A safe interface to read/write page table

[jinbpark]

• Currently, we write a page table entry in a typical way, (e.g., self.0 = RawPTE::new(0);), but this is not a safe way to modify page table entries, as it requires a strict order of instructions.

  • e.g., secure way: str something to pte; barrier_instructions; tlb_instructions; it should be read/written atomically and proper barrier or tlb instructions must be followed right after.

• As of now, this is not a big problem as we don't modify RTT after realm kernel booting. But, once it gets implemented, the current way to access page_table might cause subtle bugs.