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Subject[PATCH v5 00/13] KVM: mm: fd-based approach for supporting KVM guest private memory
This is the v5 of this series which tries to implement the fd-based KVM
guest private memory. The patches are based on latest kvm/queue branch

d5089416b7fb KVM: x86: Introduce KVM_CAP_DISABLE_QUIRKS2

In general this patch series introduce fd-based memslot which provides
guest memory through memory file descriptor fd[offset,size] instead of
hva/size. The fd can be created from a supported memory filesystem
like tmpfs/hugetlbfs etc. which we refer as memory backing store. KVM
and the the memory backing store exchange callbacks when such memslot
gets created. At runtime KVM will call into callbacks provided by the
backing store to get the pfn with the fd+offset. Memory backing store
will also call into KVM callbacks when userspace fallocate/punch hole
on the fd to notify KVM to map/unmap secondary MMU page tables.

Comparing to existing hva-based memslot, this new type of memslot allows
guest memory unmapped from host userspace like QEMU and even the kernel
itself, therefore reduce attack surface and prevent bugs.

Based on this fd-based memslot, we can build guest private memory that
is going to be used in confidential computing environments such as Intel
TDX and AMD SEV. When supported, the memory backing store can provide
more enforcement on the fd and KVM can use a single memslot to hold both
the private and shared part of the guest memory.

mm extension
Introduces new MFD_INACCESSIBLE flag for memfd_create(), the file created
with these flags cannot read(), write() or mmap() etc via normal
MMU operations. The file content can only be used with the newly
introduced memfile_notifier extension.

The memfile_notifier extension provides two sets of callbacks for KVM to
interact with the memory backing store:
- memfile_notifier_ops: callbacks for memory backing store to notify
KVM when memory gets allocated/invalidated.
- memfile_pfn_ops: callbacks for KVM to call into memory backing store
to request memory pages for guest private memory.

The memfile_notifier extension also provides APIs for memory backing
store to register/unregister itself and to trigger the notifier when the
bookmarked memory gets fallocated/invalidated.

memslot extension
Add the private fd and the fd offset to existing 'shared' memslot so that
both private/shared guest memory can live in one single memslot. A page in
the memslot is either private or shared. A page is private only when it's
already allocated in the backing store fd, all the other cases it's treated
as shared, this includes those already mapped as shared as well as those
having not been mapped. This means the memory backing store is the place
which tells the truth of which page is private.

Private memory map/unmap and conversion
Userspace's map/unmap operations are done by fallocate() ioctl on the
backing store fd.
- map: default fallocate() with mode=0.
- unmap: fallocate() with FALLOC_FL_PUNCH_HOLE.
The map/unmap will trigger above memfile_notifier_ops to let KVM map/unmap
secondary MMU page tables.

To test the new functionalities of this patch TDX patchset is needed.
Since TDX patchset has not been merged so I did two kinds of test:

- Regresion test on kvm/queue (this patch)
Most new code are not covered. I only tested building and booting.

- New Funational test on latest TDX code
The patch is rebased to latest TDX code and tested the new

For TDX test please see below repos:

And an example QEMU command line:
-object tdx-guest,id=tdx \
-object memory-backend-memfd-private,id=ram1,size=2G \
-machine q35,kvm-type=tdx,pic=no,kernel_irqchip=split,memory-encryption=tdx,memory-backend=ram1

- Removed userspace visible F_SEAL_INACCESSIBLE, instead using an
in-kernel flag (SHM_F_INACCESSIBLE for shmem). Private fd can only
be created by MFD_INACCESSIBLE.
- Introduced new APIs for backing store to register itself to
memfile_notifier instead of direct function call.
- Added the accounting and restriction for MFD_INACCESSIBLE memory.
- Added KVM API doc for new memslot extensions and man page for the new
- Removed the overlap check for mapping the same file+offset into
multiple gfns due to perf consideration, warned in document.
- Addressed other comments in v4.
- Decoupled the callbacks between KVM/mm from memfd and use new
name 'memfile_notifier'.
- Supported register multiple memslots to the same backing store.
- Added per-memslot pfn_ops instead of per-system.
- Reworked the invalidation part.
- Improved new KVM uAPIs (private memslot extension and memory
error) per Sean's suggestions.
- Addressed many other minor fixes for comments from v3.
- Added locking protection when calling
invalidate_page_range/fallocate callbacks.
- Changed memslot structure to keep use useraddr for shared memory.
- Commit message improvement.
- Many small fixes for comments from the last version.

Links to previous discussions
[1] Original design proposal:
[2] Updated proposal and RFC patch v1:
[3] Patch v4:

Chao Peng (10):
mm: Introduce memfile_notifier
mm/shmem: Restrict MFD_INACCESSIBLE memory against RLIMIT_MEMLOCK
KVM: Extend the memslot to support fd-based private memory
KVM: Use kvm_userspace_memory_region_ext
KVM: Use memfile_pfn_ops to obtain pfn for private pages
KVM: Handle page fault for private memory
KVM: Register private memslot to memory backing store
KVM: Zap existing KVM mappings when pages changed in the private fd

Kirill A. Shutemov (2):
mm/memfd: Introduce MFD_INACCESSIBLE flag
mm/shmem: Support memfile_notifier

Documentation/virt/kvm/api.rst | 59 +++++++++--
arch/x86/kvm/Kconfig | 1 +
arch/x86/kvm/mmu/mmu.c | 73 +++++++++++++-
arch/x86/kvm/mmu/paging_tmpl.h | 11 ++-
arch/x86/kvm/x86.c | 12 +--
include/linux/kvm_host.h | 49 ++++++++-
include/linux/memfile_notifier.h | 64 ++++++++++++
include/linux/shmem_fs.h | 11 +++
include/uapi/linux/kvm.h | 17 ++++
include/uapi/linux/memfd.h | 1 +
mm/Kconfig | 4 +
mm/Makefile | 1 +
mm/memfd.c | 26 ++++-
mm/memfile_notifier.c | 114 +++++++++++++++++++++
mm/shmem.c | 156 +++++++++++++++++++++++++++++
virt/kvm/kvm_main.c | 165 +++++++++++++++++++++++++++----
16 files changed, 717 insertions(+), 47 deletions(-)
create mode 100644 include/linux/memfile_notifier.h
create mode 100644 mm/memfile_notifier.c


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