[PATCH v11 00/18] KVM: Mapping guest_memfd backed memory at the host for software protected VMs

[Fuad Tabba <tabba@google.com>]

Main changes since v10 [1]:
- Added bounds checking when faulting a shared page into the host, along
  with a selftest to verify the check.
- Refactored KVM/arm64's handling of guest faults (user_mem_abort()).
  I've dropped the Reviewed-by tags from "KVM: arm64: Refactor
  user_mem_abort()..." since it has changed significantly.
- Handled nested virtualization in KVM/arm64 when faulting guest_memfd
  backed pages into the guest.
- Addressed various points of feedback from the last revision.
- Still based on Linux 6.15

This patch series enables the mapping of guest_memfd backed memory in
the host. This is useful for VMMs like Firecracker that aim to run
guests entirely backed by guest_memfd [2]. When combined with Patrick's
series for direct map removal [3], this provides additional hardening
against Spectre-like transient execution attacks.

This series also lays the groundwork for restricted mmap() support for
guest_memfd backed memory in the host for Confidential Computing
platforms that permit in-place sharing of guest memory with the host
[4].

Patch breakdown:

Patches 1-7: Primarily refactoring and renaming to decouple the concept
of guest memory being "private" from it being backed by guest_memfd.

Patches 8-9: Add support for in-place shared memory and the ability for
the host to map it. This is gated by a new configuration option, toggled
by a new flag, and advertised to userspace by a new capability
(introduced in patch 16).

Patches 10-15: Implement the x86 and arm64 support for this feature.

Patch 16: Introduces the new capability to advertise this support and
updates the documentation.

Patches 17-18: Add and fix selftests for the new functionality.

For details on how to test this patch series, and on how to boot a guest
that uses the new features, please refer to v8 [5].

Cheers,
/fuad

[1] https://lore.kernel.org/all/20250527180245.1413463-1-tabba@google.com/
[2] https://github.com/firecracker-microvm/firecracker/tree/feature/secret-hiding
[3] https://lore.kernel.org/all/20250221160728.1584559-1-roypat@amazon.co.uk/
[4] https://lore.kernel.org/all/20250328153133.3504118-1-tabba@google.com/
[5] https://lore.kernel.org/all/20250430165655.605595-1-tabba@google.com/

Ackerley Tng (2):
  KVM: x86/mmu: Handle guest page faults for guest_memfd with shared
    memory
  KVM: x86: Consult guest_memfd when computing max_mapping_level

Fuad Tabba (16):
  KVM: Rename CONFIG_KVM_PRIVATE_MEM to CONFIG_KVM_GMEM
  KVM: Rename CONFIG_KVM_GENERIC_PRIVATE_MEM to
    CONFIG_KVM_GENERIC_GMEM_POPULATE
  KVM: Rename kvm_arch_has_private_mem() to kvm_arch_supports_gmem()
  KVM: x86: Rename kvm->arch.has_private_mem to kvm->arch.supports_gmem
  KVM: Rename kvm_slot_can_be_private() to kvm_slot_has_gmem()
  KVM: Fix comments that refer to slots_lock
  KVM: Fix comment that refers to kvm uapi header path
  KVM: guest_memfd: Allow host to map guest_memfd pages
  KVM: guest_memfd: Track shared memory support in memslot
  KVM: x86: Enable guest_memfd shared memory for SW-protected VMs
  KVM: arm64: Refactor user_mem_abort()
  KVM: arm64: Handle guest_memfd-backed guest page faults
  KVM: arm64: Enable host mapping of shared guest_memfd memory
  KVM: Introduce the KVM capability KVM_CAP_GMEM_SHARED_MEM
  KVM: selftests: Don't use hardcoded page sizes in guest_memfd test
  KVM: selftests: guest_memfd mmap() test when mapping is allowed

 Documentation/virt/kvm/api.rst                |   9 +
 arch/arm64/include/asm/kvm_host.h             |   5 +
 arch/arm64/kvm/Kconfig                        |   1 +
 arch/arm64/kvm/mmu.c                          | 200 +++++++++++++----
 arch/x86/include/asm/kvm_host.h               |  22 +-
 arch/x86/kvm/Kconfig                          |   5 +-
 arch/x86/kvm/mmu/mmu.c                        | 135 ++++++-----
 arch/x86/kvm/svm/sev.c                        |   4 +-
 arch/x86/kvm/svm/svm.c                        |   4 +-
 arch/x86/kvm/x86.c                            |   4 +-
 include/linux/kvm_host.h                      |  80 +++++--
 include/uapi/linux/kvm.h                      |   2 +
 tools/testing/selftests/kvm/Makefile.kvm      |   1 +
 .../testing/selftests/kvm/guest_memfd_test.c  | 212 +++++++++++++++---
 virt/kvm/Kconfig                              |  14 +-
 virt/kvm/Makefile.kvm                         |   2 +-
 virt/kvm/guest_memfd.c                        |  94 +++++++-
 virt/kvm/kvm_main.c                           |  16 +-
 virt/kvm/kvm_mm.h                             |   4 +-
 19 files changed, 645 insertions(+), 169 deletions(-)


base-commit: 0ff41df1cb268fc69e703a08a57ee14ae967d0ca
-- 
2.49.0.1266.g31b7d2e469-goog