[PATCH v2 00/13] KVM: Introduce KVM Userfault

[Sean Christopherson]

On Thu, Jan 09, 2025, James Houghton wrote:
>   KVM: Add KVM_MEM_USERFAULT memslot flag and bitmap
>   KVM: Add KVM_MEMORY_EXIT_FLAG_USERFAULT
>   KVM: Allow late setting of KVM_MEM_USERFAULT on guest_memfd memslot
>   KVM: Advertise KVM_CAP_USERFAULT in KVM_CHECK_EXTENSION
>   KVM: x86/mmu: Add support for KVM_MEM_USERFAULT
>   KVM: arm64: Add support for KVM_MEM_USERFAULT
>   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
> 
>  Documentation/virt/kvm/api.rst                |  33 +++-
>  arch/arm64/kvm/Kconfig                        |   1 +
>  arch/arm64/kvm/mmu.c                          |  26 +++-
>  arch/x86/kvm/Kconfig                          |   1 +
>  arch/x86/kvm/mmu/mmu.c                        |  27 +++-
>  arch/x86/kvm/mmu/mmu_internal.h               |  20 ++-
>  arch/x86/kvm/x86.c                            |  36 +++--
>  include/linux/kvm_host.h                      |  19 ++-
>  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                           |  54 ++++++-
>  17 files changed, 419 insertions(+), 36 deletions(-)

I didn't look at the selftests changes, but nothing in this series scares me.  We
bikeshedded most of this death this in the "exit on missing" series, so for me at
least, the only real question is whether or not we want to add the uAPI.  AFAIK,
this is best proposal for post-copy guest_memfd support (and not just because it's
the only proposal :-D).

So... yes?

Attached are a variation on the series using the common "struct kvm_page_fault"
idea.  The documentation change could be squashed with the final enablement patch.

Compile tested only.  I would not be the least bit surprised if I completely
butchered something.
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