physical.cpp

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#include <common/logger.h>
#include <memory/physical.h>
#include <system/cpu.h>
 
using namespace MaxOS::memory;
using namespace MaxOS::system;
using namespace MaxOS::common;
 
volatile extern PAGE_ALIGNED unsigned char p4_table[];        
volatile extern PAGE_ALIGNED unsigned char p3_table[];        
volatile extern PAGE_ALIGNED unsigned char p3_table_hh[];     
volatile extern PAGE_ALIGNED unsigned char p2_table[];        
volatile extern PAGE_ALIGNED unsigned char p1_table[];        
 
extern uint64_t _kernel_end;            
extern uint64_t _kernel_size;           
extern uint64_t _kernel_physical_end;   
 
PhysicalMemoryManager::PhysicalMemoryManager(Multiboot* multiboot)
: m_kernel_end((uint64_t) &_kernel_physical_end),
m_multiboot(multiboot),
m_pml4_root_address((uint64_t*) p4_table),
m_pml4_root((pte_t*) p4_table) {
 
    Logger::INFO() << "Setting up Physical Memory Manager\n";
    Logger::DEBUG() << "Kernel Memory: kernel_end = 0x" << (uint64_t) &_kernel_end << ", kernel_size = 0x"
                    << (uint64_t) &_kernel_size << ", kernel_physical_end = 0x" << (uint64_t) &_kernel_physical_end
                    << "\n";
    m_kernel_start_page = align_up_to_page((size_t) &_kernel_physical_end + PAGE_SIZE, PAGE_SIZE);
 
    // Set up the current manager
    unmap_lower_kernel();
    m_lock.unlock();
    s_current_manager = this;
    m_nx_allowed = CPU::check_nx();
 
    // Find a region of memory available to be used
    m_mmap_tag = m_multiboot->mmap();
    for(multiboot_mmap_entry* entry = m_mmap_tag->entries; (multiboot_uint8_t*) entry < (multiboot_uint8_t*) m_mmap_tag + m_mmap_tag->size; entry = (multiboot_mmap_entry*) ( (unsigned long) entry + m_mmap_tag->entry_size)) {
 
        // Skip if the region is not free or there is not enough space
        if(entry->type != MULTIBOOT_MEMORY_AVAILABLE)
            continue;
 
        // Store the entry. (note: don't break here as it is desired to find the last usable entry as that is normally biggest)
        m_mmap = entry;
    }
 
    // Store the information about the bitmap
    m_memory_size = (m_mmap->addr + m_mmap->len);
    m_bitmap_size = m_memory_size / PAGE_SIZE + 1;
    m_total_entries = m_bitmap_size / ROW_BITS + 1;
    Logger::DEBUG() << "Memory Info: size = " << (int) (m_memory_size / 1024 / 1024) << "mb, bitmap size = 0x" << (uint64_t) m_bitmap_size << ", total entries = " << (int) m_total_entries << ", page size = 0x"<< (uint64_t) PAGE_SIZE << "\n";
 
    // Map the physical memory into the virtual memory
    Logger::DEBUG() << "Mapping from 0x0 to 0x" << (uint64_t) (m_mmap->addr + m_mmap->len) << " to higher half direct map at offset 0x" << HIGHER_HALF_DIRECT_MAP << "\n";
    for(uint64_t physical_address = 0; physical_address < (m_mmap->addr + m_mmap->len); physical_address += PAGE_SIZE)
        map((physical_address_t*) physical_address, (virtual_address_t*) (HIGHER_HALF_DIRECT_MAP + physical_address),PRESENT | WRITE);
 
    // Kernel Setup
    initialise_bit_map();
    reserve_kernel_regions(multiboot);
 
    // Initialisation Done
    m_initialised = true;
    Logger::DEBUG() << "Memory used at start: " << (int) (memory_used() / 1024 / 1024) << "mb \n";
}
 
PhysicalMemoryManager::~PhysicalMemoryManager() = default;
 
void PhysicalMemoryManager::reserve_kernel_regions(Multiboot* multiboot) {
 
    // Reserve the pages used by the higher half mapping
    reserve((uint64_t) m_mmap->addr, m_setup_frames * PAGE_SIZE, "HHDM");
 
    // Reserve the area for the bitmap
    reserve((uint64_t) from_dm_region((uint64_t) m_bit_map), m_bitmap_size / 8, "Bitmap");
 
    // Calculate how much space the kernel takes up
    uint32_t kernel_entries = (m_kernel_start_page / PAGE_SIZE) + 1;
    if((((uint32_t) (m_kernel_start_page)) % PAGE_SIZE) != 0)
        kernel_entries += 1;
 
