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@xxxxxxxxxxxxxx/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
