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The new ArmMmuLib code is easier to reason about, so that is what I did:
currently, when we create mappings that cover existing table entries, we
may end up overwriting those with block entries without taking the mapping
attributes of the original table entries into account. So let's fix this.

I honestly don't know whether the original code was better at dealing with
this: I do remember some changes from Heyi that may have been related, but
the old code is not easy to follow. In any case, I didn't manage to hit this
case in practice, given that we typically start out with large mappings, and
break them down later (to set permissions), rather than the other way around.

Patch #1 adds some helpers to hide the insane way the type bits change
meaning when you change to level 3.

Patch #2 ensures that we only replace (and free) table entries with block
entries if it is guaranteed that doing so will not lose any attribute
information.

Changes since v2:
- add patch to limit recursion to levels < 3 in FreePageTablesRecursive()

Changes since v1:
- zero newly allocated pages before splitting a block entry into a table
  entry, to avoid garbage in that page being misidentified as entry type
  attributes - this should fix the crash observed by Laszlo

Cc: Laszlo Ersek <lersek@redhat.com>
Cc: Leif Lindholm <leif@nuviainc.com>
Cc: Ashish Singhal <ashishsingha@nvidia.com>

Ard Biesheuvel (3):
  ArmPkg/ArmMmuLib AARCH64: limit recursion when freeing page tables
  ArmPkg/ArmMmuLib AARCH64: use helpers to determine table entry types
  ArmPkg/ArmMmuLib AARCH64: preserve attributes when replacing a table
    entry

 .../Library/ArmMmuLib/AArch64/ArmMmuLibCore.c | 83 +++++++++++++++----
 1 file changed, 68 insertions(+), 15 deletions(-)

--
2.17.1