    // Reserve the kernel entries
    reserve(0, kernel_entries * PAGE_SIZE, "Kernel");
 
    // Reserve the area for the mmap
    uint64_t mem_end = m_mmap->addr + m_mmap->len;
    for(multiboot_mmap_entry* entry = m_mmap_tag->entries; (multiboot_uint8_t*) entry < (multiboot_uint8_t*) m_mmap_tag + m_mmap_tag->size; entry = (multiboot_mmap_entry*) ( (unsigned long) entry + m_mmap_tag->entry_size)) {
 
        // Dont reserve free regions
        if(entry->type <= MULTIBOOT_MEMORY_AVAILABLE)
            continue;
 
        // Don't reserve the memory being managed by pmm
        if(entry->addr >= mem_end)
            continue;
 
        reserve(entry->addr, entry->len, "MMap");
    }
 
    // Reserve the area for each multiboot module
    for(multiboot_tag* tag = multiboot->start_tag(); tag->type != MULTIBOOT_TAG_TYPE_END; tag = (struct multiboot_tag*) ((multiboot_uint8_t*) tag + ((tag->size + 7) & ~7))) {
 
        if(tag->type != MULTIBOOT_TAG_TYPE_MODULE)
            continue;
 
        // Reserve the module's address
        auto* module = (struct multiboot_tag_module*) tag;
        reserve(module->mod_start, module->mod_end - module->mod_start, "Module");
    }
 
    // Reserve all the tags
    auto end_tag = (multiboot_tag*) to_higher_region(multiboot->end_address);
    auto start_tag = (uint64_t) to_lower_region((uintptr_t) multiboot->start_tag());
    size_t tags_size = multiboot->end_address - start_tag + ((end_tag->size + 7) & ~7);
    reserve(start_tag, tags_size, "Tags");
}
 
size_t PhysicalMemoryManager::size_to_frames(size_t size) {
 
    return align_to_page(size) / PAGE_SIZE;
}
 
size_t PhysicalMemoryManager::align_to_page(size_t size) {
 
    return ((size + PAGE_SIZE - 1) / PAGE_SIZE) * PAGE_SIZE;
}
 
size_t PhysicalMemoryManager::align_up_to_page(size_t size, size_t page_size) {
 
    return (size + page_size - 1) & ~(page_size - 1);
}
 
bool PhysicalMemoryManager::check_aligned(size_t size) {
 
    return (size % PAGE_SIZE) == 0;
}
 
void* PhysicalMemoryManager::allocate_frame() {
 
    // Wait for the lock
    m_lock.lock();
 
    // If not initialised, cant use the bitmap or higher half mapped physical memory so use leftover kernel memory already
    // mapped in loader.s
    if(!m_initialised) {
 
        // Use frames at the start of the mmap free
        void* address = (void*) ((uintptr_t) m_mmap->addr + (m_setup_frames * PAGE_SIZE));
        m_setup_frames++;
 
        m_lock.unlock();
        return address;
    }
 
    // Check if there are enough frames
    ASSERT(m_used_frames < m_bitmap_size, "No more frames available\n");
 
    for(uint32_t row = 0; row < m_total_entries; ++row) {
 
        // If the row is full continue
        if(m_bit_map[row] == 0xFFFFFFFFFFFFFFFF)
            continue;
 
        for(uint32_t column = 0; column < ROW_BITS; ++column) {
 
            // Entry isn't free
            if(m_bit_map[row] & (1ULL << column))
                continue;
 
            // Mark the frame as used
            m_bit_map[row] |= (1ULL << column);
            m_used_frames++;
 
            // Thread safe
            m_lock.unlock();
 
            // Calculate the address
            uint64_t frame_address = (row * ROW_BITS) + column;
            frame_address *= PAGE_SIZE;
 
            // Zero the page
            void* virt = to_dm_region(frame_address);
            memset(virt, 0, PAGE_SIZE);
 
            return (void*)frame_address;
        }
    }
 
    // Error frame not found
    ASSERT(false, "Frame not found\n");
    m_lock.unlock();
    return nullptr;
}
 
void PhysicalMemoryManager::free_frame(void* address) {
 
    m_lock.lock();
 
    // Mark the frame as not used
    m_used_frames--;
    uint64_t frame_address = (uint64_t) address / PAGE_SIZE;
    m_bit_map[frame_address / ROW_BITS] &= ~(1ULL << (frame_address % ROW_BITS));
 
    m_lock.unlock();
}
 
void* PhysicalMemoryManager::allocate_area(uint64_t start_address, size_t size) {
 
    m_lock.lock();
 
