virtual page allocation

src/arch/x86_64/mem/vmm.c

@@ -13,8 +13,74 @@
 #define PTE_USER (1ULL << 2)
 #define PTE_PS (1ULL << 7)
 
+#define BITMAP_PAGE_COUNT (ADDRES_SPACE_SIZE / PAGE_SIZE)
+#define BITMAP_SIZE (BITMAP_PAGE_COUNT / 8)
+
+static u8 page_bitmap[BITMAP_SIZE];
+static u64 total_pages = 0;
+static u64 free_pages = 0;
+
 extern u64 pml4[];
 
+void vmm_init()
+{
+    // Mark first 32 pages (64mb) as unusable since it is reserved by the bootloader
+    // todo(nub31): This should be revisited. Maybe do a higher half kernel?
+    for (u64 page = 0; page < 32; page++)
+    {
+        if (page < BITMAP_PAGE_COUNT)
+        {
+            if (!(page_bitmap[page / 8] & (1 << (page % 8))))
+            {
+                page_bitmap[page / 8] |= (1 << (page % 8));
+            }
+        }
+        else
+        {
+            panic("Bitmap is not large enough to hold the bootloader reserved memory");
+        }
+    }
+}
+
+u64 vmm_alloc_address(size_t page_count)
+{
+    for (size_t i = 0; i < BITMAP_SIZE; i++)
+    {
+        if (page_bitmap[i] != 0xFF)
+        {
+            for (int bit = 0; bit < 8; bit++)
+            {
+                if (!(page_bitmap[i] & (1 << bit)))
+                {
+                    page_bitmap[i] |= (1 << bit);
+                    free_pages--;
+                    return ((i * 8 + bit) * PAGE_SIZE);
+                }
+            }
+        }
+    }
+
+    return 0;
+}
+
+u64 vmm_free_address(u64 virtual_address, size_t page_count)
+{
+    u64 page = virtual_address / PAGE_SIZE;
+    if (page < BITMAP_SIZE * 8)
+    {
+        if (page_bitmap[page / 8] & (1 << (page % 8)))
+        {
+            page_bitmap[page / 8] &= ~(1 << (page % 8));
+            free_pages++;
+        }
+        else
+        {
+            printf("Virtual address %x is already free", virtual_address);
+            panic("Failed to free virtual address");
+        }
+    }
+}
+
 static u64 create_2mb_pte(u64 physical_address, u32 flags)
 {
     if (physical_address & (PAGE_SIZE - 1))
@@ -97,16 +163,6 @@ u64 vmm_unmap(u64 virtual_address)
     return physical_address;
 }
 
-u64 vmm_alloc_address(size_t page_count)
-{
-    panic("not implemented");
-}
-
-u64 vmm_free_address(u64 virtual_address, size_t page_count)
-{
-    panic("not implemented");
-}
-
 void* vmm_alloc(size_t page_count)
 {
     u64 virtual_address = vmm_alloc_address(page_count);