On 6/15/26 19:39, Sean Christopherson wrote: > On Mon, Jun 15, 2026, Alexandru Elisei wrote: >> Hi, >> >> On Mon, Jun 15, 2026 at 11:43:14AM +0100, Alexandru Elisei wrote: >>> Hi, >>> >>> >>> I always thought that one of the nice things about using guest_memfd as a >>> memory backend, as opposed to host userspace mappings, is that the host >>> cannot unmap VM memory because of KSM, automatic NUMA balancing, hugepage >>> collapse, compaction, etc, acting on the host userspace mapping of the >>> VM memory, and outside of the VMM's or KVM's control. > > +1000. It's not just "nice to have", it's a core design principle of > guest_memfd.
Right, and I raised in the guest_memfd call also the rough idea of Alexandru's use case of having non-movable guest_memfd pages such that we can support use cases where we can hopefully guarantee that a stage-2 mapping will not just randomly go away. > >>> I think it would be useful to preserve this behaviour, even in the absence >>> of confidential VMs (i.e, guest_memfd file descriptor created with >>> GUEST_MEMFD_FLAG_MMAP). >> >> Just to be clear, I was thinking that it might be useful for both >> behaviours to exist (migratable and non-migratable) for non-confidential >> VMs, and allow KVM or userspace to decide which they prefer for a >> guest_memfd. > > For the purposes of this discussion, we should separate the physical act of > migrating pages from the features that trigger migration. As I said in last > week's > guest-memfd call, I am a-ok with supporting page migration as a mechanism, > but I > am dead set against supporting NUMA balancing, KSM, LRU-based swap/reclaim, > and > anything else that goes against the goal of guest-first memory. Right. Page migration for supporting ZONE_MOVABLE/CMA, compaction, memory offlining, virtio-mem and possibly some collapse mechanism if we were to support THP of some sorts in guest_memfd would are all reasonable. As soon as we mix in access/lru semantics, we're going into the wrong direction. Fortunately KSM is anon-only and not even worth a rant here :) -- Cheers, David
