Kernel/aarch64: Add function to MMU.cpp to unmap identity mapping

This function will be used once the kernel runs in high virtual memory
to unmap the identity mapping as userspace will later on use this memory
range instead.

Kernel/Arch/aarch64/CPU.h

@@ -13,6 +13,7 @@ namespace Kernel {
 
 void initialize_exceptions();
 void init_page_tables();
+void unmap_identity_map();
 void panic_without_mmu(StringView);
 void dbgln_without_mmu(StringView);
 

Kernel/Arch/aarch64/MMU.cpp

@@ -244,4 +244,25 @@ void init_page_tables()
     activate_mmu();
 }
 
+void unmap_identity_map()
+{
+    auto start_of_physical_memory = FlatPtr(START_OF_NORMAL_MEMORY);
+
+    u64 level0_idx = (start_of_physical_memory >> 39) & 0x1FF;
+    u64 level1_idx = (start_of_physical_memory >> 30) & 0x1FF;
+
+    u64* level1_table = (u64*)page_tables_phys_start;
+
+    auto level2_table = FlatPtr(descriptor_to_pointer(level1_table[level0_idx]));
+    if (!level2_table)
+        panic_without_mmu("Could not find table!"sv);
+
+    // NOTE: The function descriptor_to_pointer returns a physical address, but we want to unmap that range
+    //       so, the pointer must be converted to a virtual address by adding KERNEL_MAPPING_BASE.
+    level2_table += KERNEL_MAPPING_BASE;
+
+    // Unmap the complete identity map
+    ((u64*)level2_table)[level1_idx] = 0;
+}
+
 }