[PATCH v4 00/11] liveupdate: kvm: Guest_memfd preservation

[Tarun Sahu <tarunsahu@google.com>]

Hello,

This is v4 of guest_memfd preservation during liveupdate. As per
the discussion from bi-weekly guest-memfd meeting, This series
will be go to linux via kvm tree. And Planning to get it merged
in upcoming merge window. (7.2-rc1 for release in 7.3)

The changes are rebased on:
	kvm/next + liveupdate/next (merge) + [4] + [5]
	Where,
	[4]: luo: APIs to retrieve file internally from session
	[5]: selftests: liveupdate sefltests library
Here is the github repo:
	https://github.com/tar-unix/linux/tree/gmem-pre

Patches [4] and [5] are prequisite of this series and they are
rebased/merged on liveupdate/next. kvm/next is behind few patches
from liveupdate/next, hence direct cherry-pick of [4] and [5] is
not possible. On Linux 7.2 they will be aligned. If we
succeed in merging it before I can request liveupdate maintainer
to provide the tag.

Steps to test:
1. Compile Kernel with CONFIG_LIVEUPDATE_GUEST_MEMFD=y
2. boot kernel with command line: kho=on liveupdate=on
3. run the following kselftest
	$ .selftests/kvm/guest_memfd_preservation_test --s 1
	$ <kexec> --reuse-cmdline
	$ .selftests/kvm/guest_memfd_preservation_test --s 2

NOTE: Assert the following:
	$ ls /dev/liveupdate
	$ ls /dev/kvm
	$ dmesg | grep liveupdate # (should have kvm_vm_luo &&
		# guest_memfd_luo handler registered)

V4 <- V3
1. Split the refactoring patch into multiple patches [2, 3, 4]
2. Updated commits msg as per the kvm documentation and suggestions
3. Removed KHOSER_PTR and using raw pointer, as series [3] not yet
   decided to go in the next cycle
4. Implemented more tests in guest_memfd_preservation_test
   allocation during frozen gmem_inode, non-guest_memfd preservation
5. Moved the vm_create_from_fd to kvm_util.c from the
   guest_memfd_preservation_test.c
5. Minor updates as per the suggestions

V3 <- RFC V2 [2]
1. Finalize the design
2. resolve sashiko reported bugs
3. Use of KHOSER_PTR instead of raw serialized_data as per [3]

RFC V2 [2] <- RFC V1 [1]
1. Removed mem_attr_array as it is not needed for fully-shared
2. Removed pre-faulted condition
3. Added vm_type preservation for ARM64.
4. Removed liveupdate_get_file_incoming api patch as it is sent
   separately [4] by Samiullah.


**GUEST MEMFD PRESERVATION**
Below is the cover letter of the series which explains the design.

SCOPE:
1. Fully Shared Guest_memfd
2. Guest_memfd backed by PAGE_SIZE pages

Any VM whose memory is backed by such guest_memfd and satisfy the
above conditions can be preserved across liveupdate.

To Add support for liveupdate of any subsystem, we need to implement
the following calls: (e.g: subsystem: guest_memfd)

guest_memfd_luo_can_preserve: used by luo subsystem to filter the
			      guest_memfd files
guest_memfd_luo_preserve: triggered from userspace by PRESERVE ioctl
			  If can_preserve says the file is preservable
			  by guest_memfd_luo_preserve
guest_memfd_luo_retrieve: triggered from userspace by RETRIEVE ioctl,
 			  recognised by the passed token from userspace.
			  Used in new kernel after kexec.
guest_memfd_luo_unpreserve: triggered from userspace when a session is
			    closed, which inherently calls unpreserve of
			    all preserved files.
guest_memfd_luo_freeze: triggered just before kexec automatically in
			kernel. no userspace involvement.
guest_memfd_luo_finish: triggered by userspace when all files are
			retrieved and want to finish the session
			retrieval.

*PART A: KVM VM preservation*
Introdcued the kvm_luo.c to preserve the vm_file during liveupdate.
Currently it only preserve vm_type. This can be expanded to preserve
more details in future as need arises for example data related to
cocoVM (SEPT etc.).

	int kvm_fd = open("/dev/kvm", O_RDWR);
	int vm_fd = ioctl(kvm_fd, KVM_CREATE_VM, 0);
	preserve_arg = {
    	.size = sizeof(preserve_arg),
	    .fd = vm_fd,
	    .token = VM_TOKEN,
	};
	ioctl(session_fd, LIVEUPDATE_SESSION_PRESERVE_FD, &preserve_arg);

This VM_TOKEN is used by guest_memfd preservation. guest_memfd
preservation save this token (fetched internally by luo apis [4]) to
associates the guest_memfd with its vm_file. On retrieval, guest_memfd
retrieves the vm_file using this token and use its struct kvm to
retrieve itself.

