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Tsirkin" , Nelson Escobar , netdev@vger.kernel.org, Rob Clark , Suravee Suthikulpanit , virtualization@lists.linux-foundation.org, Will Deacon , Christoph Hellwig , "Tian, Kevin" Subject: Re: [PATCH 1/5] iommu: Replace uses of IOMMU_CAP_CACHE_COHERENCY with dev_is_dma_coherent() Message-ID: <20220406142432.GF2120790@nvidia.com> References: <1-v1-ef02c60ddb76+12ca2-intel_no_snoop_jgg@nvidia.com> Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-ClientProxiedBy: YT1PR01CA0044.CANPRD01.PROD.OUTLOOK.COM (2603:10b6:b01:2e::13) To MN2PR12MB4192.namprd12.prod.outlook.com (2603:10b6:208:1d5::15) MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-Office365-Filtering-Correlation-Id: 1d8ec298-b8a2-45ff-7223-08da17d92c48 X-MS-TrafficTypeDiagnostic: BYAPR12MB3238:EE_|BYAPR12MB4712:EE_ X-Microsoft-Antispam-PRVS: X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0; X-Microsoft-Antispam-Message-Info: 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 X-Forefront-Antispam-Report: CIP:255.255.255.255;CTRY:;LANG:en;SCL:1;SRV:;IPV:NLI;SFV:NSPM;H:BYAPR12MB3238.namprd12.prod.outlook.com;PTR:;CAT:NONE;SFS:(13230001)(4636009)(366004)(7416002)(8936002)(5660300002)(2906002)(4326008)(8676002)(38100700002)(66476007)(33656002)(86362001)(36756003)(6486002)(508600001)(66556008)(66946007)(6916009)(54906003)(316002)(1076003)(2616005)(6506007)(53546011)(186003)(6512007)(26005);DIR:OUT;SFP:1101; X-MS-Exchange-AntiSpam-MessageData-ChunkCount: 1 X-MS-Exchange-AntiSpam-MessageData-0: =?us-ascii?Q?j1Rdld5Fo38ZTjZ38MIPGAsmMA5NX1SD3+L7zg2BUvUp8zp8gn1uG7GZ7Cqf?= =?us-ascii?Q?p1p8L1ganek/N/PF3U+4T08mh+V53cV06IHnpXaOs7iFCZ/Qdg2ejWQ+E0ir?= =?us-ascii?Q?A01VuBe9zxrWWChmQxx4RxId1+sNfaLbfFeNR6EcMyT+y4I5civOYXSkb2Av?= =?us-ascii?Q?VEXAh+wNXv8nwUCeE2odpKNg8cxusNS38ob8bnxk6DpMWwDVk0EqiGfvu0sE?= =?us-ascii?Q?swYmaoClsa2c3+HF2Nc/BkNVYWXIm6uRhRU8IG7xwj2nPbCQzx895CXMfMOs?= =?us-ascii?Q?2gyL+Wjlm+KYy9lejPTJscm4QdaaJXP1yO7+Ig9zwJkXJHXU3yWUmGdQcRsM?= =?us-ascii?Q?bwEqrqEl8XAadOWnT52FDuKs3WJ5HkSVNoF9e9yHw9bxpZjM64FWTTXxuczq?= =?us-ascii?Q?kZj3tOLADpCsP0oZgBz9+bzj8cDQn8cqgfFVkgV+Ez7B1Fmy5lqMnRpbqHsC?= =?us-ascii?Q?6HVdOw1Cvn5/6aybMEkgqVBqwOrrJMcYD4jcVPEdpRolmd2K7vDTC1zYTdPa?= =?us-ascii?Q?trtd9g1v+jxw62zYgdyKPpUkiXQgkL7qHqzhwLI9dPcG7grA/jUV/l6xQYS/?= =?us-ascii?Q?m45VA5OebJeGNapmv4FJ4F+fDQEWP1iQr3nN6czi9JyEnWKaE5JtsW1wiKmB?= =?us-ascii?Q?RkHzLAFKJywQnDLI8rJUWl3LjpNpz6/mc3pdItQ8a6b23MUhx2h99W7WMlmZ?= =?us-ascii?Q?ofjCUs2j99hZTvPiiqPFFm5UQUo5mGQ6X8Ui59ENeJoRLhKk/Sb93cd//+Yy?= =?us-ascii?Q?3K5IJK8+cQQ+P3uoRa8vkTp0eivTo6/WeRxLZil5oCQMxIUdu0L+qZ4RFsJi?= =?us-ascii?Q?rp5RbYeNZ0IDvPLcHe9IBw5sTdANsbf2kj4+BBLrqy/5Ke78bwHH/VBWjpL4?= =?us-ascii?Q?BtZHm5k/sVtzqGgCCZaCS4CukhjWX5lrxi9QDlythZz/javtfMt5MXBu2XBM?= =?us-ascii?Q?Tr1S2wPCaev90Q/SfRQjnvsPa4pk1z8IQHrQAQZ9aLiCBeyolZjBBbJzK9YD?= =?us-ascii?Q?y+/KPscj829uDe2go1OXAs+B42V5iDaebBeGDjoBsuzJML+4drpBwWaH7jqt?= =?us-ascii?Q?mEBF2Bykqa6gRuoTNlS/f6FoQpF0e1xiZe9DiRmdtf9Vha7kQ55HtpfVRzBL?