- * 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.
- *
- * We end up with the following available bits: 1..3 and 7..31.
- * We don't bother with 1..3 for now (we can use them later if needed),
- * and chose to allocate 6 bits for SWP_TYPE and the remaining 19 bits
- * for SWP_OFFSET. At least 5 bits are needed for SWP_TYPE, because it
- * is currently implemented as an index into swap_info[MAX_SWAPFILES]
- * and MAX_SWAPFILES is currently defined as 32 in <linux/swap.h>.
- * However, for some reason all other architectures in the 2.4 kernel
- * reserve either 6, 7, or 8 bits so I'll not detract from that for now. :)
- * SWP_OFFSET is an offset into the swap file in page-size units, so
- * with 4 kB pages, 19 bits supports a maximum swap file size of 2 GB.
- *
- * FIXME: 2 GB isn't very big. Other bits can be used to allow
- * larger swap sizes. In the meantime, it appears relatively easy to get
- * around the 2 GB limitation by simply using multiple swap files.
+ * Format of swap pte:
+ * bit 0 MBZ
+ * bit 1 page-file (must be zero)
+ * bits 2 - 3 page hw access mode (must be 11: _PAGE_INVALID)
+ * bits 4 - 5 ring protection (must be 01: _PAGE_USER)
+ * bits 6 - 10 swap type (5 bits -> 32 types)
+ * bits 11 - 31 swap offset / PAGE_SIZE (21 bits -> 8GB)
+
+ * Format of file pte:
+ * bit 0 MBZ
+ * bit 1 page-file (must be one: _PAGE_FILE)
+ * bits 2 - 3 page hw access mode (must be 11: _PAGE_INVALID)
+ * bits 4 - 5 ring protection (must be 01: _PAGE_USER)
+ * bits 6 - 31 file offset / PAGE_SIZE