kernel has gdt now and loads segment register and we can allocate pages

1 files changed, 96 insertions, 0 deletions

diff --git a/kernel/memory.c b/kernel/memory.c
new file mode 100644
index 0000000..b6fa5b2
--- /dev/null
+++ b/kernel/memory.c
@@ -0,0 +1,96 @@
+#include "memory.h"
+
+#include <kernel/memory/paging.h>
+
+/*
+ * kernel virtual space guarantees
+ *
+ * 1. there are two kernel page directories, one each for the lower and upper
+ *    GiB of space. they are self-mapped as tables in the trailing entries
+ *    of the table for the upper space. these must exist.
+ * 2. because of (1.) the kernel page tables (directoires inclusive) occupy
+ *    the upper 4MiB of kernel address space.
+ * 3. the kernel is always at the beginning of its virtual space.
+ *    currently this is 0xffffffff80000000 but that may change
+ * 4. the entry stack's head is always -4MiB from the end of the address space.
+ *    its size is currently 64KiB. this may change.
+ */
+
+#define kpde_index(v) ((v >> (PAGE_MAP_BITS + PAGE_OFFSET_BITS)) & 0x3ff)
+#define kpte_index(v) ((v >> PAGE_OFFSET_BITS) & 0x7ffff)
+
+extern char k_virt_base;
+extern char k_text_begin;
+extern char k_text_end;
+extern char k_data_begin;
+extern char k_data_end;
+
+static uint64_t *const kernel_ptes = (uint64_t *)0xffffffffffc00000;
+static uint64_t *const kernel_pdes = (uint64_t *)0xffffffffffffe000;
+
+// we can track 4GiB of physical pages with this bitmap.
+#define PAGE_BITMAP_ENTRIES 16384
+#define PAGE_BITMAP_WIDTH 64
+static uint64_t page_bitmap[PAGE_BITMAP_ENTRIES];
+
+static phys_addr_t bitmap_alloc(void)
+{
+    static uint64_t *page_bitmap_last = NULL;
+
+    if (!page_bitmap_last || !*page_bitmap_last)
+    {
+        for (int i = 0; i < PAGE_BITMAP_ENTRIES; i++)
+        {
+            if (page_bitmap[i])
+            {
+                page_bitmap_last = &page_bitmap[i];
+                break;
+            }
+        }
+    }
+
+    int index = page_bitmap_last - page_bitmap;
+    int bit = __builtin_ffsll(*page_bitmap_last);
+
+    return (index * PAGE_BITMAP_WIDTH + bit) * PAGE_SIZE;
+}
+
+static void bitmap_free(phys_addr_t page)
+{
+    int index = (page >> PAGE_OFFSET_BITS) / PAGE_BITMAP_WIDTH;
+    int bit = (page >> PAGE_OFFSET_BITS) % PAGE_BITMAP_WIDTH;
+    page_bitmap[index] |= (1 << bit);
+}
+/*
+static virt_addr_t virtual_alloc(void)
+{
+}
+
+static void virtual_free(virt_addr_t vpage)
+{
+}
+*/
+static uint64_t *get_kernel_pde(virt_addr_t vaddr)
+{
+    return &kernel_pdes[kpde_index(vaddr)];
+}
+
+static uint64_t *get_kernel_pte(uint64_t vaddr)
+{
+    return &kernel_ptes[kpte_index(vaddr)];
+}
+
+void memory_init()
+{
+}
+
+phys_addr_t page_alloc(void)
+{
+    return bitmap_alloc();
+}
+
+void page_free(phys_addr_t page)
+{
+    bitmap_free(page);
+}
+