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Tue, 1 Sep 2026 10:36:07 -0700 Date: Tue, 1 Sep 2026 10:36:06 -0700 From: Nicolin Chen To: Aneesh Kumar K.V CC: , , , , Alexey Kardashevskiy , Catalin Marinas , Dan Williams , "Jason Gunthorpe" , Joerg Roedel , Jonathan Cameron , Marc Zyngier , Pranjal Shrivastava , Robin Murphy , Samuel Ortiz , Steven Price , Suzuki K Poulose , Will Deacon , Xu Yilun Subject: Re: [RFC PATCH v4 03/16] iommu/arm-smmu-v3: Add initial pSMMU realm viommu plumbing Message-ID: References: <20260427085344.941627-1-aneesh.kumar@kernel.org> <20260427085344.941627-4-aneesh.kumar@kernel.org> Precedence: bulk X-Mailing-List: linux-coco@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Disposition: inline In-Reply-To: X-NV-OnPremToCloud: ExternallySecured X-EOPAttributedMessage: 0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: SJ5PEPF00000203:EE_|SA1PR12MB9548:EE_ X-MS-Office365-Filtering-Correlation-Id: 893bb952-a558-4599-280f-08df084f8def X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|82310400026|1800799024|376014|7416014|23010399003|36860700016|6133799003|3023799007|10067099003|11063799006|5023799004|56012099006|4143699003|22082099003|18002099003; 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X-MS-Exchange-AntiSpam-MessageData-ChunkCount: 1 X-MS-Exchange-AntiSpam-MessageData-0: LcWiNv1Rt3t/GEW26HTyypHg+wNB9bvTBhl3Gf4GYcC9x0IQk5lG+nJEO1zsY42NCWdjywmL1Ag+/sw4p3dleJGTXARjPVi2tq5a8wBSMLPZvbu1RKgrMpymM/0T3adg067bUQrVBQP6gXXmehx8FQS9HKVMZ3rfH8IhdqYhMHFenikxFjGqc3UdKXAhQRWq6LBgYouWwyYAaE8PgM64E8s/XPmmnGMzvT32sxz+dmn5fHGw6UFy2tbl66mGdu5a84BqURi8IhCX0ytRUdZdnt/3delUJ8y3Pc7Akc7/bXs6rTy9gUCyTfqdY8JlCTcrH4yPKKzxUSAD4cgTkwrOdicaVPYE5wnKKDR4LZC1u2ejNEVjHvcINR4NpGnV9vHxniD5vV/Kw74GAiWxwTsiSmdLdDOWmlbZ/JwIiuWCl3dmDpQ6chi1fg+m6w76psdZ X-OriginatorOrg: Nvidia.com X-MS-Exchange-CrossTenant-OriginalArrivalTime: 01 Sep 2026 17:36:28.6394 (UTC) X-MS-Exchange-CrossTenant-Network-Message-Id: 893bb952-a558-4599-280f-08df084f8def X-MS-Exchange-CrossTenant-Id: 43083d15-7273-40c1-b7db-39efd9ccc17a X-MS-Exchange-CrossTenant-OriginalAttributedTenantConnectingIp: TenantId=43083d15-7273-40c1-b7db-39efd9ccc17a;Ip=[216.228.118.232];Helo=[mail.nvidia.com] X-MS-Exchange-CrossTenant-AuthSource: SJ5PEPF00000203.namprd05.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Anonymous X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem X-MS-Exchange-Transport-CrossTenantHeadersStamped: SA1PR12MB9548 On Tue, Sep 01, 2026 at 02:47:04PM +0530, Aneesh Kumar K.V wrote: > Nicolin Chen writes: > > But, on the other hand, I can imagine that a Realm VSMMU would add > > a new flag with a user_data to this VIOMMU. Then, this flow would > > give some troubles to VMM (QEMU for example): > > > > - For VM with a guest-level SMMU, QEMU creates a realm instance > > where IOMMU_VIOMMU_TYPE_ARM_REALM_SMMUV3 (with vsmmu) can be > > allocated. > > - For VM w/o a guest-level SMMU, QEMU won't create such a realm > > instance, while still required to invoke the ioctl (w/o vsmmu). > > > > Taking a step back, I wonder if we really need to use iommufd for > > PSMMU activation and its stream table allocations? > > > > Here are some facts: > > - An iommufd has a ctx, that's one per VM. Similarly, a Realm > > has an RD. > > - For an RMI command that needs an RD, it makes sense to be per > > iommufd ctx, e.g. RMI_VSMMU_* or RMI_VDEV_* commands. > > - PSMMU commands are global; they don't need RD. So they don't > > seem necessary to tie to an iommufd ctx. > > > > Instead, could the PSMMU activation be done after RMI_PSMMU_INFO > > check? Is there any reason not to do that? A safer timing might > > be at the device assignment stage? > > > > One of the reasons I added IOMMU_VIOMMU_TYPE_ARM_REALM_SMMUV3 was to > avoid creating a psmmu object when we are not using a PCI passthrough > VM. That is also the reason for all the refcounting around the psmmu > objects. > > If we are okay with creating psmmu objects early, then I guess we can go > with the above approach. Do you see any trouble if we implement in this way: - Add "struct arm_smmu_realm *psmmu" in struct arm_smmu_device - RMI_PSMMU_INFO in probe(): + allocate a smmu->psmmu on success + keep smmu->psmmu->users = 0; psmmu is not activated - Device binds to iommufd, creating idev + invoke an iommu op to a driver-level callback: psmmu = smmu->psmmu; if (!psmmu) return 0; guard(mutex)(&psmmu->mutex); if (!smmu->psmmu->users) rmi_psmmu_activate(); psmmu->users++; rmi_psmmu_st_l2_create(); ? Nicolin