On Fri, Apr 04, 2025 at 05:56:58PM +0100, Nikita Kalyazin wrote: > > > On 04/04/2025 17:33, Lorenzo Stoakes wrote: > > 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? > > Hi, > > I dropped the RFC tag because I saw similar examples before, but I'm happy > to bring it back next time if the dependency is not merged until then. Yeah really sorry to be a pain haha, I realise this particular situation is a bit unclear, but I think just for the sake of getting our ducks in a row and ensuring things are settled on the baseline (and it's sort of a fairly big baseline), it'd be best to bring it back! > > > > > Thanks! > > Thanks! Cheers! > > > > > > > > > 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 > > > >
