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Tue, 11 Jun 2024 12:11:46 -0700 Date: Tue, 11 Jun 2024 12:11:44 -0700 From: Nicolin Chen To: Jason Gunthorpe CC: "Tian, Kevin" , "will@kernel.org" , "robin.murphy@arm.com" , "suravee.suthikulpanit@amd.com" , "joro@8bytes.org" , "linux-kernel@vger.kernel.org" , "iommu@lists.linux.dev" , "linux-arm-kernel@lists.infradead.org" , "linux-tegra@vger.kernel.org" , "Liu, Yi L" , "eric.auger@redhat.com" , "vasant.hegde@amd.com" , "jon.grimm@amd.com" , "santosh.shukla@amd.com" , "Dhaval.Giani@amd.com" , "shameerali.kolothum.thodi@huawei.com" Subject: Re: [PATCH RFCv1 08/14] iommufd: Add IOMMU_VIOMMU_SET_DEV_ID ioctl Message-ID: References: <20240607144917.GK19897@nvidia.com> <20240610120446.GP19897@nvidia.com> <20240610220110.GQ19897@nvidia.com> <20240611002839.GS19897@nvidia.com> <20240611121756.GT19897@nvidia.com> MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Disposition: inline In-Reply-To: <20240611121756.GT19897@nvidia.com> X-NV-OnPremToCloud: ExternallySecured X-EOPAttributedMessage: 0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: DS3PEPF000099DE:EE_|MN0PR12MB5883:EE_ X-MS-Office365-Filtering-Correlation-Id: 06349bdf-2ee8-4f00-7baa-08dc8a4a5f88 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230032|1800799016|7416006|376006|36860700005|82310400018; 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X-OriginatorOrg: Nvidia.com X-MS-Exchange-CrossTenant-OriginalArrivalTime: 11 Jun 2024 19:12:01.3510 (UTC) X-MS-Exchange-CrossTenant-Network-Message-Id: 06349bdf-2ee8-4f00-7baa-08dc8a4a5f88 X-MS-Exchange-CrossTenant-Id: 43083d15-7273-40c1-b7db-39efd9ccc17a X-MS-Exchange-CrossTenant-OriginalAttributedTenantConnectingIp: TenantId=43083d15-7273-40c1-b7db-39efd9ccc17a;Ip=[216.228.118.233];Helo=[mail.nvidia.com] X-MS-Exchange-CrossTenant-AuthSource: DS3PEPF000099DE.namprd04.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Anonymous X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem X-MS-Exchange-Transport-CrossTenantHeadersStamped: MN0PR12MB5883 X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20240611_122655_745120_731D2B66 X-CRM114-Status: GOOD ( 28.43 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On Tue, Jun 11, 2024 at 09:17:56AM -0300, Jason Gunthorpe wrote: > On Mon, Jun 10, 2024 at 05:44:16PM -0700, Nicolin Chen wrote: > > On Mon, Jun 10, 2024 at 09:28:39PM -0300, Jason Gunthorpe wrote: > > > On Mon, Jun 10, 2024 at 04:04:30PM -0700, Nicolin Chen wrote: > > > > > > > > > Actually, even now as we put a dispatcher in VMM, VMM still does > > > > > > decode the CD table to link ASID to s1_hwpt. Otherwise, it could > > > > > > only broadcast a TLBI cmd to all pSMMUs. > > > > > > > > > > No, there should be no CD table decoding and no linking ASID to > > > > > anything by the VMM. > > > > > > > > > > The ARM architecture is clean, the ASID can remain private to the VM, > > > > > there is no reason for the VMM to understand it. > > > > > > > > But a guest-level TLBI command usually has only ASID available to > > > > know which pSMMU to dispatch the command. Without an ASID lookup > > > > table, how could VMM then dispatch a command to the corresponding > > > > pSMMU? > > > > > > It can broadcast. The ARM architecture does not expect a N:1 mapping > > > of SMMUs. This is why I think it is not such a good idea.. > > > > Hmm, I thought we had an agreed idea that we shouldn't broadcast > > a TLBI (except global NH_ALL/VAA) for invalidation performance? > > I wouldn't say agree, there are just lots of different trade offs to > be made here. You can reduce broadcast by parsing the CD table from > the VMM. You can reduce broadcast with multiple vSMMUs. > > VMM needs to pick a design. I favour multiple vSMMUs. Yea, having multiple vSMMUs for nesting too seems to be a cleaner design. The thing is that we have to put a certain complexity in the VMM, and it should be more efficient by having it at the boot stage (creating multi-vSMMUs/PCIs and IORT nodes) v.s. runtime (trappings and distributing at every command). > > CD table walkthrough would be always done only by VMM, while the > > lookup table could be created/maintained by the kernel. I feel a > > vasid table could make sense since we maintain the vdev_id table > > in the kernel space too. > > I'm not convinced we should put such a micro optimization in the > kernel. If the VMM cares about performance then split the vSMMU, > otherwise lets just put all the mess in the VMM and give it the tools > to manage the invalidation distribution. > > In the long run things like vCMDQ are going to force the choice to > multi-smmu, which is why I don't see too much value with investing in > optimizing the single vSMMU case. The optimization can be done later > if someone does have a use case. I see. That makes sense to me. Nicolin