[v8,00/13] KVM: Mapping guest_memfd backed memory at the host for software protected VMs

Main changes since v7 [1]:
- Renaming/refactoring to decouple guest memory from whether the
  underlying memory is private vs being backed by guest_memfd
- Drop folio_put() callback patches
- Fixes based on feedback from the previous series
- Rebase on Linux 6.15-rc4

The purpose of this series is to allow mapping guest_memfd backed memory
at the host. This support enables VMMs like Firecracker to run VM guests
backed completely by guest_memfd [2]. Combined with Patrick's series for
direct map removal in guest_memfd [3], this would allow running VMs that
offer additional hardening against Spectre-like transient execution
attacks.

This series will also serve as a base for _restricted_ mmap() support
for guest_memfd backed memory at the host for CoCos that allow sharing
guest memory in-place with the host [4].

Patches 1 to 7 are mainly about decoupling the concept of guest memory
being private vs guest memory being backed by guest_memfd. They are
mostly refactoring and renaming.

Patch 8 adds support for in-place shared memory, as well as the ability
to map it by the host as long as it is shared, gated by a new
configuration option, and adviertised to userspace by a new capability.

Patches 9 to 12 add arm64 and x86 support for in-place shared memory.

Patch 13 expands the guest_memfd selftest to test in-place shared memory
when avaialble.

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
Additional config options to enable:
	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 [5] 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 [5] 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 [6].

Cheers,
/fuad

[1] https://lore.kernel.org/all/20250318161823.4005529-1-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://android-kvm.googlesource.com/kvmtool/+/refs/heads/tabba/guestmem-basic-6.15
[6] https://mirrors.edge.kernel.org/pub/linux/kernel/people/will/docs/qemu/qemu-arm64-howto.html

Fuad Tabba (13):
  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: x86: Generalize private fault lookups to guest_memfd fault
    lookups
  KVM: Fix comments that refer to slots_lock
  KVM: guest_memfd: Allow host to map guest_memfd() pages
  KVM: arm64: Refactor user_mem_abort() calculation of force_pte
  KVM: arm64: Handle guest_memfd()-backed guest page faults
  KVM: arm64: Enable mapping guest_memfd in arm64
  KVM: x86: KVM_X86_SW_PROTECTED_VM to support guest_memfd shared memory
  KVM: guest_memfd: selftests: guest_memfd mmap() test when mapping is
    allowed

 arch/arm64/include/asm/kvm_host.h             | 12 +++
 arch/arm64/kvm/Kconfig                        |  1 +
 arch/arm64/kvm/mmu.c                          | 76 +++++++++------
 arch/x86/include/asm/kvm_host.h               | 17 ++--
 arch/x86/kvm/Kconfig                          |  4 +-
 arch/x86/kvm/mmu/mmu.c                        | 31 +++---
 arch/x86/kvm/svm/sev.c                        |  4 +-
 arch/x86/kvm/svm/svm.c                        |  4 +-
 arch/x86/kvm/x86.c                            |  3 +-
 include/linux/kvm_host.h                      | 44 +++++++--
 include/uapi/linux/kvm.h                      |  1 +
 tools/testing/selftests/kvm/Makefile.kvm      |  1 +
 .../testing/selftests/kvm/guest_memfd_test.c  | 75 +++++++++++++--
 virt/kvm/Kconfig                              | 15 ++-
 virt/kvm/Makefile.kvm                         |  2 +-
 virt/kvm/guest_memfd.c                        | 96 ++++++++++++++++++-
 virt/kvm/kvm_main.c                           | 21 ++--
 virt/kvm/kvm_mm.h                             |  4 +-
 18 files changed, 316 insertions(+), 95 deletions(-)


base-commit: b4432656b36e5cc1d50a1f2dc15357543add530e

Fuad Tabba wrote:
> The option KVM_PRIVATE_MEM enables guest_memfd in general. Subsequent
> patches add shared memory support to guest_memfd. Therefore, rename it
> to KVM_GMEM to make its purpose clearer.
> 
> Co-developed-by: David Hildenbrand <david@redhat.com>
> Signed-off-by: David Hildenbrand <david@redhat.com>
> Signed-off-by: Fuad Tabba <tabba@google.com>

Reviewed-by: Ira Weiny <ira.weiny@intel.com>

Fuad Tabba wrote:
> The function kvm_arch_has_private_mem() is used to indicate whether
> guest_memfd is supported by the architecture, which until now implies
> that its private. To decouple guest_memfd support from whether the
> memory is private, rename this function to kvm_arch_supports_gmem().
> 
> Co-developed-by: David Hildenbrand <david@redhat.com>
> Signed-off-by: David Hildenbrand <david@redhat.com>
> Signed-off-by: Fuad Tabba <tabba@google.com>

Reviewed-by: Ira Weiny <ira.weiny@intel.com>

[snip]

Fuad Tabba wrote:
> The function kvm_slot_can_be_private() is used to check whether a memory
> slot is backed by guest_memfd. Rename it to kvm_slot_has_gmem() to make
> that clearer and to decouple memory being private from guest_memfd.
> 
> Co-developed-by: David Hildenbrand <david@redhat.com>
> Signed-off-by: David Hildenbrand <david@redhat.com>
> Signed-off-by: Fuad Tabba <tabba@google.com>

Reviewed-by: Ira Weiny <ira.weiny@intel.com>

[snip]

Fuad Tabba wrote:
> Fix comments so that they refer to slots_lock instead of slots_locks
> (remove trailing s).
> 
> Signed-off-by: Fuad Tabba <tabba@google.com>

Reviewed-by: Ira Weiny <ira.weiny@intel.com>

[snip]

On Thu, 1 May 2025 at 22:43, Ira Weiny <ira.weiny@intel.com> wrote:
>
> Fuad Tabba wrote:
> > Fix comments so that they refer to slots_lock instead of slots_locks
> > (remove trailing s).
> >
> > Signed-off-by: Fuad Tabba <tabba@google.com>
>
> Reviewed-by: Ira Weiny <ira.weiny@intel.com>

Thank you for the reviews!
/fuad

> [snip]