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Silva" , Ankit Agrawal , Yishai Hadas , Shameer Kolothum , Kevin Tian , Alex Williamson , Krishnakant Jaju , Matt Ochs , linux-pci@vger.kernel.org, linux-kernel@vger.kernel.org, linux-block@vger.kernel.org, iommu@lists.linux.dev, linux-mm@kvack.org, linux-doc@vger.kernel.org, linux-media@vger.kernel.org, dri-devel@lists.freedesktop.org, linaro-mm-sig@lists.linaro.org, kvm@vger.kernel.org, linux-hardening@vger.kernel.org, Alex Mastro , Nicolin Chen References: <20251111-dmabuf-vfio-v8-0-fd9aa5df478f@nvidia.com> <20251111-dmabuf-vfio-v8-6-fd9aa5df478f@nvidia.com> <8a11b605-6ac7-48ac-8f27-22df7072e4ad@amd.com> <20251119132511.GK17968@ziepe.ca> <69436b2a-108d-4a5a-8025-c94348b74db6@amd.com> <20251119193114.GP17968@ziepe.ca> Content-Language: en-US From: =?UTF-8?Q?Christian_K=C3=B6nig?= In-Reply-To: <20251119193114.GP17968@ziepe.ca> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-ClientProxiedBy: FR4P281CA0178.DEUP281.PROD.OUTLOOK.COM (2603:10a6:d10:b7::6) To PH7PR12MB5685.namprd12.prod.outlook.com (2603:10b6:510:13c::22) Precedence: bulk X-Mailing-List: linux-block@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: PH7PR12MB5685:EE_|CH3PR12MB8850:EE_ X-MS-Office365-Filtering-Correlation-Id: 65b461c9-e10f-4c57-ddb4-08de28039fd7 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|366016|1800799024|7416014|376014; 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>>>>> + if (!dma->state) { >>>>> + ret = -ENOMEM; >>>>> + goto err_free_dma; >>>>> + } >>>>> + >>>>> + dma_iova_try_alloc(attach->dev, dma->state, 0, size); >>>> >>>> Oh, that is a clear no-go for the core DMA-buf code. >>>> >>>> It's intentionally up to the exporter how to create the DMA >>>> addresses the importer can work with. >>> >>> I can't fully understand this remark? >> >> The exporter should be able to decide if it actually wants to use >> P2P when the transfer has to go through the host bridge (e.g. when >> IOMMU/bridge routing bits are enabled). > > Sure, but this is a simplified helper for exporters that don't have > choices where the memory comes from. That is extremely questionable as justification to put that in common DMA-buf code. > I fully expet to see changes to this to support more use cases, > including the one above. We should do those changes along with users > making use of them so we can evaluate what works best. Yeah, exactly that's my concern. >> But only take that as Acked-by, I would need at least a day (or >> week) of free time to wrap my head around all the technical details >> again. And that is something I won't have before January or even >> later. > > Sure, it is alot, and I think DRM community in general should come up > to speed on the new DMA API and how we are pushing to see P2P work > within Linux. > > So thanks, we can take the Acked-by and progress here. Interested > parties can pick it up from this point when time allows. Wait a second. After sleeping a night over it I think my initial take that we really should not put that into common DMA-buf code seems to hold true. This is the use case for VFIO, but I absolutely want to avoid other drivers from re-using this code until be have more experience with that. So to move forward I now strongly think we should keep that in VFIO until somebody else comes along and needs that helper. Regards, Christian. > > We can also have a mini-community call to give a summary/etc on these > topics. > > Thanks, > Jason