Re: [PATCH RFC 0/3] KVM: guest_memfd: folio migration for non-confidential VMs

["David Hildenbrand (Arm)" <david@kernel.org>]

On 6/16/26 20:09, Ackerley Tng wrote:
> "David Hildenbrand (Arm)" <david@kernel.org> writes:
> 
>> On 6/15/26 19:39, Sean Christopherson wrote:
>>>
>>> +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.
>>
>>>
> 
> More concretely, are y'all pointing towards a
> GUEST_MEMFD_FLAG_MIGRATABLE, which will set .migrate =
> kvm_gmem_migrate_folio, and for now, error out for CoCo VMs?
> 
>>>
>>> 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.
>>
> 
> Background question: how would virtio-mem use migration in the host/guest_memfd?

Good question! As long as there is no nested-virt support (and virtio-mem
support for coco still being in the making) that wouldn't apply, only ordinary
memory hot(un)plug (incl CXL).

-- 
Cheers,

David