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McKenney" To: Will Deacon Cc: linux-arch@vger.kernel.org, linux-kernel@vger.kernel.org, Benjamin Herrenschmidt , Arnd Bergmann , Peter Zijlstra , Andrea Parri , Daniel Lustig , David Howells , Alan Stern , Linus Torvalds Subject: Re: [RFC PATCH] docs/memory-barriers.txt: Rewrite "KERNEL I/O BARRIER EFFECTS" section Reply-To: paulmck@linux.ibm.com References: <20190211172948.3322-1-will.deacon@arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190211172948.3322-1-will.deacon@arm.com> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 x-cbid: 19021120-2213-0000-0000-0000034F22A7 X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00010578; HX=3.00000242; KW=3.00000007; PH=3.00000004; SC=3.00000279; SDB=6.01159650; UDB=6.00605182; IPR=6.00940197; MB=3.00025530; MTD=3.00000008; XFM=3.00000015; UTC=2019-02-11 20:22:22 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 19021120-2214-0000-0000-00005D52DAC5 Message-Id: <20190211202218.GQ4240@linux.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:,, definitions=2019-02-11_14:,, signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 priorityscore=1501 malwarescore=0 suspectscore=0 phishscore=0 bulkscore=0 spamscore=0 clxscore=1015 lowpriorityscore=0 mlxscore=0 impostorscore=0 mlxlogscore=999 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1810050000 definitions=main-1902110148 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Feb 11, 2019 at 05:29:48PM +0000, Will Deacon wrote: > 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 x86-centric, out-of-date, incomplete and demonstrably incorrect in places.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: Arnd Bergmann > Cc: Peter Zijlstra > Cc: Andrea Parri > Cc: Daniel Lustig > Cc: David Howells > Cc: Alan Stern > cc: Linus Torvalds > Signed-off-by: Will Deacon Hello, Will, The intent is to replace commit 3f305018dcf3 ("docs/memory-barriers.txt: Enforce heavy ordering for port I/O accesses"), correct? Either way is fine, just guessing based on the conflicts when applying this one. ;-) Thanx, Paul > --- > Documentation/memory-barriers.txt | 115 ++++++++++++++++++++------------------ > 1 file changed, 62 insertions(+), 53 deletions(-) > > diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt > index 1c22b21ae922..d08b49b2c011 100644 > --- a/Documentation/memory-barriers.txt > +++ b/Documentation/memory-barriers.txt > @@ -2599,72 +2599,81 @@ likely, then interrupt-disabling locks should be used to guarantee ordering. > KERNEL I/O BARRIER EFFECTS > ========================== > > -When accessing I/O memory, drivers should use the appropriate accessor > -functions: > - > - (*) inX(), outX(): > - > - These are intended to talk to I/O space rather than memory space, but > - that's primarily a CPU-specific concept. The i386 and x86_64 processors > - do indeed have special I/O space access cycles and instructions, but many > - CPUs don't have such a concept. > - > - The PCI bus, amongst others, defines an I/O space concept which - on such > - CPUs as i386 and x86_64 - readily maps to the CPU's concept of I/O > - space. However, it may also be mapped as a virtual I/O space in the CPU's > - memory map, particularly on those CPUs that don't support alternate I/O > - spaces. > - > - Accesses to this space may be fully synchronous (as on i386), but > - intermediary bridges (such as the PCI host bridge) may not fully honour > - that. > - > - They are guaranteed to be fully ordered with respect to each other. > - > - They are not guaranteed to be fully ordered with respect to other types of > - memory and I/O operation. > +Interfacing with peripherals via I/O accesses is deeply architecture and device > +specific. Therefore, drivers which are inherently non-portable may rely on > +specific behaviours of their target systems in order to achieve synchronization > +in the most lightweight manner possible. For drivers intending to be portable > +between multiple architectures and bus implementations, the kernel offers a > +series of accessor functions that provide various degrees of ordering > +guarantees: > > (*) readX(), writeX(): > > - Whether these are guaranteed to be fully ordered and uncombined with > - respect to each other on the issuing CPU depends on the characteristics > - defined for the memory window through which they're accessing. On later > - i386 architecture machines, for example, this is controlled by way of the > - MTRR registers. > + The readX() and writeX() MMIO accessors take a pointer to the peripheral > + being accessed as an __iomem * parameter. For pointers mapped with the > + default I/O attributes (e.g. those returned by ioremap()), then the > + ordering guarantees are as follows: > + > + 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. > + > + 2. A writeX() by the CPU to the peripheral will first wait for the > + completion of all prior CPU writes to memory. For example, this ensures > + that writes by the CPU to an outbound DMA buffer allocated by > + dma_alloc_coherent() will be visible to a DMA engine when the CPU writes > + to its MMIO control register to trigger the transfer. > + > + 3. A readX() by the CPU from the peripheral will complete before any > + subsequent CPU reads from memory can begin. For example, this ensures > + that reads by the CPU from an incoming DMA buffer allocated by > + dma_alloc_coherent() will not see stale data after reading from the DMA > + engine's MMIO status register to establish that the DMA transfer has > + completed. > + > + 4. A readX() by the CPU from the peripheral will complete before any > + subsequent delay() loop can begin execution. For example, this ensures > + that two MMIO register writes by the CPU to a peripheral will arrive at > + least 1us apart if the first write is immediately read back with readX() > + and udelay(1) is called prior to the second writeX(). > + > + __iomem pointers obtained with non-default attributes (e.g. those returned > + by ioremap_wc()) are unlikely to provide many of these guarantees. If > + ordering is required for such mappings, then the mandatory barriers should > + be used in conjunction with the _relaxed() accessors defined below. > + > + (*) readX_relaxed(), writeX_relaxed(): > > - Ordinarily, these will be guaranteed to be fully ordered and uncombined, > - provided they're not accessing a prefetchable device. > - > - However, intermediary hardware (such as a PCI bridge) may indulge in > - deferral if it so wishes; to flush a store, a load from the same location > - is preferred[*], but a load from the same device or from configuration > - space should suffice for PCI. > - > - [*] NOTE! attempting to load from the same location as was written to may > - cause a malfunction - consider the 16550 Rx/Tx serial registers for > - example. > - > - Used with prefetchable I/O memory, an mmiowb() barrier may be required to > - force stores to be ordered. > + These are similar to readX() and writeX(), but provide weaker memory > + ordering guarantees. Specifically, they do not guarantee ordering with > + respect to normal memory accesses or delay() loops (i.e bullets 2-4 above) > + but they are still guaranteed to be ordered with respect to other accesses > + to the same peripheral when operating on __iomem pointers mapped with the > + default I/O attributes. > > - Please refer to the PCI specification for more information on interactions > - between PCI transactions. > + (*) inX(), outX(): > > - (*) readX_relaxed(), writeX_relaxed() > + The inX() and outX() accessors are intended to access legacy port-mapped > + I/O peripherals, which may require special instructions on some > + architectures (notably x86). The port number of the peripheral being > + accessed is passed as an argument. > > - These are similar to readX() and writeX(), but provide weaker memory > - ordering guarantees. Specifically, they do not guarantee ordering with > - respect to normal memory accesses (e.g. DMA buffers) nor do they guarantee > - ordering with respect to LOCK or UNLOCK operations. If the latter is > - required, an mmiowb() barrier can be used. Note that relaxed accesses to > - the same peripheral are guaranteed to be ordered with respect to each > - other. > + Since many CPU architectures ultimately access these peripherals via an > + internal virtual memory mapping, the portable ordering guarantees provided > + by inX() and outX() are the same as those provided by readX() and writeX() > + 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 > -- > 2.11.0 >