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Fri, 29 May 2026 07:36:07 +0000 Message-ID: <8d9bb0b7-182d-4930-b683-d5d24da6b2ab@amd.com> Date: Fri, 29 May 2026 09:36:00 +0200 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v5 0/4] vfio/dma-buf: add TPH support for peer-to-peer access To: Zhiping Zhang Cc: Jason Gunthorpe , Alex Williamson , Leon Romanovsky , Sumit Semwal , Bjorn Helgaas , kvm@vger.kernel.org, linux-rdma@vger.kernel.org, linux-pci@vger.kernel.org, netdev@vger.kernel.org, dri-devel@lists.freedesktop.org, Keith Busch , Yochai Cohen , Yishai Hadas , Linus Torvalds References: <20260526144401.1485788-1-zhipingz@meta.com> <20260527121438.GJ2487554@ziepe.ca> <20260527123634.GK2487554@ziepe.ca> <71302a7a-6b9f-40da-af81-b1862dbd637a@amd.com> Content-Language: en-US From: =?UTF-8?Q?Christian_K=C3=B6nig?= In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-ClientProxiedBy: FR4P281CA0419.DEUP281.PROD.OUTLOOK.COM (2603:10a6:d10:d0::18) To PH7PR12MB5685.namprd12.prod.outlook.com (2603:10b6:510:13c::22) MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: PH7PR12MB5685:EE_|SJ2PR12MB8011:EE_ X-MS-Office365-Filtering-Correlation-Id: b34ab829-1a26-481e-ea69-08debd54f259 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0; 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DMA-buf is an in kernel interface for buffer sharing between drivers and any change to it needs an in kernel driver as justification for the added complexity. > — the device is a Meta MTIA > accelerator managed entirely from userspace via VFIO passthrough. When you have a complete open source driver stack which utilizes VFIO passthrough as the interface to communicate with the kernel drivers then we can eventually talk about that. But as far as I can see without upstreaming or at least open sourcing the full stack to utilize this functionality it's a clear NAK to upstreaming this. Regards, Christian. > That's why the ST has to flow through a uAPI: userspace owns the > device and its ST table, so it's the only entity that can publish a > meaningful value for a given dma-buf. > > On the effect: the endpoint's PCIe ingress block uses the 8-bit ST as > an in-band instruction for the incoming P2P TLP — selecting a target > cache partition and, on writes, an in-flight operation on the data > before it lands. The dma-buf callback keeps this opaque to the > framework — only the producer (userspace owner of the VFIO device) and > the consumer (endpoint block) need to interpret the value. > > will include these words into v6's cover letter. > > Thanks, > Zhiping