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[159.2.239.150]) by smtp.gmail.com with ESMTPSA id d75a77b69052e-53839709ed6sm1185741cf.16.2026.10.23.12.00.37 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 23 Oct 2026 12:00:37 -0700 (PDT) Received: from jgg by wakko with local (Exim 4.97) (envelope-from ) id 1xKKVI-0000000GxzN-4BG5; Fri, 23 Oct 2026 16:00:36 -0300 Date: Fri, 23 Oct 2026 16:00:36 -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: <20261023190036.GL3977851@ziepe.ca> References: <20261023155914.GK3977851@ziepe.ca> <20261023182715.GA2798722@bhelgaas> X-Mailing-List: iommu@lists.linux.dev 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: <20261023182715.GA2798722@bhelgaas> 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. > If that's a possibility, do we have any notes in dmesg that we are > depending on fabric behavior that the OS can't discover and verify? Maybe, it could be interesting if the P2P logic found asymetric paths it refuses to permit it. Jason