From mboxrd@z Thu Jan 1 00:00:00 1970 From: Akira Yokosawa Subject: Re: [PATCH tip/core/rcu 04/21] docs/memory-barriers.txt: Rewrite "KERNEL I/O BARRIER EFFECTS" section Date: Fri, 5 Apr 2019 07:23:59 +0900 Message-ID: <1a30f6ec-cbf9-7bb0-3b08-ab785156a144@gmail.com> References: <20190326234114.GA23843@linux.ibm.com> <20190326234133.24962-4-paulmck@linux.ibm.com> <20190402130346.GA14559@fuggles.cambridge.arm.com> <7c0b1afc-9308-e060-d1cc-7389a2330e97@gmail.com> <20190404164022.GB28932@fuggles.cambridge.arm.com> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <20190404164022.GB28932@fuggles.cambridge.arm.com> Content-Language: en-US Sender: linux-kernel-owner@vger.kernel.org To: Will Deacon Cc: "Paul E. McKenney" , linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org, mingo@kernel.org, stern@rowland.harvard.edu, andrea.parri@amarulasolutions.com, peterz@infradead.org, boqun.feng@gmail.com, npiggin@gmail.com, dhowells@redhat.com, j.alglave@ucl.ac.uk, luc.maranget@inria.fr, Benjamin Herrenschmidt , Michael Ellerman , Arnd Bergmann , Palmer Dabbelt , Daniel Lustig , Linus Torvalds , "Maciej W. Rozycki" , Mikulas Patocka List-Id: linux-arch.vger.kernel.org On Thu, 4 Apr 2019 17:40:22 +0100, Will Deacon wrote: > Hi Akira, > > On Fri, Apr 05, 2019 at 12:58:36AM +0900, Akira Yokosawa wrote: >> On Tue, 2 Apr 2019 14:03:46 +0100, Will Deacon wrote: >>> On Tue, Mar 26, 2019 at 04:41:16PM -0700, Paul E. McKenney wrote: >>>> From: Will Deacon >>>> >>>> The "KERNEL I/O BARRIER EFFECTS" section of memory-barriers.txt is vague, >>>> x86-centric, out-of-date, incomplete and demonstrably incorrect in places. >>>> This is largely because I/O ordering is a horrible can of worms, but also >>>> because the document has stagnated as our understanding has evolved. >>>> >>>> Attempt to address some of that, by rewriting the section based on >>>> recent(-ish) discussions with Arnd, BenH and others. Maybe one day we'll >>>> find a way to formalise this stuff, but for now let's at least try to >>>> make the English easier to understand. >>>> >>>> Cc: "Paul E. McKenney" >>>> Cc: Benjamin Herrenschmidt >>>> Cc: Michael Ellerman >>>> Cc: Arnd Bergmann >>>> Cc: Peter Zijlstra >>>> Cc: Andrea Parri >>>> Cc: Palmer Dabbelt >>>> Cc: Daniel Lustig >>>> Cc: David Howells >>>> Cc: Alan Stern >>>> Cc: Linus Torvalds >>>> Cc: "Maciej W. Rozycki" >>>> Cc: Mikulas Patocka >>>> Signed-off-by: Will Deacon >>>> Signed-off-by: Paul E. McKenney >>>> --- >>>> Documentation/memory-barriers.txt | 115 ++++++++++++++++++------------ >>>> 1 file changed, 70 insertions(+), 45 deletions(-) >>> >>> If somebody could provide an Ack on this patch, I'd really appreciate it, >>> please. Whilst the portable ordering guarantees that I've documented are >>> fairly conservative, I do think that this change is a big improvement and >>> gives you what you need if you're writing a portable device driver for a new >>> piece of hardware. I'm tackling the removal of MMIOWB as a separate series. >>> >>> I think Paul now requires an Ack before he'll send a patch to mainline, >>> hence the grovelling. >> >> I'm afraid I'm not that qualified to provide an Ack to this patch, >> but please find a nit fix below. > > Oh well, thanks for having a look anyway! > >>>> + (*) insX(), outsX(): >>>> + >>>> + As above, the insX() and outX() accessors provide the same ordering >> outsX() > > Thanks; I'll fix that. > >>>> + guarantees as readsX() and writesX() respectively when accessing a mapping >>>> + with the default I/O attributes. >>>> >>>> (*) ioreadX(), iowriteX() >>>> >>>> These will perform appropriately for the type of access they're actually >>>> doing, be