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[159.2.239.150]) by smtp.gmail.com with ESMTPSA id af79cd13be357-940f078a762sm18520585a.30.2026.10.23.15.01.49 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 23 Oct 2026 15:01:49 -0700 (PDT) Received: from jgg by wakko with local (Exim 4.97) (envelope-from ) id 1xKNKe-0000000H7Vk-3jIV; Fri, 23 Oct 2026 19:01:48 -0300 Date: Fri, 23 Oct 2026 19:01:48 -0300 From: Jason Gunthorpe To: Bjorn Helgaas Cc: Leon Romanovsky , Bjorn Helgaas , Logan Gunthorpe , "Joerg Roedel (AMD)" , Will Deacon , Robin Murphy , Christian =?utf-8?B?S8O2bmln?= , Thomas =?utf-8?Q?Hellstr=C3=B6m?= , linux-pci@vger.kernel.org, linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org, iommu@lists.linux.dev, Tushar Dave , linux-media@vger.kernel.org, dri-devel@lists.freedesktop.org, linaro-mm-sig@lists.linaro.org, linux-rdma@vger.kernel.org, kvm@vger.kernel.org, Chaitanya Kulkarni , Greg Kroah-Hartman , Jens Axboe , Alex Williamson , Ankit Agrawal , Jonathan Corbet , Shuah Khan , Randy Dunlap , Sumit Semwal Subject: Re: [PATCH v9 02/18] PCI/P2PDMA: Derive routing from directional ACS controls Message-ID: <20261023220148.GN3977851@ziepe.ca> References: <20261023190036.GL3977851@ziepe.ca> <20261023201531.GA2805244@bhelgaas> X-Mailing-List: kvm@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20261023201531.GA2805244@bhelgaas> On Fri, Oct 23, 2026 at 03:15:31PM -0500, Bjorn Helgaas wrote: > On Fri, Oct 23, 2026 at 04:00:36PM -0300, Jason Gunthorpe wrote: > > On Fri, Oct 23, 2026 at 01:27:15PM -0500, Bjorn Helgaas wrote: > > > > > > Yes, the ACS combination is selected to avoid ordering issues > > > > > > Does a user have any way to know whether the configuration is safe? > > > > No, unfortunately in PCIe so much is really difficult when it comes to > > deployment :( > > > > > I'm wondering whether users might use "pci=config_acs=" to configure > > > ACS so things appear to work as they prefer but may not provide the > > > ordering guarantees drivers depend on. > > > > I imagine that could happen, but on most practical systems the root > > complex will blow up if you do asymmetric routing so there is an early > > indication. > > > > I know this because I've help debug such configurations at > > customers. Some servers helpfully MCE if you do this. Lovely. > > I'm still confused about this because earlier in this thread, I > thought Leon said you do asymmetric routing: > > Bjorn> And I guess asymmetric fabrics must mean fabrics where > Bjorn> Request Redirect and Completion Redirect are not set the same > Bjorn> way? > > Leon> Yes, in some of our systems, deviceA to deviceB is going > Leon> through different route than deviceB to deviceA. Yeah, that's what I clarifed, it isn't quite accurate. The asymetric routing is between Strict Order and Relaxed Order TLPs, not request/response. Jason