[RFC PATCH v2 00/37] guest_memfd: In-place conversion support

[Ackerley Tng <ackerleytng@google.com>]

Here's a second revision of guest_memfd In-place conversion support.

In this version, other than addressing comments from RFCv1 [1], the largest
change is that guest_memfd now does not avoid participation in LRU; it
participates in LRU by joining the unevictable list (no change from before this
series).

While checking for elevated refcounts during shared to private conversions,
guest_memfd will now do an lru_add_drain_all() if elevated refcounts were found,
before concluding that there are true users of the shared folio and erroring
out.

I'd still like feedback on these points, if any:

1. Having private/shared status stored in a maple tree (Thanks Michael for your
   support of using maple trees over xarrays for performance! [5]).
2. Having a new guest_memfd ioctl (not a vm ioctl) that performs conversions.
3. Using ioctls/structs/input attribute similar to the existing vm ioctl
   KVM_SET_MEMORY_ATTRIBUTES to perform conversions.
4. Storing requested attributes directly in the maple tree.
5. Using a KVM module-wide param to toggle between setting memory attributes via
   vm and guest_memfd ioctls (making them mututally exclusive - a single loaded
   KVM module can only do one of the two.).

This series is based on kvm/next as at 2026-01-21, and here's the tree for your
convenience:

https://github.com/googleprodkernel/linux-cc/commits/guest_memfd-inplace-conversion-v2

The "Don't set FGP_ACCESSED when getting folios" patch from RFCv1 is still
useful but no longer related to conversion, and was posted separately [6].

Older series:

+ RFCv1 is at [1]
+ Previous versions of this feature, part of other series, are available at
  [2][3][4].

[1] https://lore.kernel.org/all/cover.1760731772.git.ackerleytng@google.com/T/
[2] https://lore.kernel.org/all/bd163de3118b626d1005aa88e71ef2fb72f0be0f.1726009989.git.ackerleytng@google.com/
[3] https://lore.kernel.org/all/20250117163001.2326672-6-tabba@google.com/
[4] https://lore.kernel.org/all/b784326e9ccae6a08388f1bf39db70a2204bdc51.1747264138.git.ackerleytng@google.com/
[5] https://lore.kernel.org/all/20250529054227.hh2f4jmyqf6igd3i@amd.com/
[6] https://lore.kernel.org/all/20260129172646.2361462-1-ackerleytng@google.com/

Ackerley Tng (19):
  KVM: guest_memfd: Update kvm_gmem_populate() to use gmem attributes
  KVM: Introduce KVM_SET_MEMORY_ATTRIBUTES2
  KVM: guest_memfd: Add support for KVM_SET_MEMORY_ATTRIBUTES2
  KVM: guest_memfd: Handle lru_add fbatch refcounts during conversion
    safety check
  KVM: selftests: Update framework to use KVM_SET_MEMORY_ATTRIBUTES2
  KVM: selftests: Test using guest_memfd for guest private memory
  KVM: selftests: Test basic single-page conversion flow
  KVM: selftests: Test conversion flow when INIT_SHARED
  KVM: selftests: Test indexing in guest_memfd
  KVM: selftests: Test conversion before allocation
  KVM: selftests: Convert with allocated folios in different layouts
  KVM: selftests: Test precision of conversion
  KVM: selftests: Test that truncation does not change shared/private
    status
  KVM: selftests: Test conversion with elevated page refcount
  KVM: selftests: Reset shared memory after hole-punching
  KVM: selftests: Provide function to look up guest_memfd details from
    gpa
  KVM: selftests: Make TEST_EXPECT_SIGBUS thread-safe
  KVM: selftests: Update private_mem_conversions_test to mmap()
    guest_memfd
  KVM: selftests: Add script to exercise private_mem_conversions_test

Sean Christopherson (18):
  KVM: guest_memfd: Introduce per-gmem attributes, use to guard user
    mappings
  KVM: Rename KVM_GENERIC_MEMORY_ATTRIBUTES to KVM_VM_MEMORY_ATTRIBUTES
  KVM: Enumerate support for PRIVATE memory iff kvm_arch_has_private_mem
    is defined
  KVM: Stub in ability to disable per-VM memory attribute tracking
  KVM: guest_memfd: Wire up kvm_get_memory_attributes() to per-gmem
    attributes
  KVM: guest_memfd: Enable INIT_SHARED on guest_memfd for x86 Coco VMs
  KVM: Move KVM_VM_MEMORY_ATTRIBUTES config definition to x86
  KVM: Let userspace disable per-VM mem attributes, enable per-gmem
    attributes
  KVM: selftests: Create gmem fd before "regular" fd when adding memslot
  KVM: selftests: Rename guest_memfd{,_offset} to gmem_{fd,offset}
  KVM: selftests: Add support for mmap() on guest_memfd in core library
  KVM: selftests: Add selftests global for guest memory attributes
    capability
  KVM: selftests: Add helpers for calling ioctls on guest_memfd
  KVM: selftests: Test that shared/private status is consistent across
    processes
  KVM: selftests: Provide common function to set memory attributes
  KVM: selftests: Check fd/flags provided to mmap() when setting up
    memslot
  KVM: selftests: Update pre-fault test to work with per-guest_memfd
    attributes
  KVM: selftests: Update private memory exits test work with per-gmem
    attributes

 Documentation/virt/kvm/api.rst                |  72 ++-
 arch/x86/include/asm/kvm_host.h               |   2 +-
 arch/x86/kvm/Kconfig                          |  15 +-
 arch/x86/kvm/mmu/mmu.c                        |   4 +-
 arch/x86/kvm/x86.c                            |  13 +-
 include/linux/kvm_host.h                      |  53 +-
 include/trace/events/kvm.h                    |   4 +-
 include/uapi/linux/kvm.h                      |  17 +
 tools/testing/selftests/kvm/.gitignore        |   1 +
 tools/testing/selftests/kvm/Makefile.kvm      |   1 +
 .../kvm/guest_memfd_conversions_test.c        | 486 ++++++++++++++++++
 .../testing/selftests/kvm/guest_memfd_test.c  |  57 +-
 .../testing/selftests/kvm/include/kvm_util.h  | 128 ++++-
 .../testing/selftests/kvm/include/test_util.h |  31 +-
 tools/testing/selftests/kvm/lib/kvm_util.c    | 130 +++--
 tools/testing/selftests/kvm/lib/test_util.c   |   7 -
 .../selftests/kvm/pre_fault_memory_test.c     |   2 +-
 .../kvm/x86/private_mem_conversions_test.c    |  48 +-
 .../kvm/x86/private_mem_conversions_test.py   | 152 ++++++
 .../kvm/x86/private_mem_kvm_exits_test.c      |  36 +-
 virt/kvm/Kconfig                              |   4 +-
 virt/kvm/guest_memfd.c                        | 399 +++++++++++++-
 virt/kvm/kvm_main.c                           | 104 +++-
 23 files changed, 1590 insertions(+), 176 deletions(-)
 create mode 100644 tools/testing/selftests/kvm/guest_memfd_conversions_test.c
 create mode 100755 tools/testing/selftests/kvm/x86/private_mem_conversions_test.py

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2.53.0.rc1.225.gd81095ad13-goog