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.
Thanks!
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
