From f18e73a522578e5672e7e5bcfa96d85fd4f6ce28 Mon Sep 17 00:00:00 2001
From: yangminz <yangminz@outlook.com>
Date: Sun, 11 Jul 2021 12:13:36 +0800
Subject: [PATCH] reversed mapping for physical page

---
 src/hardware/cpu/mmu.c | 46 ++++++++++++++++++++++++++++++++++++++++++
 src/headers/memory.h   | 19 +++++++++++++++--
 2 files changed, 63 insertions(+), 2 deletions(-)

diff --git a/src/hardware/cpu/mmu.c b/src/hardware/cpu/mmu.c
index d701903..550ee96 100644
--- a/src/hardware/cpu/mmu.c
+++ b/src/hardware/cpu/mmu.c
@@ -18,6 +18,7 @@
 #include "headers/address.h"
 
 static uint64_t page_walk(uint64_t vaddr_value);
+static void page_fault_handler(pte4_t *pte, address_t vaddr);
 
 // consider this function va2pa as functional
 uint64_t va2pa(uint64_t vaddr)
@@ -73,6 +74,12 @@ static uint64_t page_walk(uint64_t vaddr_value)
 #ifdef DBUEG_PAGE_WALK
                     printf("page walk level 4: pt[%lx].present == 0\n\tmalloc new page table for it\n", vaddr.vpn1);
 #endif
+                    // search paddr from main memory and disk
+                    // TODO: raise exception 14 (page fault) here
+                    // switch privilege from user mode (ring 3) to kernel mode (ring 0)
+                    page_fault_handler(&pt[vaddr.vpn4], vaddr);
+
+                    /*
                     pte4_t *pt = malloc(page_table_size);
                     memset(pt, 0, page_table_size);
 
@@ -83,6 +90,7 @@ static uint64_t page_walk(uint64_t vaddr_value)
                     // TODO: page fault here
                     // map the physical page and the virtual page
                     exit(0);
+                    */
                 }
             }
             else
@@ -138,4 +146,42 @@ static uint64_t page_walk(uint64_t vaddr_value)
         // map the physical page and the virtual page
         exit(0);
     }
+}
+
+static void page_fault_handler(pte4_t *pte, address_t vaddr)
+{
+    // select one victim physical page to swap to disk
+
+    // this is the selected ppn for vaddr
+    int ppn = -1;
+
+    // 1. try to request one free physical page from DRAM
+    // kernel's responsibility
+    for (int i = 0; i < MAX_NUM_PHYSICAL_PAGE; ++ i)
+    {
+        if (page_map[i].pte4->present == 0)
+        {
+            printf("PageFault: use free ppn %d\n", i);
+
+            // found i as free ppn
+            ppn = i;
+
+            page_map[ppn].allocated = 1;    // allocate for vaddr
+            page_map[ppn].dirty = 0;    // allocated as clean
+            page_map[ppn].time = 0;    // most recently used physical page
+            page_map[ppn].pte4 = pte;
+
+            pte->present = 1;
+            pte->ppn = ppn;
+            pte->dirty = 0;
+
+            return;
+        }
+    }
+
+    // 2. no free physical page: select one clean page (LRU) and overwrite
+    // in this case, there is no DRAM - DISK transaction
+
+    // 3. no free nor clean physical page: select one LRU victim
+    // write back (swap out) the DIRTY victim to disk
 }
\ No newline at end of file
diff --git a/src/headers/memory.h b/src/headers/memory.h
index 45ff6f0..c55031c 100644
--- a/src/headers/memory.h
+++ b/src/headers/memory.h
@@ -25,6 +25,7 @@
 // total 16 physical memory
 #define PHYSICAL_MEMORY_SPACE   (65536)
 #define MAX_INDEX_PHYSICAL_PAGE (15)
+#define MAX_NUM_PHYSICAL_PAGE (16)    // 1 + MAX_INDEX_PHYSICAL_PAGE
 
 #define PAGE_TABLE_ENTRY_NUM    (512)
 
@@ -77,7 +78,7 @@ typedef union
 
     struct
     {
-        uint64_t present            : 1;
+        uint64_t present            : 1;    // present = 1
         uint64_t readonly           : 1;
         uint64_t usermode           : 1;
         uint64_t writethough        : 1;
@@ -94,11 +95,25 @@ typedef union
 
     struct
     {
-        uint64_t _present           : 1;
+        uint64_t _present           : 1;    // present = 0
         uint64_t swap_id            : 63;   // disk address
     };
 } pte4_t;   // PT
 
+// physical page descriptor
+typedef struct
+{
+    int allocated;
+    int dirty;
+    int time;   // LRU cache
+
+    pte4_t *pte4;       // the reversed mapping: from PPN to page table entry
+} pd_t;
+
+// for each pagable (swappable) physical page
+// create one reversed mapping
+pd_t page_map[MAX_NUM_PHYSICAL_PAGE]; 
+
 /*======================================*/
 /*      memory R/W                      */
 /*======================================*/