Re: [PATCH v3 0/6] KVM: guest_memfd: support for uffd minor

[Lorenzo Stoakes]

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
> > >
>