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 >
