[PATCH v13 00/20] KVM: Enable host userspace mapping for guest_memfd-backed memory for non-CoCo VMs

[Fuad Tabba <tabba@google.com>]

Main changes since v12 [1]:
* Rename various functions and variables
* Expand and clarify commit messages
* Rebase on Linux 6.16-rc5

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 1-7: Primarily infrastructure refactorings and renames to decouple
guest_memfd from the concept of "private" memory.

Patches 8-9: 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 18).

Patches 10-14: Implement x86 guest_memfd mmap support.

Patches 15-17: Implement arm64 guest_memfd mmap support.

Patch 18: Introduce the new capability to advertise this support and
update the documentation.

Patches 19-20: 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/20250611133330.1514028-3-tabba@google.com/T/
[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 (4):
  KVM: x86/mmu: Generalize private_max_mapping_level x86 op to
    max_mapping_level
  KVM: x86/mmu: Allow NULL-able fault in kvm_max_private_mapping_level
  KVM: x86/mmu: Consult guest_memfd when computing max_mapping_level
  KVM: x86/mmu: Handle guest page faults for guest_memfd with shared
    memory

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: Introduce kvm_arch_supports_gmem()
  KVM: x86: Introduce 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 guest_memfd mmap support in memslot
  KVM: x86: Enable guest_memfd mmap for default VM type
  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_MMAP
  KVM: selftests: Do not use hardcoded page sizes in guest_memfd test
  KVM: selftests: guest_memfd mmap() test when mmap is supported

 Documentation/virt/kvm/api.rst                |   9 +
 arch/arm64/include/asm/kvm_host.h             |   4 +
 arch/arm64/kvm/Kconfig                        |   1 +
 arch/arm64/kvm/mmu.c                          | 190 ++++++++++++----
 arch/x86/include/asm/kvm-x86-ops.h            |   2 +-
 arch/x86/include/asm/kvm_host.h               |  18 +-
 arch/x86/kvm/Kconfig                          |   7 +-
 arch/x86/kvm/mmu/mmu.c                        | 115 ++++++----
 arch/x86/kvm/svm/sev.c                        |  12 +-
 arch/x86/kvm/svm/svm.c                        |   3 +-
 arch/x86/kvm/svm/svm.h                        |   4 +-
 arch/x86/kvm/vmx/main.c                       |   6 +-
 arch/x86/kvm/vmx/tdx.c                        |   6 +-
 arch/x86/kvm/vmx/x86_ops.h                    |   2 +-
 arch/x86/kvm/x86.c                            |   5 +-
 include/linux/kvm_host.h                      |  64 +++++-
 include/uapi/linux/kvm.h                      |   2 +
 tools/testing/selftests/kvm/Makefile.kvm      |   1 +
 .../testing/selftests/kvm/guest_memfd_test.c  | 208 +++++++++++++++---
 virt/kvm/Kconfig                              |  14 +-
 virt/kvm/Makefile.kvm                         |   2 +-
 virt/kvm/guest_memfd.c                        |  96 +++++++-
 virt/kvm/kvm_main.c                           |  14 +-
 virt/kvm/kvm_mm.h                             |   4 +-
 24 files changed, 622 insertions(+), 167 deletions(-)


base-commit: d7b8f8e20813f0179d8ef519541a3527e7661d3a
-- 
2.50.0.727.gbf7dc18ff4-goog