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Message-ID: <de7da4d8-0e9d-46f2-88ec-cfd5dc14421c@amazon.com> Date: Thu, 4 Sep 2025 17:43:29 +0100 From: Nikita Kalyazin <kalyazin@...zon.com> To: James Houghton <jthoughton@...gle.com>, Paolo Bonzini <pbonzini@...hat.com>, Sean Christopherson <seanjc@...gle.com>, Oliver Upton <oliver.upton@...ux.dev> CC: Jonathan Corbet <corbet@....net>, Marc Zyngier <maz@...nel.org>, Yan Zhao <yan.y.zhao@...el.com>, Anish Moorthy <amoorthy@...gle.com>, Peter Gonda <pgonda@...gle.com>, Peter Xu <peterx@...hat.com>, David Matlack <dmatlack@...gle.com>, <wei.w.wang@...el.com>, <kvm@...r.kernel.org>, <linux-doc@...r.kernel.org>, <linux-kernel@...r.kernel.org>, <linux-arm-kernel@...ts.infradead.org>, <kvmarm@...ts.linux.dev> Subject: Re: [PATCH v3 00/15] KVM: Introduce KVM Userfault On 18/06/2025 05:24, James Houghton wrote: > Hi Sean, Paolo, Oliver, + others, > > Here is a v3 of KVM Userfault. Thanks for all the feedback on the v2, > Sean. I realize it has been 6 months since the v2; I hope that isn't an > issue. > > I am working on the QEMU side of the changes as I get time. Let me know > if it's important for me to send those patches out for this series to be > merged. Hi Sean and others, Are there any blockers for merging this series? We would like to use the functionality in Firecracker for restoring guest_memfd-backed VMs from snapshots via UFFD [1]. [2] is a Firecracker feature branch that builds on top of KVM userfault, along with direct map removal [3], write syscall [4] and UFFD support [5] in guest_memfd (currently in discussion with MM at [6]) series. Thanks, Nikita [1]: https://github.com/firecracker-microvm/firecracker/blob/main/docs/snapshotting/handling-page-faults-on-snapshot-resume.md [2]: https://github.com/firecracker-microvm/firecracker/tree/feature/secret-hiding [3]: https://lore.kernel.org/kvm/20250828093902.2719-1-roypat@amazon.co.uk [4]: https://lore.kernel.org/kvm/20250902111951.58315-1-kalyazin@amazon.com [5]: https://lore.kernel.org/kvm/20250404154352.23078-1-kalyazin@amazon.com [6]: https://lore.kernel.org/linux-mm/20250627154655.2085903-1-peterx@redhat.com > Be aware that this series will have non-trivial conflicts with Fuad's > user mapping support for guest_memfd series[1]. For example, for the > arm64 change he is making, the newly introduced gmem_abort() would need > to be enlightened to handle KVM Userfault exits. > > Changelog: > v2[2]->v3: > - Pull in Sean's changes to genericize struct kvm_page_fault and use it > for arm64. Many of these patches now have Sean's SoB. > - Pull in Sean's small rename and squashing of the main patches. > - Add kvm_arch_userfault_enabled() in place of calling > kvm_arch_flush_shadow_memslot() directly from generic code. > - Pull in Xin Li's documentation section number fix for > KVM_CAP_ARM_WRITABLE_IMP_ID_REGS[3]. > v1[4]->v2: > - For arm64, no longer zap stage 2 when disabling KVM_MEM_USERFAULT > (thanks Oliver). > - Fix the userfault_bitmap validation and casts (thanks kernel test > robot). > - Fix _Atomic cast for the userfault bitmap in the selftest (thanks > kernel test robot). > - Pick up Reviewed-by on doc changes (thanks Bagas). > > Below is the cover letter from v1, mostly unchanged: > > Please see the RFC[5] for the problem description. In summary, > guest_memfd VMs have no mechanism for doing post-copy live migration. > KVM Userfault provides such a mechanism. > > There is a second problem that KVM Userfault solves: userfaultfd-based > post-copy doesn't scale very well. KVM Userfault when used with > userfaultfd can scale much better in the common case that most post-copy > demand fetches are a result of vCPU access violations. This is a > continuation of the solution Anish was working on[6]. This aspect of > KVM Userfault is important for userfaultfd-based live migration when > scaling up to hundreds