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

[Alexandru Elisei <alexandru.elisei@arm.com>]

Hi,

On Mon, Jun 15, 2026 at 11:43:14AM +0100, Alexandru Elisei wrote:
> Hi,
> 
> On Thu, Jun 11, 2026 at 01:05:07PM +0000, Shivank Garg wrote:
> > guest_memfd folios are currently marked unmovable, so the kernel cannot
> > perform NUMA-balancing, memory compaction, etc. This is unavoidable for
> > confidential VMs (SEV-SNP, TDX), since memory is encrypted and copying it
> > needs firmware assistance. However, for non-confidential VMs (like
> > Firecracker), we can migrate the folios.
> > 
> > This series enables folio migration for non-confidential guest_memfd and
> > also lays the groundwork for migrating confidential guest_memfd later.
> > Once firmware-assisted copying support is available, those VMs can be
> > made movable, the confidential folio content can be copied separately,
> > and the destination folio marked with FOLIO_CONTENT_COPIED so
> > __migrate_folio() skips the host-side folio_mc_copy().
> 
> 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.
> 
> 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.

Thanks,
Alex