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

[Nikita Kalyazin <kalyazin@amazon.com>]

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@google.com/T/
>> [2] https://lore.kernel.org/kvm/20250109204929.1106563-1-jthoughton@google.com/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@amazon.com/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
>>