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[159.2.239.150]) by smtp.gmail.com with ESMTPSA id 6a1803df08f44-91260a60f21sm67324286d6.19.2026.09.21.06.25.59 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 21 Sep 2026 06:25:59 -0700 (PDT) Received: from jgg by wakko with local (Exim 4.97) (envelope-from ) id 1x8e1v-00000006Knm-1R2E; Mon, 21 Sep 2026 10:25:59 -0300 Date: Mon, 21 Sep 2026 10:25:59 -0300 From: Jason Gunthorpe To: Leon Romanovsky Cc: Thomas =?utf-8?Q?Hellstr=C3=B6m?= , Bjorn Helgaas , Logan Gunthorpe , Chaitanya Kulkarni , Greg Kroah-Hartman , Jens Axboe , Alex Williamson , Ankit Agrawal , Jonathan Corbet , Shuah Khan , "Joerg Roedel (AMD)" , Will Deacon , Robin Murphy , Randy Dunlap , Sumit Semwal , Christian =?utf-8?B?S8O2bmln?= , 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 Subject: Re: [PATCH v6 18/18] RDMA/mlx5: Ask P2PDMA whether ATS takes a direct peer-to-peer route Message-ID: <20260921132559.GQ11599@ziepe.ca> References: <20260914-fix-p2p-acs-v4-0-v6-0-5ef07ec9ef06@nvidia.com> <20260914-fix-p2p-acs-v4-0-v6-18-5ef07ec9ef06@nvidia.com> <321890690ce83d1943b2f678bd9bee9b8c895b66.camel@linux.intel.com> <20260918121500.GV13683@unreal> Precedence: bulk X-Mailing-List: linux-doc@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260918121500.GV13683@unreal> On Fri, Sep 18, 2026 at 03:15:00PM +0300, Leon Romanovsky wrote: > > That is, a flag to tell the topology check that some transactions > > *will* take the host-bridge path due to IOVA being used, and that the > > computations including pci_p2pdma_distance() need to check whether that > > is possible (checking whitelist etc.) and return the corresponding > > THRU_HOST_BRIDGE mapping type. Translated transactions taking a short- > > cut using the bus-address would then be hidden from the driver. > > The word *will* puzzles me. As I understand it, if your device has ATS > enabled all the time, it should always get THRU_HOST_BRIDGE. The P2P subsystem currently does not understand ATS, it assumes the device will use untranslated requests and makes the routing calculation accordingly. If the device knows it will use ATS it should ask P2P for an ATS path, and P2P should only return THRU_HOST_BRIDGE or failure. The purpose of involving the P2P subsystem is to verify that the ACS flags for the ATS translated path are going to work. There are many ACS configurations where the fabric will hang :( For something like mlx5 it is ideal if P2P chooses between ATS or !ATS based on what provides an optimal transfer. eg there is usually no reason to use ATS to access system memory for streaming (ie non cache hitting) transfers. Jason