it inX()/outX() or readX()/writeX(). >>>> >>>> +All of these accessors assume that the underlying peripheral is little-endian, >>>> +and will therefore perform byte-swapping operations on big-endian architectures. >>>> + >>>> +Composing I/O ordering barriers with SMP ordering barriers and LOCK/UNLOCK >>>> +operations is a dangerous sport which may require the use of mmiowb(). See the >>>> +subsection "Acquires vs I/O accesses" for more information. >>>> >>>> ======================================== >>>> ASSUMED MINIMUM EXECUTION ORDERING MODEL >>>> -- >>>> 2.17.1 >>>> >> >> JFYI, there is another document Documentation/driver-api/device-io.rst, >> which is somewhat related to this update. It looks like this one also needs >> some update, as Jon commented in transforming to .rst format in commit >> 8a8a602fdb83 ("docs: Convert the deviceio template to RST"): >> >> Like the rest of our documentation, this one could use some work. There's >> no mention of ioremap() and friends, no mention of io_read*() and friends. >> But we have nice documentation for all those folks writing new drivers that >> do port I/O :). >> >> >> This commit was merged in v4.11 cycle. And there has been no update whatsoever >> since. mmiowb() is lightly mentioned therein. IMHO, just updating >> memory-barriers.txt would widen the gap of information. >> >> Thoughts? > > I have a subsequent patch which kills mmiowb() entirely: > > https://git.kernel.org/pub/scm/linux/kernel/git/arm64/linux.git/commit/?h=for-next/mmiowb&id=3c1a2050c08fb8193777b60b49e60320254a156c > > and that one does hit device-io.rst. Ah, I see. So can somebody else have a look at this patch and provide an Ack, please? Thanks, Akira > > Will > From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pl1-f196.google.com ([209.85.214.196]:42808 "EHLO mail-pl1-f196.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727441AbfDDWYI (ORCPT ); Thu, 4 Apr 2019 18:24:08 -0400 Subject: Re: [PATCH tip/core/rcu 04/21] docs/memory-barriers.txt: Rewrite "KERNEL I/O BARRIER EFFECTS" section References: <20190326234114.GA23843@linux.ibm.com> <20190326234133.24962-4-paulmck@linux.ibm.com> <20190402130346.GA14559@fuggles.cambridge.arm.com> <7c0b1afc-9308-e060-d1cc-7389a2330e97@gmail.com> <20190404164022.GB28932@fuggles.cambridge.arm.com> From: Akira Yokosawa Message-ID: <1a30f6ec-cbf9-7bb0-3b08-ab785156a144@gmail.com> Date: Fri, 5 Apr 2019 07:23:59 +0900 MIME-Version: 1.0 In-Reply-To: <20190404164022.GB28932@fuggles.cambridge.arm.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-arch-owner@vger.kernel.org List-ID: To: Will Deacon Cc: "Paul E. McKenney" , linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org, mingo@kernel.org, stern@rowland.harvard.edu, andrea.parri@amarulasolutions.com, peterz@infradead.org, boqun.feng@gmail.com, npiggin@gmail.com, dhowells@redhat.com, j.alglave@ucl.ac.uk, luc.maranget@inria.fr, Benjamin Herrenschmidt , Michael Ellerman , Arnd Bergmann , Palmer Dabbelt , Daniel Lustig , Linus Torvalds , "Maciej W. Rozycki" , Mikulas Patocka Message-ID: <20190404222359.xKdDJnI0N_wlhqlDsffDWERbysJOMMLjYlLuKvI0aIo@z> On Thu, 4 Apr 2019 17:40:22 +0100, Will Deacon wrote: > Hi Akira, > > On Fri, Apr 05, 2019 at 12:58:36AM +0900, Akira Yokosawa wrote: >> On Tue, 2 Apr 2019 14:03:46 +0100, Will Deacon wrote: >>> On Tue, Mar 26, 2019 at 04:41:16PM -0700, Paul E. McKenney wrote: >>>> From: Will Deacon >>>> >>>> The "KERNEL I/O BARRIER EFFECTS" section of memory-barriers.txt is vague, >>>> x86-centric, out-of-date, incomplete and demonstrably incorrect in places. >>>> This is largely because I/O ordering is a horrible can of worms, but also >>>> because the document has stagnated as our understanding has evolved. >>>> >>>> Attempt to address some of that, by rewriting the section based on >>>> recent(-ish) discussions with Arnd, BenH and others. Maybe one day we'll >>>> find a way to formalise this stuff, but for now let's at least try to >>>> make the English easier to understand. >>>> >>>> Cc: "Paul E. McKenney" >>>> Cc: Benjamin Herrenschmidt >>>> Cc: Michael Ellerman >>>> Cc: Arnd Bergmann >>>> Cc: Peter Zijlstra >>>> Cc: Andrea Parri >>>> Cc: Palmer Dabbelt >>>> Cc: Daniel Lustig >>>> Cc: David Howells >>>> Cc: Alan Stern >>>> Cc: Linus Torvalds >>>> Cc: "Maciej W. Rozycki" >>>> Cc: Mikulas Patocka >>>> Signed-off-by: Will Deacon >>>> Signed-off-by: Paul E. McKenney >>>> --- >>>> Documentation/memory-barriers.txt | 115 ++++++++++++++++++------------ >>>> 1 file changed, 70 insertions(+), 45 deletions(-) >>> >>> If somebody could provide an Ack on this patch, I'd really appreciate it, >>> please. Whilst the portable ordering guarantees that I've documented are >>> fairly conservative, I do think that this change is a big improvement and >>> gives you what you need if you're writing a portable device driver for a new >>> piece of hardware. I'm tackling the removal of MMIOWB as a separate series. >>> >>> I think Paul now requires an Ack before he'll send a patch to mainline, >>> hence the grovelling. >> >> I'm afraid I'm not that qualified to provide an Ack to this patch, >> but please find a nit fix below. > > Oh well, thanks for having a look anyway! > >>>> + (*) insX(), outsX(): >>>> + >>>> + As above, the insX() and outX() accessors provide the same ordering >> outsX() > > Thanks; I'll fix that. > >>>> + guarantees as readsX() and writesX() respectively when accessing a mapping >>>> + with the default I/O attributes. >>>> >>>> (*) ioreadX(), iowriteX() >>>> >>>> These will perform appropriately for the type of access they're actually >>>> doing, be it inX()/outX() or readX()/writeX(). >>>> >>>> +All of these accessors assume that the underlying peripheral is little-endian, >>>> +and will therefore perform byte-swapping operations on big-endian architectures. >>>> + >>>> +Composing I/O ordering barriers with SMP ordering barriers and LOCK/UNLOCK >>>> +operations is a dangerous sport which may require the use of mmiowb(). See the >>>> +subsection "Acquires vs I/O accesses" for more information. >>>> >>>> ======================================== >>>> ASSUMED MINIMUM EXECUTION ORDERING MODEL >>>> -- >>>> 2.17.1 >>>> >> >> JFYI, there is another document Documentation/driver-api/device-io.rst, >> which is somewhat related to this update. It looks like this one also needs >> some update, as Jon commented in transforming to .rst format in commit >> 8a8a602fdb83 ("docs: Convert the deviceio template to RST"): >> >> Like the rest of our documentation, this one could use some work. There's >> no mention of ioremap() and friends, no mention of io_read*() and friends. >> But we have nice documentation for all those folks writing new drivers that >> do port I/O :). >> >> >> This commit was merged in v4.11 cycle. And there has been no update whatsoever >> since. mmiowb() is lightly mentioned therein. IMHO, just updating >> memory-barriers.txt would widen the gap of information. >> >> Thoughts? > > I have a subsequent patch which kills mmiowb() entirely: > > https://git.kernel.org/pub/scm/linux/kernel/git/arm64/linux.git/commit/?h=for-next/mmiowb&id=3c1a2050c08fb8193777b60b49e60320254a156c > > and that one does hit device-io.rst. Ah, I see. So can somebody else have a look at this patch and provide an Ack, please? Thanks, Akira > > Will >