[RFC PATCH 0/4] KVM: ioctl for populating guest_memfd

[Nikita Kalyazin <kalyazin@amazon.com>]

Firecracker currently allows to populate guest memory from a separate
process via UserfaultFD [1].  This helps keep the VMM codebase and
functionality concise and generic, while offloading the logic of
obtaining guest memory to another process.  UserfaultFD is currently not
supported for guest_memfd, because it binds to a VMA, while guest_memfd
does not need to (or cannot) be necessarily mapped to userspace,
especially for private memory.  [2] proposes an alternative to
UserfaultFD for intercepting stage-2 faults, while this series
conceptually compliments it with the ability to populate guest memory
backed by guest_memfd for `KVM_X86_SW_PROTECTED_VM` VMs.

Patches 1-3 add a new ioctl, `KVM_GUEST_MEMFD_POPULATE`, that uses a
vendor-agnostic implementation of `post_populate` callback.

Patch 4 allows to call the ioctl from a separate (non-VMM) process.  It
has been prohibited by [3], but I have not been able to locate the exact
justification for the requirement.

Questions:
 - Does exposing a generic population interface via ioctl look
   sensible in this form?
 - Is there a path where "only VMM can call KVM API" requirement is
   relaxed? If not, what is the recommended efficient alternative for
   populating guest memory from outside the VMM?

[1]: https://github.com/firecracker-microvm/firecracker/blob/main/docs/snapshotting/handling-page-faults-on-snapshot-resume.md
[2]: https://lore.kernel.org/kvm/CADrL8HUHRMwUPhr7jLLBgD9YLFAnVHc=N-C=8er-x6GUtV97pQ@mail.gmail.com/T/
[3]: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=6d4e4c4fca5be806b888d606894d914847e82d78

Nikita

Nikita Kalyazin (4):
  KVM: guest_memfd: add generic post_populate callback
  KVM: add KVM_GUEST_MEMFD_POPULATE ioctl for guest_memfd
  KVM: allow KVM_GUEST_MEMFD_POPULATE in another mm
  KVM: document KVM_GUEST_MEMFD_POPULATE ioctl

 Documentation/virt/kvm/api.rst | 23 +++++++++++++++++++++++
 include/linux/kvm_host.h       |  3 +++
 include/uapi/linux/kvm.h       |  9 +++++++++
 virt/kvm/guest_memfd.c         | 28 ++++++++++++++++++++++++++++
 virt/kvm/kvm_main.c            | 19 ++++++++++++++++++-
 5 files changed, 81 insertions(+), 1 deletion(-)


base-commit: c8d430db8eec7d4fd13a6bea27b7086a54eda6da
-- 
2.40.1