KVM: Mapping guest_memfd backed memory at the host for software protected VMs

Main changes since v1 [1]:
- Added x86 support for mapping guest_memfd at the host, enabled
 only for the KVM_X86_SW_PROTECTED_VM type.
- Require setting memslot userspace_addr for guest_memfd slots
 even if shared, and remove patches that worked around that.
- Brought in more of the infrastructure from the patch series
 that allows restricted mapping of guest_memfd backed memory.
- Renamed references to "mappable" -> "shared".
- Expanded the selftests.
- Added instructions to test on x86 and arm64 (below).
- Rebased on Linux 6.13.

The purpose of this series is to serve as a base for _restricted_
mmap() support for guest_memfd backed memory at the host [2]. It
would allow experimentation with what that support would be like
in the safe environment of the software VM types, which are meant
for testing and experimentation.

This series adds a new VM type for arm64,
KVM_VM_TYPE_ARM_SW_PROTECTED, analogous to the x86
KVM_X86_SW_PROTECTED_VM. This type is to serve as a development
and testing vehicle for Confidential (CoCo) VMs.

Similar to its x86 counterpart, SW_PROTECTED is meant only for
development and testing. It's not meant to be used for "real"
VMs, and especially not in production. The behavior and effective
ABI for software-protected VMs is unstable.

This series enables mmap() and fault() support for guest_memfd
backed memory specifically for the software-protected VM types
(in x86 and arm64), only when explicitly enabled in the config.

The series is based on Linux 6.13 and much of the code within
is a subset of the latest series I sent [2], with the addition of
the new software protected vm type.

To test this series, I've pushed a kvmtool branch with support
for guest_memfd for x86 and arm64 and the new runtime options of
--guest_memfd and --sw_protected, which marks the VM as software
protected [3]. I plan on upstreaming this branch once I've tested
it more and tidied it up a bit (or a lot).

To test this patch series on x86 (I use a standard Debian image):

Build:

- Build the kernel with the following config options enabled:
defconfigs:
	x86_64_defconfig
	kvm_guest.config
config options:
	KVM
	KVM_INTEL
	KVM_PRIVATE_MEM
	KVM_SW_PROTECTED_VM
	KVM_GMEM_SHARED_MEM

- Build the kernel kvm selftest tools/testing/selftests/kvm, you
only need guest_memfd_test, e.g.:
	make EXTRA_CFLAGS="-static -DDEBUG" -C tools/testing/selftests/kvm

- Build kvmtool [3] lkvm-static (I build it on a different machine).
	make lkvm-static

Run:
Boot your Linux image with the kernel you built above.

The selftest you can run as it is:
	./guest_memfd_test

For kvmtool, where bzImage is the same as the host's:
	./lkvm-static run -c 2 -m 512 -p "break=mount" --kernel bzImage --debug --guest_memfd --sw_protected

To test this patch series on arm64 (I use a standard Debian image):

Build:

- Build the kernel with defconfig

- Build the kernel kvm selftest tools/testing/selftests/kvm, you
only need guest_memfd_test.

- Build kvmtool [3] lkvm-static (I cross compile it on a different machine).
You are likely to need libfdt as well.

For libfdt (in the same directory as kvmtool):
	git clone git://git.kernel.org/pub/scm/utils/dtc/dtc.git
	cd dtc
	export CC=aarch64-linux-gnu-gcc
	make
	cd ..

Then for kvmtool:
	make ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- LIBFDT_DIR=./dtc/libfdt/ lkvm-static

Run:
Boot your Linux image with the kernel you built above.

The selftest you can run as it is:
	./guest_memfd_test

For kvmtool, where Image is the same as the host's, and rootfs is
your rootfs image (in case kvmtool can't figure it out):
	./lkvm-static run -c 2 -m 512 -d rootfs --kernel Image --force-pci --irqchip gicv3 --debug --guest_memfd --sw_protected

You can find (potentially slightly outdated) instructions on how
to a full arm64 system stack under QEMU here [4].

Cheers,
/fuad

[1] https://lore.kernel.org/all/20250122152738.1173160-1-tabb...
[2] https://lore.kernel.org/all/20250117163001.2326672-1-tabb...
[3] https://android-kvm.googlesource.com/kvmtool/+/refs/heads/tabba/guestmem-6.13
[4] https://mirrors.edge.kernel.org/pub/linux/kernel/people/w...

Fuad Tabba (11):
  mm: Consolidate freeing of typed folios on final folio_put()
  KVM: guest_memfd: Handle final folio_put() of guest_memfd pages
  KVM: guest_memfd: Allow host to map guest_memfd() pages
  KVM: guest_memfd: Add KVM capability to check if guest_memfd is shared
  KVM: guest_memfd: Handle in-place shared memory as guest_memfd backed
    memory
  KVM: x86: Mark KVM_X86_SW_PROTECTED_VM as supporting guest_memfd
    shared memory
  KVM: arm64: Refactor user_mem_abort() calculation of force_pte
  KVM: arm64: Handle guest_memfd()-backed guest page faults
  KVM: arm64: Introduce KVM_VM_TYPE_ARM_SW_PROTECTED machine type
  KVM: arm64: Enable mapping guest_memfd in arm64
  KVM: guest_memfd: selftests: guest_memfd mmap() test when mapping is
    allowed

 Documentation/virt/kvm/api.rst                |  5 +
 arch/arm64/include/asm/kvm_host.h             | 10 ++
 arch/arm64/kvm/Kconfig                        |  1 +
 arch/arm64/kvm/arm.c                          |  5 +
 arch/arm64/kvm/mmu.c                          | 91 ++++++++++++-------
 arch/x86/include/asm/kvm_host.h               |  5 +
 arch/x86/kvm/Kconfig                          |  3 +-
 include/linux/kvm_host.h                      | 19 +++-
 include/linux/page-flags.h                    | 22 +++++
 include/uapi/linux/kvm.h                      |  7 ++
 mm/debug.c                                    |  1 +
 mm/swap.c                                     | 27 +++++-
 tools/testing/selftests/kvm/Makefile          |  1 +
 .../testing/selftests/kvm/guest_memfd_test.c  | 75 +++++++++++++--
 tools/testing/selftests/kvm/lib/kvm_util.c    |  3 +-
 virt/kvm/Kconfig                              |  4 +
 virt/kvm/guest_memfd.c                        | 90 ++++++++++++++++++
 virt/kvm/kvm_main.c                           |  9 +-
 18 files changed, 326 insertions(+), 52 deletions(-)


base-commit: ffd294d346d185b70e28b1a28abe367bbfe53c04
-- 
2.48.1.262.g85cc9f2d1e-goog