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X-MS-Exchange-AntiSpam-MessageData-ChunkCount: 1 X-MS-Exchange-AntiSpam-MessageData-0: XtNhPppKdOf4rn+sPe02/nFSKRlfenY9OJVXahdrGAp8Y9HGcFrfVbnHwJBSh9QZn029+UMYRCABzKujMSL1H02PpigDz9OBDRKwNYwB9Szwq7W/R40rtAfSjHticO2BsxZ0PXiq0sDxNhdAMkUI8vqqxisSwBQJnIfdiqDrOt/JAuIFwNEt5uqhcV5kihj9Sqv4KjCS0JAzToZnBCxJM/rri22W7bdn5njQ8FjglOTNxLlVFuEPyIezOrxHcD5c5dTeB3mWkdb57k2OiALeXjG255f2/VBvOVPqMqCzKWtoJQ1CW0YKsKfkZz+cO3vLxdZFE4ffB/iQ04QM0mreYegtm5wtbJstCTT/60bnMfsLlBm8freoRdRu6j6LWZfTCbyhtBOlPNR70TvNAG1LYLX7LPHMEZ5N0Jetda3f7Q34eplUrIGe0LI6b0H0qRly X-OriginatorOrg: arm.com X-MS-Exchange-CrossTenant-OriginalArrivalTime: 13 Aug 2026 20:06:36.9426 (UTC) X-MS-Exchange-CrossTenant-Network-Message-Id: ee36a406-541f-47a3-339b-08def9766183 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: DB5PEPF00014B8E.eurprd02.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Anonymous X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem X-MS-Exchange-Transport-CrossTenantHeadersStamped: VE1PR08MB5598 On 8/13/26 18:09, Alexandru Elisei wrote: > Hi Sean, David, Hi, > > On Mon, Jul 13, 2026 at 04:11:57PM +0200, David Hildenbrand wrote: >> >>> >>> Yeah. I agree (with the caveats you mention). Arm folk need to go figure >>> out if it's worthwhile to support with all those caveats. >> >> Right, disabling migration (once gmem supports it) was also what I discussed >> with Alexandru when that topic comes up. >> >> How to communicate to gmem that it wants these fixed mappings is a good question. > > There's already a proposal for how to do this in the migratable guest_memfd > series [1] - it's a new guest_memfd creation flag that disables migration. > In the guest_memfd call I was arguing against the flag in the first version, and instead adding it when actually required. I was also raising whether KVM couldn't tell guest_memfd (e.g., at creation time?) that it supports a CPU feature that requires S2 to be always mapped to disable migration. It would then be a contract between KVM and guest_memfd without user space having to be involved on that level. > With Sean's comment that he expects swap/reclaim to be fully userspace > driven, I believe that would be enough to guarantee on the _kernel_ side > that SPE will work as intended for a guest. That's my understanding. > > I'm a slightly concerned though that all of this will work by chance, and > not by design, and in the future the behaviour might change to allow > guest_memfd memory to be unmapped from stage 2 without the VMM or KVM > explicitly allowing it or initiating it. Thus my idea of the explicit contract between KVM and guest_memfd. Instead of being a "this doesn't support migration" it would be a "S2 always mapped" kind-of contract. > > My understanding from the conversation so far is that the plan for the > future of guest_memfd is to support an option/mode where the memory is > effectively "pinned" at stage 2 (but which allows userspace to explicitly > free/unmap it, of course). Is that correct, or am I being overly optimistic > in my interpretation? We could then even disallow fallocate() to punch holes if that contract is negotiated. -- Cheers, David