+ /*
+ * If the SPTE is not MMU-present, there is no backing
+ * page associated with the SPTE and so no side effects
+ * that need to be recorded, and exclusive ownership of
+ * mmu_lock ensures the SPTE can't be made present.
+ * Note, zapping MMIO SPTEs is also unnecessary as they
+ * are guarded by the memslots generation, not by being
+ * unreachable.
+ */