Re: [PATCH v3 0/6] KVM: guest_memfd: support for uffd minor

[Lorenzo Stoakes]

On Fri, Apr 04, 2025 at 03:43:46PM +0000, Nikita Kalyazin wrote:
> This series is built on top of the Fuad's v7 "mapping guest_memfd backed
> memory at the host" [1].

Hm if this is based on an unmerged series this seems quite speculative and
should maybe be an RFC? I mean that series at least still seems quite under
discussion/experiencing issues?

Maybe worth RFC'ing until that one settles down first to avoid complexity
in review/application to tree?

Thanks!

>
> With James's KVM userfault [2], it is possible to handle stage-2 faults
> in guest_memfd in userspace.  However, KVM itself also triggers faults
> in guest_memfd in some cases, for example: PV interfaces like kvmclock,
> PV EOI and page table walking code when fetching the MMIO instruction on
> x86.  It was agreed in the guest_memfd upstream call on 23 Jan 2025 [3]
> that KVM would be accessing those pages via userspace page tables.  In
> order for such faults to be handled in userspace, guest_memfd needs to
> support userfaultfd.
>
> Changes since v2 [4]:
>  - James: Fix sgp type when calling shmem_get_folio_gfp
>  - James: Improved vm_ops->fault() error handling
>  - James: Add and make use of the can_userfault() VMA operation
>  - James: Add UFFD_FEATURE_MINOR_GUEST_MEMFD feature flag
>  - James: Fix typos and add more checks in the test
>
> Nikita
>
> [1] https://lore.kernel.org/kvm/20250318161823.4005529-1-tabba@xxxxxxxxxx/T/
> [2] https://lore.kernel.org/kvm/20250109204929.1106563-1-jthoughton@xxxxxxxxxx/T/
> [3] https://docs.google.com/document/d/1M6766BzdY1Lhk7LiR5IqVR8B8mG3cr-cxTxOrAosPOk/edit?tab=t.0#heading=h.w1126rgli5e3
> [4] https://lore.kernel.org/kvm/20250402160721.97596-1-kalyazin@xxxxxxxxxx/T/
>
> Nikita Kalyazin (6):
>   mm: userfaultfd: generic continue for non hugetlbfs
>   mm: provide can_userfault vma operation
>   mm: userfaultfd: use can_userfault vma operation
>   KVM: guest_memfd: add support for userfaultfd minor
>   mm: userfaultfd: add UFFD_FEATURE_MINOR_GUEST_MEMFD
>   KVM: selftests: test userfaultfd minor for guest_memfd
>
>  fs/userfaultfd.c                              |  3 +-
>  include/linux/mm.h                            |  5 +
>  include/linux/mm_types.h                      |  4 +
>  include/linux/userfaultfd_k.h                 | 10 +-
>  include/uapi/linux/userfaultfd.h              |  8 +-
>  mm/hugetlb.c                                  |  9 +-
>  mm/shmem.c                                    | 17 +++-
>  mm/userfaultfd.c                              | 47 ++++++---
>  .../testing/selftests/kvm/guest_memfd_test.c  | 99 +++++++++++++++++++
>  virt/kvm/guest_memfd.c                        | 10 ++
>  10 files changed, 188 insertions(+), 24 deletions(-)
>
>
> base-commit: 3cc51efc17a2c41a480eed36b31c1773936717e0
> --
> 2.47.1
>