Add a basic kernel heap object.

Currently allocation always fails because we don't
have a way to allocate a physical page.

zion/memory/paging_util.cpp

@@ -1,9 +1,45 @@
 #include "memory/paging_util.h"
 
 #include "boot/boot_info.h"
+#include "debug/debug.h"
 
 #define PRESENT_BIT 0x1
 #define READ_WRITE_BIT 0x2
+#define SIGN_EXT 0xFFFF0000'00000000
+#define RECURSIVE ((uint64_t)0x1FE)
+#define PML_OFFSET 39
+#define PDP_OFFSET 30
+#define PD_OFFSET 21
+#define PT_OFFSET 12
+
+// How to recursively index into each page table structure assuming
+// the PML4 is recursively mapped at the 510th entry (0x1FE).
+#define PML_RECURSE 0xFFFFFF7F'BFDFE000
+#define PDP_RECURSE 0xFFFFFF7F'BFC00000
+#define PD_RECURSE 0xFFFFFF7F'80000000
+#define PT_RECURSE 0xFFFFFF00'00000000
+
+namespace {
+
+uint64_t PageAlign(uint64_t addr) { return addr & ~0xFFF; }
+uint64_t* PageAlign(uint64_t* addr) {
+  return reinterpret_cast<uint64_t*>(reinterpret_cast<uint64_t>(addr) & ~0xFFF);
+}
+
+void ZeroOutPage(uint64_t* ptr) {
+  ptr = PageAlign(ptr);
+  for (uint64_t i = 0; i < 512; i++) {
+    ptr[i] = 0;
+  }
+}
+
+uint64_t ShiftForEntryIndexing(uint64_t addr, uint64_t offset) {
+  addr &= ~0xFFFF0000'00000000;
+  addr >>= offset;
+  addr <<= 3;
+  return addr;
+}
+}  // namespace
 
 void InitPaging() {
   uint64_t pml4_addr = 0;
@@ -18,3 +54,56 @@ void InitializePml4(uint64_t pml4_physical_addr) {
   uint64_t recursive_entry = pml4_physical_addr | PRESENT_BIT | READ_WRITE_BIT;
   pml4_virtual[0x1FE] = recursive_entry;
 }
+
+void AllocatePageDirectoryPointer(uint64_t addr);
+void AllocatePageDirectory(uint64_t addr);
+void AllocatePageTable(uint64_t addr);
+void AllocatePage(uint64_t addr) { panic("Page Allocation Not Implemented."); }
+
+void EnsureResident(uint64_t addr, uint64_t size) {
+  uint64_t max = addr + size;
+  addr = PageAlign(addr);
+  while (addr < max) {
+    if (!PageLoaded(addr)) {
+      AllocatePage(addr);
+    }
+    addr += 0x1000;
+  }
+}
+
+uint64_t* Pml4Entry(uint64_t addr) {
+  return reinterpret_cast<uint64_t*>(PML_RECURSE |
+                                     ShiftForEntryIndexing(addr, PML_OFFSET));
+}
+
+uint64_t* PageDirectoryPointerEntry(uint64_t addr) {
+  return reinterpret_cast<uint64_t*>(PDP_RECURSE |
+                                     ShiftForEntryIndexing(addr, PDP_OFFSET));
+}
+
+uint64_t* PageDirectoryEntry(uint64_t addr) {
+  return reinterpret_cast<uint64_t*>(PD_RECURSE |
+                                     ShiftForEntryIndexing(addr, PD_OFFSET));
+}
+
+uint64_t* PageTableEntry(uint64_t addr) {
+  return reinterpret_cast<uint64_t*>(PT_RECURSE |
+                                     ShiftForEntryIndexing(addr, PT_OFFSET));
+}
+
+bool PageDirectoryPointerLoaded(uint64_t addr) {
+  return *Pml4Entry(addr) & PRESENT_BIT;
+}
+
+bool PageDirectoryLoaded(uint64_t addr) {
+  return PageDirectoryPointerLoaded(addr) &&
+         (*PageDirectoryPointerEntry(addr) & PRESENT_BIT);
+}
+
+bool PageTableLoaded(uint64_t addr) {
+  return PageDirectoryLoaded(addr) && (*PageDirectoryEntry(addr) & PRESENT_BIT);
+}
+
+bool PageLoaded(uint64_t addr) {
+  return PageTableLoaded(addr) && (*PageTableEntry(addr) & PRESENT_BIT);
+}