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[v7,0/7] KVM: Restricted mapping of guest_memfd at the host and arm64 support

Message ID 20250328153133.3504118-1-tabba@google.com (mailing list archive)
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Series KVM: Restricted mapping of guest_memfd at the host and arm64 support | expand

Message

Fuad Tabba March 28, 2025, 3:31 p.m. UTC 
This series adds restricted mmap() support to guest_memfd, as well as
support for guest_memfd on arm64. Please see v3 for the context [1].

Main change since v6 [2]:
Protected the shared_offsets array with a rwlock instead of hopping on
the invalidate_lock. The main reason for this is that the final put
callback (kvm_gmem_handle_folio_put()) could be called from an atomic
context, and the invalidate_lock is an rw_semaphore (Vishal).

The other reason is, in hindsight, it didn't make sense to use the
invalidate_lock since they're not quite protecting the same thing.

I did consider using the folio lock to implicitly protect the array, and
even have another series that does that. The result was more
complicated, and not obviously race free. One of the difficulties with
relying on the folio lock is that there are cases (e.g., on
initilization and teardown) when we want to toggle the state of an
offset that doesn't have a folio in the cache. We could special case
these, but the result was more complicated (and to me not obviously
better) than adding a separate lock for the shared_offsets array.

Based on the `KVM: Mapping guest_memfd backed memory at the host for
software protected VMs` series [3], which in turn is based on Linux
6.14-rc7.

The state diagram that uses the new states in this patch series,
and how they would interact with sharing/unsharing in pKVM [4].

Cheers,
/fuad

[1] https://lore.kernel.org/all/20241010085930.1546800-1-tabba@google.com/
[2] https://lore.kernel.org/all/20250318162046.4016367-1-tabba@google.com/
[3] https://lore.kernel.org/all/20250318161823.4005529-1-tabba@google.com/
[4] https://lpc.events/event/18/contributions/1758/attachments/1457/3699/Guestmemfd%20folio%20state%20page_type.pdf

Ackerley Tng (2):
  KVM: guest_memfd: Make guest mem use guest mem inodes instead of
    anonymous inodes
  KVM: guest_memfd: Track folio sharing within a struct kvm_gmem_private

Fuad Tabba (5):
  KVM: guest_memfd: Introduce kvm_gmem_get_pfn_locked(), which retains
    the folio lock
  KVM: guest_memfd: Folio sharing states and functions that manage their
    transition
  KVM: guest_memfd: Restore folio state after final folio_put()
  KVM: guest_memfd: Handle invalidation of shared memory
  KVM: guest_memfd: Add a guest_memfd() flag to initialize it as shared

 Documentation/virt/kvm/api.rst                |   4 +
 include/linux/kvm_host.h                      |  56 +-
 include/uapi/linux/kvm.h                      |   1 +
 include/uapi/linux/magic.h                    |   1 +
 .../testing/selftests/kvm/guest_memfd_test.c  |   7 +-
 virt/kvm/guest_memfd.c                        | 613 +++++++++++++++++-
 virt/kvm/kvm_main.c                           |  62 ++
 7 files changed, 706 insertions(+), 38 deletions(-)


base-commit: 62aff24816ac463bf3f754a15b2e7cdff59976ea