KVM: guest_memfd: support for uffd missing

This series is built on top of the v3 write syscall support [1].

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.

This series proposes a limited support for userfaultfd in guest_memfd:
 - userfaultfd support is conditional to `CONFIG_KVM_GMEM_SHARED_MEM`
   (as is fault support in general)
 - Only `page missing` event is currently supported
 - Userspace is supposed to respond to the event with the `write`
   syscall followed by `UFFDIO_CONTINUE` ioctl to unblock the faulting
   process.   Note that we can't use `UFFDIO_COPY` here because
   userfaulfd code does not know how to prepare guest_memfd pages, eg
   remove them from direct map [4].

Not included in this series:
 - Proper interface for userfaultfd to recognise guest_memfd mappings
 - Proper handling of truncation cases after locking the page

Request for comments:
 - Is it a sensible workflow for guest_memfd to resolve a userfault
   `page missing` event with `write` syscall + `UFFDIO_CONTINUE`?  One
   of the alternatives is teaching `UFFDIO_COPY` how to deal with
   guest_memfd pages.
 - What is a way forward to make userfaultfd code aware of guest_memfd?
   I saw that Patrick hit a somewhat similar problem in [5] when trying
   to use direct map manipulation functions in KVM and was pointed by
   David at Elliot's guestmem library [6] that might include a shim for that.
   Would the library be the right place to expose required interfaces like
   `vma_is_gmem`?

Nikita

[1] https://lore.kernel.org/kvm/20250303130838.28812-1-kalyaz...
[2] https://lore.kernel.org/kvm/20250109204929.1106563-1-jtho...
[3] https://docs.google.com/document/d/1M6766BzdY1Lhk7LiR5IqV...
[4] https://lore.kernel.org/kvm/20250221160728.1584559-1-royp...
[4] https://lore.kernel.org/kvm/20250221160728.1584559-1-royp...
[5] https://lore.kernel.org/kvm/20241122-guestmem-library-v5-...

Nikita Kalyazin (5):
  KVM: guest_memfd: add kvm_gmem_vma_is_gmem
  KVM: guest_memfd: add support for uffd missing
  mm: userfaultfd: allow to register userfaultfd for guest_memfd
  mm: userfaultfd: support continue for guest_memfd
  KVM: selftests: add uffd missing test for guest_memfd

 include/linux/userfaultfd_k.h                 |  9 ++
 mm/userfaultfd.c                              | 23 ++++-
 .../testing/selftests/kvm/guest_memfd_test.c  | 88 +++++++++++++++++++
 virt/kvm/guest_memfd.c                        | 17 +++-
 virt/kvm/kvm_mm.h                             |  1 +
 5 files changed, 136 insertions(+), 2 deletions(-)


base-commit: 592e7531753dc4b711f96cd1daf808fd493d3223
-- 
2.47.1