- * Each PTE in a process VM's page table is either:
- * "present" -- valid and not swapped out, protection bits are meaningful;
- * "not present" -- which further subdivides in these two cases:
- * "none" -- no mapping at all; identified by pte_none(), set by pte_clear(
- * "swapped out" -- the page is swapped out, and the SWP macros below
- * are used to store swap file info in the PTE itself.
- *
- * In the Xtensa processor MMU, any PTE entries in user space (or anywhere
- * in virtual memory that can map differently across address spaces)
- * must have a correct ring value that represents the RASID field that
- * is changed when switching address spaces. Eg. such PTE entries cannot
- * be set to ring zero, because that can cause a (global) kernel ASID
- * entry to be created in the TLBs (even with invalid cache attribute),
- * potentially causing a multihit exception when going back to another
- * address space that mapped the same virtual address at another ring.
- *
- * SO: we avoid using ring bits (_PAGE_RING_MASK) in "not present" PTEs.
- * We also avoid using the _PAGE_VALID bit which must be zero for non-present
- * pages.