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

[David Hildenbrand (Arm)]

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