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X-MS-Exchange-AntiSpam-MessageData-ChunkCount: 1 X-MS-Exchange-AntiSpam-MessageData-0: oVZiSe8kKFovBW/pcQziq3o2GBdyeU37u18SGK22LS/vXRnpCH0eGIC0KlRNt0fwijfE3yUMlOtDP3ZnCxb48OYveeWiW+fR7mMvOzVBNJUV4P9JDWeHm6WJyCiM3p8KkjHSp58tqV1KgrVpI0mV8iNo+EY/O1b6b2Mqe4WVnKIiRtRKY+B81uyWUoCySilaqqn6bTQevQPGxrA/s/Vnw0Ju+aaTZSwyMDc8+Vr0vMczez+uBFO6bNpBDaoR7Jt5usCssUbOfY7ucLP6FpZG0EGqMqPqjBXFPkRx3L8KUUbeL6E+hgIw0w+zStscM9ngFBCUis6po8xd2kPMT5dV1mQwEDKu7TQEUsAnNJdZ4YyTnGuwgPIyjEVXK3RutB/ow5CI2ZgmrrrvJaA65Vr6XN89PC9VBHlgRVuTezPE27+zBN4jL6ip2ewtePVuJc7E X-OriginatorOrg: arm.com X-MS-Exchange-CrossTenant-OriginalArrivalTime: 19 Aug 2026 15:39:30.2064 (UTC) X-MS-Exchange-CrossTenant-Network-Message-Id: e7c74822-d8ff-46a9-b48d-08defe080f4d X-MS-Exchange-CrossTenant-Id: f34e5979-57d9-4aaa-ad4d-b122a662184d X-MS-Exchange-CrossTenant-OriginalAttributedTenantConnectingIp: TenantId=f34e5979-57d9-4aaa-ad4d-b122a662184d;Ip=[4.158.2.129];Helo=[outbound-uk1.az.dlp.m.darktrace.com] X-MS-Exchange-CrossTenant-AuthSource: DB1PEPF00039231.eurprd03.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Anonymous X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem X-MS-Exchange-Transport-CrossTenantHeadersStamped: AS8PR08MB6102 On 8/14/26 18:58, Alexandru Elisei wrote: > Hi, > > On Fri, Aug 14, 2026 at 06:03:54AM -0700, Sean Christopherson wrote: >> On Fri, Aug 14, 2026, David Hildenbrand wrote: >>> We have a hardware feature that requires pages to always be mapped into S2. Some >>> things I had in mind: >>> >>> 1) Page migration would not be a problem as long as hardware could be paused >>> while migrating (e.g., kick all vCPUs). I doubt someone would implement that >>> right now, but you could consider it an implementation detail that page >>> migration cannot be supported right now. >>> >>> 2) Newer hardware could mitigate this problem, allowing the feature to support >>> pages temporarily being unmapped from S2. >>> >>> 3) Disallowing page migration is really just one implication of "pages must >>> always be mapped into S2". >>> >>> So what we really want is "if feature X is enabled and hardware requires it, >>> always keep pages mapped into S2, which currently implies that page migration >>> cannot be supported." >>> >>> Which isn't all that different to "if a confidential VM is run on current TDX >>> hardware, always keep pages mapped into S2, which currently implies that page >>> migration cannot be supported." >>> >>> So I was wondering whether the flow could be: >>> >>> User space enabled CPU feature for VM -> KVM knows that current hardware >>> requires for that CPU feature to have S2 always mapped -> KVM tells guest_memfd >>> that S2 must be always mapped / disables page migration. >> >> I'm a-ok with adding a flag to guest_memfd to communicate whether or not page >> migration is allowed, because guest_memfd needs to actively support page migration. >> >> I'm not ok adding a flag telling guest_memfd that memory must always be mapped >> in S2, because guest_memfd doesn't care. E.g. KVM doesn't yet support page >> migration for SNP, but SNP tracks page ownership in an out-of-band table and so >> KVM can map/unmap all guest memory from S2 at will. >> >>> That would be in contrast to user space having to guess that page migration on >>> the current hardware with the current guest_memfd implementation does not >>> support page migration, to then disable exactly that. >>> >>> Does that explanation makes sense? I don't know the exact mechanism to do that, >>> but that's just my high-level thinking. >> >> Yes, I'm supportive of KVM expressing to guest_memfd that page migration isn't >> supported by the VM. I'm only objecting to expressing that memory must stay >> mapped in S2, because guest_memfd doesn't care *why* page migration is or isn't >> supported/allowed by a particular VM. > > My naive contribution is this idea I had: > > 1. Userspace queries support in KVM for feature xyz by checking the > capability KVM_CAP_xyz. > > 2. Userspace knows that for feature xyz to work correctly, it is required > that memory remains mapped at stage 2. > > 3. Userspace creates a guest_memfd instance with the right combination of > flags set and _unset_ for feature xyz to work correctly - i.e, to keep > memory mapped at stage 2. > > For this to work, new guest_memfd features that might lead to memory being > unmapped are enabled via a flag, and KVM keeps memory mapped at stage 2 by > default, to maintain compatibility with an userspace not updated for the > new features/flags. I was wondering whether KVM could be driving that setting in guest_memfd, with less userspace intervention. 1. Userspace enables support in KVM for feature xyz through capability KVM_CAP_xyz. 2. KVM knows that the feature, on the current hardware requires permanent S2 mapping, so it instructs guest_memfd to disable any features that could lead to a temporary unmapping (e.g.,migration support). -- Cheers, David