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Thu, 21 May 2026 07:23:38 +0000 Message-ID: <80e7e1be-c384-470f-9949-8c0dbad165ac@intel.com> Date: Thu, 21 May 2026 15:31:46 +0800 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v4 1/3] PCI: Allow ATS to be always on for CXL.cache capable devices To: Jason Gunthorpe CC: Nicolin Chen , , , , , , , , , , , , , , , , , References: <20260520143410.GV3602937@nvidia.com> Content-Language: en-US From: Yi Liu In-Reply-To: <20260520143410.GV3602937@nvidia.com> Content-Type: text/plain; charset="UTF-8"; format=flowed Content-Transfer-Encoding: 8bit X-ClientProxiedBy: TPYP295CA0051.TWNP295.PROD.OUTLOOK.COM (2603:1096:7d0:8::11) To SA1PR11MB8492.namprd11.prod.outlook.com (2603:10b6:806:3a3::7) MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: SA1PR11MB8492:EE_|DM4PR11MB6549:EE_ X-MS-Office365-Filtering-Correlation-Id: a4874eab-c0d4-450a-6761-08deb709e054 X-LD-Processed: 46c98d88-e344-4ed4-8496-4ed7712e255d,ExtAddr X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|7416014|376014|366016|1800799024|42112799006|56012099003|11063799006|22082099003|18002099003|3023799007|4143699003; 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This is working >>> even when a device has no translation on its RID (i.e., the RID is IOMMU >>> bypassed). >> >> nit: this description seems not accurate. Intel iommu driver enables ATS >> in the probe_device() phase. mind tweak a bit to avoid misleading >> message. :) > > It probably shouldn't do this, it should follow ARM and have it > dynamic during domain attach. Agreed that making it dynamic during domain attach is a better direction. However, even framing it that way, the description tying ATS enablement to PASID attachment is still architecturally specific to ARM SMMUv3, and doesn't hold as a general statement. :) > For security we need ATS disabled for blocking domains at a minimum. Agreed on the security model. One more data point worth discussing: today Intel's IOMMU driver enables ATS at probe time, which has two effects — enabling the PCI ATS capability on the device, and setting the DTE bit in the scalable-mode PASID-table entry. When a RID or PASID is subsequently attached to a blocking domain, the corresponding PASID-table entry has its Present (P) bit cleared. Per the VT-d spec (condition SPT.2), with P=0: - Translation Requests (with or without PASID) complete successfully, but return R=W=U=S=0 to the device — effectively a no-access result. - Untranslated Requests receive UR. - Translated Requests are N/A. So while neither the PCI ATS capability nor the DTE bit is explicitly cleared when a blocking domain is attached, ATS-related transactions don't produce any usable result from the device's perspective. Does this hardware behavior satisfy the security expectation you have in mind? Or do you still require that both the DTE bit and the PCI ATS capability be explicitly disabled when a blocking domain is in effect? Regards, Yi Liu