= =?us-ascii?Q?oy+LM5GKOV1OI4r9VqLSVPsbGc9uMufyQ89qSn/z0npzPqmn/CPD+9DEanSp?= =?us-ascii?Q?+czvtnsa/2ZaWs39mWjTi0rn0nTfvN0N9MKFZH2l04XkVtS08IJgoOGMDD9y?= =?us-ascii?Q?xbNnXlSNWUzVhFfTvZsgWEYCLUmTho5knPA9POPAQfTWbwao/BIsMsGjvYbe?= =?us-ascii?Q?siozhacB1VGR3473pHktK07LQH915ou6lAa+P3fpnABYNxdex082vKOU7Dg3?= =?us-ascii?Q?/9j2IVvYGL0CzYAY8S2QLQC0KZpUAIiLY2eOipwhYGZjO4tlx3rvopo5rlFX?= =?us-ascii?Q?XFLC7QiPAqMwiPYm/S2jeKNNWtBrAoo6hoderQALU6DYAp8YtT4Zo0yS6mEL?= =?us-ascii?Q?Imv9KbJfHBj21ol8woLA1nBtwzhO2RUXvBL2rkeIE9c2Jv1Xy24Qu6pagkn6?= =?us-ascii?Q?4aCL0GdlpPs8Hc0syEJIChGyQ2H+t6KAzNbdhGIYZT4vwfV2wX7eWZ+1HjK0?= =?us-ascii?Q?jfzIypDQEIAImv08bfBZjEMZzQKY5wxQGx9HZeGeFlTPtDLPMZF3RdQ8svmF?= =?us-ascii?Q?SGxg9vG6mGsnxmqKDZ3WOz0LjqwmeyztO3JmebxMWL6u+EFE9TvuYKImEhhG?= =?us-ascii?Q?7joljw21kA=3D=3D?= X-OriginatorOrg: Nvidia.com X-MS-Exchange-CrossTenant-Network-Message-Id: 1d8ec298-b8a2-45ff-7223-08da17d92c48 X-MS-Exchange-CrossTenant-AuthSource: MN2PR12MB4192.namprd12.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Internal X-MS-Exchange-CrossTenant-OriginalArrivalTime: 06 Apr 2022 14:24:34.5883 (UTC) X-MS-Exchange-CrossTenant-FromEntityHeader: Hosted X-MS-Exchange-CrossTenant-Id: 43083d15-7273-40c1-b7db-39efd9ccc17a X-MS-Exchange-CrossTenant-MailboxType: HOSTED X-MS-Exchange-CrossTenant-UserPrincipalName: to0YNbI5NCs1iQcSoMToWSaVLgDn9KA2Utie6CPdXXB5PeX8CcZNQ+XkDmn9wvMQ X-MS-Exchange-Transport-CrossTenantHeadersStamped: BYAPR12MB4712 Precedence: bulk List-ID: X-Mailing-List: linux-s390@vger.kernel.org On Wed, Apr 06, 2022 at 02:56:56PM +0100, Robin Murphy wrote: > On 2022-04-05 17:16, Jason Gunthorpe wrote: > > vdpa and usnic are trying to test if IOMMU_CACHE is supported. The correct > > way to do this is via dev_is_dma_coherent() > > Not necessarily... > > Disregarding the complete disaster of PCIe No Snoop on Arm-Based systems, > there's the more interesting effectively-opposite scenario where an SMMU > bridges non-coherent devices to a coherent interconnect. It's not something > we take advantage of yet in Linux, and it can only be properly described in > ACPI, but there do exist situations where IOMMU_CACHE is capable of making > the device's traffic snoop, but dev_is_dma_coherent() - and > device_get_dma_attr() for external users - would still say non-coherent > because they can't assume that the SMMU is enabled and programmed in just > the right way. Oh, I didn't know about device_get_dma_attr().. Considering your future issue, maybe this: /* * true if the given domain supports IOMMU_CACHE and when dev is attached to * that domain it will have coherent DMA and require no cache * maintenance when IOMMU_CACHE is used. */ bool iommu_domain_supports_coherent_dma(struct iommu_domain *domain, struct device *dev) { return device_get_dma_attr(dev) == DEV_DMA_COHERENT; } ? In future it could become a domain op and the SMMU driver could figure out the situation you described? Jason