of vCPUs with ~30us network latency for a > PAGE_SIZE demand-fetch. > > The implementation in this series is version than the RFC[5]. It adds... > 1. a new memslot flag is added: KVM_MEM_USERFAULT, > 2. a new parameter, userfault_bitmap, into struct kvm_memory_slot, > 3. a new KVM_RUN exit reason: KVM_MEMORY_EXIT_FLAG_USERFAULT, > 4. a new KVM capability KVM_CAP_USERFAULT. > > KVM Userfault does not attempt to catch KVM's own accesses to guest > memory. That is left up to userfaultfd. > > When enabling KVM_MEM_USERFAULT for a memslot, the second-stage mappings > are zapped, and new faults will check `userfault_bitmap` to see if the > fault should exit to userspace. > > When KVM_MEM_USERFAULT is enabled, only PAGE_SIZE mappings are > permitted. > > When disabling KVM_MEM_USERFAULT, huge mappings will be reconstructed > consistent with dirty log disabling. So on x86, huge mappings will be > reconstructed, but on arm64, they won't be. > > KVM Userfault is not compatible with async page faults. Nikita has > proposed a new implementation of async page faults that is more > userspace-driven that *is* compatible with KVM Userfault[7]. > > See v1 for more performance details[4]. They are unchanged in this > version. > > This series is based on the latest kvm-x86/next. > > [1]: https://lore.kernel.org/kvm/20250611133330.1514028-1-tabba@google.com/ > [2]: https://lore.kernel.org/kvm/20250109204929.1106563-1-jthoughton@google.com/ > [3]: https://lore.kernel.org/kvm/20250414165146.2279450-1-xin@zytor.com/ > [4]: https://lore.kernel.org/kvm/20241204191349.1730936-1-jthoughton@google.com/ > [5]: https://lore.kernel.org/kvm/20240710234222.2333120-1-jthoughton@google.com/ > [6]: https://lore.kernel.org/all/20240215235405.368539-1-amoorthy@google.com/ > [7]: https://lore.kernel.org/kvm/20241118123948.4796-1-kalyazin@amazon.com/#t > > James Houghton (11): > KVM: Add common infrastructure for KVM Userfaults > KVM: x86: Add support for KVM userfault exits > KVM: arm64: Add support for KVM userfault exits > KVM: Enable and advertise support for KVM userfault exits > KVM: selftests: Fix vm_mem_region_set_flags docstring > KVM: selftests: Fix prefault_mem logic > KVM: selftests: Add va_start/end into uffd_desc > KVM: selftests: Add KVM Userfault mode to demand_paging_test > KVM: selftests: Inform set_memory_region_test of KVM_MEM_USERFAULT > KVM: selftests: Add KVM_MEM_USERFAULT + guest_memfd toggle tests > KVM: Documentation: Add KVM_CAP_USERFAULT and KVM_MEM_USERFAULT > details > > Sean Christopherson (3): > KVM: x86/mmu: Move "struct kvm_page_fault" definition to > asm/kvm_host.h > KVM: arm64: Add "struct kvm_page_fault" to gather common fault > variables > KVM: arm64: x86: Require "struct kvm_page_fault" for memory fault > exits > > Xin Li (Intel) (1): > KVM: Documentation: Fix section number for > KVM_CAP_ARM_WRITABLE_IMP_ID_REGS > > Documentation/virt/kvm/api.rst | 35 ++++- > arch/arm64/include/asm/kvm_host.h | 9 ++ > arch/arm64/kvm/Kconfig | 1 + > arch/arm64/kvm/mmu.c | 48 +++--- > arch/x86/include/asm/kvm_host.h | 68 +++++++- > arch/x86/kvm/Kconfig | 1 + > arch/x86/kvm/mmu/mmu.c | 13 +- > arch/x86/kvm/mmu/mmu_internal.h | 77 +--------- > arch/x86/kvm/x86.c | 27 ++-- > include/linux/kvm_host.h | 49 +++++- > include/uapi/linux/kvm.h | 6 +- > .../selftests/kvm/demand_paging_test.c | 145 ++++++++++++++++-- > .../testing/selftests/kvm/include/kvm_util.h | 5 + > .../selftests/kvm/include/userfaultfd_util.h | 2 + > tools/testing/selftests/kvm/lib/kvm_util.c | 42 ++++- > .../selftests/kvm/lib/userfaultfd_util.c | 2 + > .../selftests/kvm/set_memory_region_test.c | 33 ++++ > virt/kvm/Kconfig | 3 + > virt/kvm/kvm_main.c | 57 ++++++- > 19 files changed, 489 insertions(+), 134 deletions(-) > > > base-commit: 19272b37aa4f83ca52bdf9c16d5d81bdd1354494 > -- > 2.50.0.rc2.692.g299adb8693-goog >
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