PART B: Guest_memfd preservation
Introduce guest_memfd_luo.c to preserve the guest_memfd file during
liveupdate. Currently it only preserve guest_memfd files that have:

1. INIT_SHARED flag ensures intially all the memory is shared
2. !kvm_arch_has_private_mem() which ensures no shared pages can be
   converted to private after [6] lands.

Preservation first freezes the gmem_inode introduced in PATCH [07/11]
and preserve each allocated folios from guest_memfd page_cache using
KHO.

luo_freeze (not the gmem_inode freeze PATCH[07/11]) call update the
vm_token which is preserved across kexec.

Retrieval retrieves the vm_file using vm_token and create the
guest_memfd with vm_file->kvm and populate the preserved folios
to guest_memfd page_cache.

Added the Documentation for VMM on guest_memfd preservation guidelines
To know more about, PATCH[10/11] can be referred which has the more
detailed documentation.

Future Plan:
	1. Make preservation immune to new allocation and fallocate
	calls so that we can remove freeze call
	2. In future, When IOMMU will use guest_memfd and can allocate
	page tables for DMA in that area, Later it will be a problem if
	other user of guest_memfd do punch hole before preservation,
	that will make the pages to be lost after preservation. This
	will be taken care of when IOMMU will add support for
	guest_memfd.
	3. guest_memfd preservation for private pages + in-place
	conversion.
	4. In future, when guest_memfd will have hugetlb page support,
	add support its preservation.

[1]: https://lore.kernel.org/all/cover.1779080766.git.tarunsahu@google.com/
[2]: https://lore.kernel.org/all/cover.1780667929.git.tarunsahu@google.com/
     https://lore.kernel.org/all/c054ba0fb2639932bbe354420d3f4f84cce84905.1780676742.git.tarunsahu@google.com/
[3]: https://lore.kernel.org/all/20260622184851.2309827-1-tarunsahu@google.com/
[4]: https://git.kernel.org/pub/scm/linux/kernel/git/liveupdate/linux.git/commit/?h=next&id=f4d2e4f0d12d88155cd1128ed3294f7827bf4c5d
[5]: https://git.kernel.org/pub/scm/linux/kernel/git/liveupdate/linux.git/commit/?h=next&id=2f3f53a3735a7e67bbe9e580cb49372bf9261967
     https://git.kernel.org/pub/scm/linux/kernel/git/liveupdate/linux.git/commit/?h=next&id=df1a0d068d481b7d175c5af3970043206409a062
[6]: https://lore.kernel.org/all/20260618-gmem-inplace-conversion-v8-0-9d2959357853@google.com/

Tarun Sahu (11):
  liveupdate: Add LIVEUPDATE_GUEST_MEMFD config option
  KVM: Introduce kvm_create_vm_file() helper
  KVM: Export kvm_uevent_notify_vm_create()
  KVM: Track weak reference to vm_file in struct kvm
  KVM: LUO: Support VM preservation across live updates
  KVM: guest_memfd: Move internal definitions to internal header
  KVM: guest_memfd: Add support for freezing mappings
  KVM: guest_memfd: Add support for preservation via LUO
  docs: liveupdate: Add documentation for VM and guest_memfd
    preservation
  KVM: selftests: Split ____vm_create() and add vm_create_from_fd()
  KVM: selftests: Add guest_memfd_preservation_test

 Documentation/core-api/liveupdate.rst         |   1 +
 Documentation/liveupdate/vmm.rst              | 107 ++++
 MAINTAINERS                                   |  14 +
 include/linux/kho/abi/kvm.h                   | 106 ++++
 include/linux/kvm_host.h                      |  14 +
 kernel/liveupdate/Kconfig                     |  15 +
 tools/testing/selftests/kvm/Makefile.kvm      |   6 +-
 .../kvm/guest_memfd_preservation_test.c       | 357 ++++++++++++
 .../testing/selftests/kvm/include/kvm_util.h  |   3 +
 tools/testing/selftests/kvm/lib/kvm_util.c    |  49 +-
 virt/kvm/Makefile.kvm                         |   1 +
 virt/kvm/guest_memfd.c                        | 185 +++++--
 virt/kvm/guest_memfd.h                        |  44 ++
 virt/kvm/guest_memfd_luo.c                    | 515 ++++++++++++++++++
 virt/kvm/kvm_luo.c                            | 195 +++++++
 virt/kvm/kvm_main.c                           |  94 +++-
 virt/kvm/kvm_mm.h                             |  15 +
 17 files changed, 1640 insertions(+), 81 deletions(-)
 create mode 100644 Documentation/liveupdate/vmm.rst
 create mode 100644 include/linux/kho/abi/kvm.h
 create mode 100644 tools/testing/selftests/kvm/guest_memfd_preservation_test.c
 create mode 100644 virt/kvm/guest_memfd.h
 create mode 100644 virt/kvm/guest_memfd_luo.c
 create mode 100644 virt/kvm/kvm_luo.c


base-commit: a204badd8432f93b7e862e7dac6db0fe3d65f370
-- 
2.55.0.229.g6434b31f56-goog