
Subject: Re: [PATCH RFC LKMM 5/7] docs/memory-barriers.txt: Enforce heavy ordering for port I/O accesses
Date: Mon, 11 Feb 2019 18:11:48 +0100 [thread overview]
Message-ID: <CAK8P3a3bkQoVfkV-+pNniirLN5Z_Wv3eG2TXW35kdJixCOH9_Q@mail.gmail.com> (raw)
In-Reply-To: <20190211153043.GC32385@fuggles.cambridge.arm.com>
On Mon, Feb 11, 2019 at 4:30 PM Will Deacon <will.deacon@arm.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
next prev parent reply other threads:[~2019-02-11 17:12 UTC|newest]
Thread overview: 28+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-01-09 21:07 [PATCH RFC memory-model 0/6] LKMM updates Paul E. McKenney
2019-01-09 21:07 ` [PATCH RFC LKMM 1/7] tools/memory-model: Rename some RCU relations Paul E. McKenney
2019-01-09 21:07 ` [PATCH RFC LKMM 2/7] tools/memory-model: Refactor " Paul E. McKenney
2019-01-09 21:07 ` [PATCH RFC LKMM 3/7] tools/memory-model: Add SRCU support Paul E. McKenney
2019-01-09 21:07 ` [PATCH RFC LKMM 4/7] tools/memory-model: Update README for addition of SRCU Paul E. McKenney
2019-01-09 21:07 ` [PATCH RFC LKMM 5/7] docs/memory-barriers.txt: Enforce heavy ordering for port I/O accesses Paul E. McKenney
2019-01-11 9:53 ` Peter Zijlstra
2019-02-11 15:30 ` Will Deacon
2019-02-11 17:11 ` Arnd Bergmann [this message]
2019-02-11 17:32 ` Will Deacon
2019-01-09 21:07 ` [PATCH RFC LKMM 6/7] tools/memory-model: Update Documentation/explanation.txt to include SRCU support Paul E. McKenney
2019-01-09 21:07 ` [PATCH RFC LKMM 7/7] tools/memory-model: Dynamically check SRCU lock-to-unlock matching Paul E. McKenney
2019-01-10 9:41 ` Andrea Parri
2019-01-10 14:40 ` Paul E. McKenney
2019-01-10 23:20 ` Andrea Parri
2019-01-11 21:44 ` Paul E. McKenney
2019-01-11 21:57 ` Alan Stern
2019-01-09 23:18 ` [PATCH RFC memory-model 0/6] LKMM updates Andrea Parri
2019-01-09 23:40 ` Paul E. McKenney
2019-01-10 0:39 ` Andrea Parri
2019-01-10 4:20 ` Paul E. McKenney
2019-01-10 8:40 ` Andrea Parri
2019-01-10 14:31 ` Paul E. McKenney
2019-01-10 15:41 ` Alan Stern
2019-01-10 16:31 ` Paul E. McKenney
2019-01-10 22:46 ` Andrea Parri
2019-01-10 15:47 ` Alan Stern
2019-01-10 19:03 ` Paul E. McKenney
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=CAK8P3a3bkQoVfkV-+pNniirLN5Z_Wv3eG2TXW35kdJixCOH9_Q@mail.gmail.com \
--to=arnd@arndb.de \
--cc=David.Laight@aculab.com \
--cc=benh@kernel.crashing.org \
--cc=boqun.feng@gmail.com \
--cc=dhowells@redhat.com \
--cc=j.alglave@ucl.ac.uk \
--cc=linux-arch@vger.kernel.org \
--cc=linux-doc@vger.kernel.org \
--cc=linux-kernel@vger.kernel.org \
--cc=luc.maranget@inria.fr \
--cc=mingo@kernel.org \
--cc=npiggin@gmail.com \
--cc=parri.andrea@gmail.com \
--cc=paulmck@linux.ibm.com \
--cc=peterz@infradead.org \
--cc=stern@rowland.harvard.edu \
--cc=will.deacon@arm.com \
--cc=willy@infradead.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).