[RFC PATCH 0/8] KVM: Prepopulate guest memory API

[isaku . yamahata]

From: Isaku Yamahata <isaku.yamahata@xxxxxxxxx>

The objective of this RFC patch series is to develop a uAPI aimed at
(pre)populating guest memory for various use cases and underlying VM
technologies.

- Pre-populate guest memory to mitigate excessive KVM page faults during guest
  boot [1], a need not limited to any specific technology.

- Pre-populating guest memory (including encryption and measurement) for
  confidential guests [2].  SEV-SNP, TDX, and SW-PROTECTED VM.  Potentially
  other technologies and pKVM.

The patches are organized as follows.
- 1: documentation on uAPI KVM_MAP_MEMORY.
- 2: archtechture-independent implementation part.
- 3-4: refactoring of x86 KVM MMU as preparation.
- 5: x86 Helper function to map guest page.
- 6: x86 KVM arch implementation.
- 7: Add x86-ops necessary for TDX and SEV-SNP.
- 8: selftest for validation.

Discussion point:

uAPI design:
- access flags
  Access flags are needed for the guest memory population.  We have options for
  their exposure to uAPI.
  - option 1. Introduce access flags, possibly with the addition of a private
              access flag.
  - option 2. Omit access flags from UAPI.
    Allow the kernel to deduce the necessary flag based on the memory slot and
    its memory attributes.

- SEV-SNP and byte vs. page size
  The SEV correspondence is SEV_LAUNCH_UPDATE_DATA.  Which dictates memory
  regions to be in 16-byte alignment, not page size.  Should we define struct
  kvm_memory_mapping in bytes rather than page size?

  struct kvm_sev_launch_update_data {
        __u64 uaddr;
        __u32 len;
  };

- TDX and measurement
  The TDX correspondence is TDH.MEM.PAGE.ADD and TDH.MR.EXTEND.  TDH.MEM.EXTEND
  extends its measurement by the page contents.
  Option 1. Add an additional flag like KVM_MEMORY_MAPPING_FLAG_EXTEND to issue
            TDH.MEM.EXTEND
  Option 2. Don't handle extend. Let TDX vendor specific API
            KVM_EMMORY_ENCRYPT_OP to handle it with the subcommand like
            KVM_TDX_EXTEND_MEMORY.

- TDX and struct kvm_memory_mapping:source
  While the current patch series doesn't utilize
  kvm_memory_mapping::source member.  TDX needs it to specify the source of
  memory contents.

Implementation:
- x86 KVM MMU
  In x86 KVM MMU, I chose to use kvm_mmu_do_page_fault().  It's not confined to
  KVM TDP MMU.  We can restrict it to KVM TDP MMU and introduce an optimized
  version.

[1] https://lore.kernel.org/all/65262e67-7885-971a-896d-ad9c0a760907@xxxxxxxxx/
[2] https://lore.kernel.org/all/6a4c029af70d41b63bcee3d6a1f0c2377f6eb4bd.1690322424.git.isaku.yamahata@xxxxxxxxx

Thanks,

Isaku Yamahata (8):
  KVM: Document KVM_MAP_MEMORY ioctl
  KVM: Add KVM_MAP_MEMORY vcpu ioctl to pre-populate guest memory
  KVM: x86/mmu: Introduce initialier macro for struct kvm_page_fault
  KVM: x86/mmu: Factor out kvm_mmu_do_page_fault()
  KVM: x86/mmu: Introduce kvm_mmu_map_page() for prepopulating guest
    memory
  KVM: x86: Implement kvm_arch_{, pre_}vcpu_map_memory()
  KVM: x86: Add hooks in kvm_arch_vcpu_map_memory()
  KVM: selftests: x86: Add test for KVM_MAP_MEMORY

 Documentation/virt/kvm/api.rst                |  36 +++++
 arch/x86/include/asm/kvm-x86-ops.h            |   2 +
 arch/x86/include/asm/kvm_host.h               |   6 +
 arch/x86/kvm/mmu.h                            |   3 +
 arch/x86/kvm/mmu/mmu.c                        |  30 ++++
 arch/x86/kvm/mmu/mmu_internal.h               |  70 +++++----
 arch/x86/kvm/x86.c                            |  83 +++++++++++
 include/linux/kvm_host.h                      |   4 +
 include/uapi/linux/kvm.h                      |  15 ++
 tools/include/uapi/linux/kvm.h                |  14 ++
 tools/testing/selftests/kvm/Makefile          |   1 +
 .../selftests/kvm/x86_64/map_memory_test.c    | 136 ++++++++++++++++++
 virt/kvm/kvm_main.c                           |  74 ++++++++++
 13 files changed, 448 insertions(+), 26 deletions(-)
 create mode 100644 tools/testing/selftests/kvm/x86_64/map_memory_test.c


base-commit: 6a108bdc49138bcaa4f995ed87681ab9c65122ad
-- 
2.25.1