A bitmap indicating which sptes in spt point (directly or indirectly) at
pages that may be unsynchronized. Used to quickly locate all unsychronized
pages reachable from a given page.
+ clear_spte_count:
+ Only present on 32-bit hosts, where a 64-bit spte cannot be written
+ atomically. The reader uses this while running out of the MMU lock
+ to detect in-progress updates and retry them until the writer has
+ finished the write.
Reverse map
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