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-kalyazin@amazon.com/T/ [2] https://lore.kernel.org/kvm/20250109204929.1106563-1-jthoughton@google.com/T... [3] https://docs.google.com/document/d/1M6766BzdY1Lhk7LiR5IqVR8B8mG3cr-cxTxOrAos... [4] https://lore.kernel.org/kvm/20250221160728.1584559-1-roypat@amazon.co.uk/T/ [4] https://lore.kernel.org/kvm/20250221160728.1584559-1-roypat@amazon.co.uk/T/#... [5] https://lore.kernel.org/kvm/20241122-guestmem-library-v5-2-450e92951a15@quic...
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