[PATCH v16 00/22] KVM: Enable host userspace mapping for guest_memfd-backed memory for non-CoCo VMs

[Fuad Tabba <tabba@google.com>]

Main changes since v15 [1]:

* Rework and simplify Kconfig selection and dependencies.
* Always enable guest_memfd for KVM x86 (64-bit) and arm64, which
  simplifies the enablement checks.
* Based on kvm-x86/next: commit 33f843444e28 ("Merge branch 'vmx'").

This patch series enables host userspace mapping of guest_memfd-backed
memory for non-CoCo VMs. This is required for several evolving KVM use
cases:

* Allows VMMs like Firecracker to run guests entirely backed by
  guest_memfd [2]. This provides a unified memory management model for
  both confidential and non-confidential guests, simplifying VMM design.

* Enhanced Security via direct map removal: When combined with Patrick's
  series for direct map removal [3], this provides additional hardening
  against Spectre-like transient execution attacks by eliminating the
  need for host kernel direct maps of guest memory.

* Lays the groundwork for *restricted* mmap() support for
  guest_memfd-backed memory on CoCo platforms [4] that permit in-place
  sharing of guest memory with the host.

Patch breakdown:

* Patches 1-9: Primarily infrastructure refactoring, Kconfig cleaning,
  and renames to decouple guest_memfd from the concept of "private"
  memory.

* Patches 10-11: Add support for the host to map guest_memfd backed
  memory for non-CoCo VMs, which includes support for mmap() and fault
  handling. This is gated by a new configuration option, toggled by a
  new flag, and advertised to userspace by a new capability (introduced
  in patch 20).

* Patches 12-15: Implement x86 guest_memfd mmap support.

* Patches 16-19: Implement arm64 guest_memfd mmap support.

* Patch 20: Introduce the new capability to advertise this support and
  update the documentation.

* Patches 21-22: Update and expand selftests for guest_memfd to include
  mmap functionality and improve portability.

To test this patch series and boot a guest utilizing the new features,
please refer to the instructions in v8 of the series [5]. Note that
kvmtool for Linux 6.16 (available at [6]) is required, as the
KVM_CAP_GMEM_MMAP capability number has changed, additionally, drop the
--sw_protected kvmtool parameter to test with the default VM type.

Cheers,
/fuad

[1] https://lore.kernel.org/all/20250717162731.446579-1-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/
[6] https://android-kvm.googlesource.com/kvmtool/+/refs/heads/tabba/guestmem-basic-6.16

Ackerley Tng (1):
  KVM: x86/mmu: Rename .private_max_mapping_level() to
    .gmem_max_mapping_level()

Fuad Tabba (15):
  KVM: Rename CONFIG_KVM_PRIVATE_MEM to CONFIG_KVM_GUEST_MEMFD
  KVM: Rename CONFIG_KVM_GENERIC_PRIVATE_MEM to
    CONFIG_HAVE_KVM_ARCH_GMEM_POPULATE
  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: x86: Enable KVM_GUEST_MEMFD for all 64-bit builds
  KVM: guest_memfd: Add plumbing to host to map guest_memfd pages
  KVM: guest_memfd: Track guest_memfd mmap support in memslot
  KVM: arm64: Refactor user_mem_abort()
  KVM: arm64: Handle guest_memfd-backed guest page faults
  KVM: arm64: nv: Handle VNCR_EL2-triggered faults backed by guest_memfd
  KVM: arm64: Enable support for guest_memfd backed memory
  KVM: Allow and advertise support for host mmap() on guest_memfd files
  KVM: selftests: Do not use hardcoded page sizes in guest_memfd test
  KVM: selftests: guest_memfd mmap() test when mmap is supported

Sean Christopherson (6):
  KVM: x86: Have all vendor neutral sub-configs depend on KVM_X86, not
    just KVM
  KVM: x86: Select KVM_GENERIC_PRIVATE_MEM directly from
    KVM_SW_PROTECTED_VM
  KVM: x86: Select TDX's KVM_GENERIC_xxx dependencies iff
    CONFIG_KVM_INTEL_TDX=y
  KVM: x86/mmu: Hoist guest_memfd max level/order helpers "up" in mmu.c
  KVM: x86/mmu: Enforce guest_memfd's max order when recovering
    hugepages
  KVM: x86/mmu: Extend guest_memfd's max mapping level to shared
    mappings

 Documentation/virt/kvm/api.rst                |   9 +
 arch/arm64/kvm/Kconfig                        |   1 +
 arch/arm64/kvm/mmu.c                          | 203 ++++++++++++-----
 arch/arm64/kvm/nested.c                       |  41 +++-
 arch/x86/include/asm/kvm-x86-ops.h            |   2 +-
 arch/x86/include/asm/kvm_host.h               |   6 +-
 arch/x86/kvm/Kconfig                          |  26 ++-
 arch/x86/kvm/mmu/mmu.c                        | 143 +++++++-----
 arch/x86/kvm/mmu/mmu_internal.h               |   2 +-
 arch/x86/kvm/mmu/tdp_mmu.c                    |   2 +-
 arch/x86/kvm/svm/sev.c                        |   6 +-
 arch/x86/kvm/svm/svm.c                        |   2 +-
 arch/x86/kvm/svm/svm.h                        |   4 +-
 arch/x86/kvm/vmx/main.c                       |   7 +-
 arch/x86/kvm/vmx/tdx.c                        |   5 +-
 arch/x86/kvm/vmx/x86_ops.h                    |   2 +-
 arch/x86/kvm/x86.c                            |  11 +
 include/linux/kvm_host.h                      |  38 ++--
 include/uapi/linux/kvm.h                      |   2 +
 tools/testing/selftests/kvm/Makefile.kvm      |   1 +
 .../testing/selftests/kvm/guest_memfd_test.c  | 205 +++++++++++++++---
 virt/kvm/Kconfig                              |  15 +-
 virt/kvm/Makefile.kvm                         |   2 +-
 virt/kvm/guest_memfd.c                        |  81 ++++++-
 virt/kvm/kvm_main.c                           |  12 +-
 virt/kvm/kvm_mm.h                             |   4 +-
 26 files changed, 623 insertions(+), 209 deletions(-)


base-commit: 33f843444e28920d6e624c6c24637b4bb5d3c8de
-- 
2.50.1.470.g6ba607880d-goog