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Fri, 4 Apr 2025 16:59:40 +0000 Received: from BL4PR10MB8229.namprd10.prod.outlook.com ([fe80::552b:16d2:af:c582]) by BL4PR10MB8229.namprd10.prod.outlook.com ([fe80::552b:16d2:af:c582%3]) with mapi id 15.20.8583.041; Fri, 4 Apr 2025 16:59:40 +0000 Date: Fri, 4 Apr 2025 17:59:37 +0100 From: Lorenzo Stoakes To: Nikita Kalyazin Cc: akpm@linux-foundation.org, pbonzini@redhat.com, shuah@kernel.org, viro@zeniv.linux.org.uk, brauner@kernel.org, muchun.song@linux.dev, hughd@google.com, kvm@vger.kernel.org, linux-kselftest@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-fsdevel@vger.kernel.org, jack@suse.cz, Liam.Howlett@oracle.com, jannh@google.com, ryan.roberts@arm.com, david@redhat.com, jthoughton@google.com, peterx@redhat.com, graf@amazon.de, jgowans@amazon.com, roypat@amazon.co.uk, derekmn@amazon.com, nsaenz@amazon.es, xmarcalx@amazon.com Subject: Re: [PATCH v3 0/6] KVM: guest_memfd: support for uffd minor Message-ID: <6bba1a40-76e2-44fb-8f9c-e15f89ff1d0f@lucifer.local> References: <20250404154352.23078-1-kalyazin@amazon.com> <7b8d9aa6-99ff-4986-bd16-664de63ecb58@amazon.com> Content-Type: text/plain; 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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@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 > > > >