    // Store the information about the frames needed to be allocated for this size
    size_t frame_count = size_to_frames(size);
    uint32_t start_row = 0;
    uint32_t start_column = 0;
    size_t adjacent_frames = 0;
 
    for(uint32_t row = 0; row < m_total_entries; ++row) {
 
        // Skip full rows
        if(m_bit_map[row] == 0xFFFFFFFFFFFFFFFF) {
            adjacent_frames = 0;
            continue;
        }
 
        for(uint32_t column = 0; column < ROW_BITS; ++column) {
 
            // Not enough adjacent frames
            if(m_bit_map[row] & (1ULL << column)) {
                adjacent_frames = 0;
                continue;
            }
 
            // Store the address of the first frame in set of adjacent ones
            if(adjacent_frames == 0) {
                start_row = row;
                start_column = column;
            }
 
            // Make sure there are enough frames in a row found
            adjacent_frames++;
            if(adjacent_frames != frame_count)
                continue;
 
            // Mark the frames as used
            m_used_frames += frame_count;
            for(uint32_t i = 0; i < frame_count; ++i) {
 
                // Get the location of the bit
                uint32_t index = start_row + (start_column + i) / ROW_BITS;
                uint32_t bit = (start_column + i) % ROW_BITS;
 
                // Check bounds
                ASSERT(index < m_total_entries && bit < ROW_BITS, "Index out of bounds\n");
 
                // Mark the bit as used
                m_bit_map[index] |= (1ULL << bit);
            }
 
            // Return start of the block of adjacent frames
            m_lock.unlock();
            return (void*) (start_address + (start_row * ROW_BITS + start_column) * PAGE_SIZE);
 
        }
    }
 
    // Not enough free frames adjacent to each other
    m_lock.unlock();
    ASSERT(false, "Cannot allocate that much memory\n");
    return nullptr;
}
 
void PhysicalMemoryManager::free_area(uint64_t start_address, size_t size) {
 
    // Convert address into frames
    size_t frame_count = size_to_frames(size);
    uint64_t frame_address = start_address / PAGE_SIZE;
 
    // Check bounds
    if(frame_address >= m_bitmap_size)
        return;
 
    // Wait until other threads have finished other memory operations
    m_lock.lock();
 
    // Mark the frames as not used
    m_used_frames -= frame_count;
    for(uint32_t i = 0; i < frame_count; ++i)
        m_bit_map[(frame_address + i) / ROW_BITS] &= ~(1ULL << ((frame_address + i) % ROW_BITS));
 
    m_lock.unlock();
}
 
pml_t* PhysicalMemoryManager::get_higher_half_table(uint64_t index, uint64_t index2, uint64_t index3) {
 
    return (pml_t*) (0xFFFF000000000000 | ((510UL << 39) | (index3 << 30) | (index2 << 21) | (index << 12)));
}
 
 
pml_t* PhysicalMemoryManager::get_or_create_table(pml_t* table, size_t index, size_t flags) {
 
    // Table is already created so just find the entry
    if(table->entries[index].present)
        return (pml_t*) to_dm_region(physical_address_of_entry(&table->entries[index]));
 
    // Create the table
    auto* new_table = (uint64_t*) allocate_frame();
    table->entries[index] = create_page_table_entry((uint64_t) new_table, flags);
 
    // Move the table to the higher half
    new_table = (uint64_t*) to_dm_region((uintptr_t) new_table);
 
    // Reset the memory contents at the address
    clean_page_table(new_table);
 
    return (pml_t*) new_table;
}
 
pml_t* PhysicalMemoryManager::get_and_create_table(pml_t* parent_table, uint64_t table_index, pml_t* table) {
 
    // Table already created so don't need to do anything
    if(parent_table->entries[table_index].present)
        return table;
 
    // Create the table
    auto* new_table = (uint64_t*) allocate_frame();
    parent_table->entries[table_index] = create_page_table_entry((uint64_t) new_table, PRESENT | WRITE);
 
    // Move the table to higher half
    // Except this doesn't need to be done because using anom memory
 
    // Reset the memory contents at the address
    clean_page_table((uint64_t*) table);
 
    return table;
}
 
 
pte_t* PhysicalMemoryManager::get_entry(virtual_address_t* virtual_address, pml_t* pml4_table) {
 
    // Kernel memory must be in the higher half
    size_t flags = PRESENT | WRITE;
    if(!in_higher_region((uint64_t) virtual_address))
        flags |= (USER | GLOBAL);
 
    uint16_t pml4_index = PML4_GET_INDEX((uint64_t) virtual_address);
    uint16_t pdpr_index = PML3_GET_INDEX((uint64_t) virtual_address);
    uint16_t pd_index = PML2_GET_INDEX((uint64_t) virtual_address);
    uint16_t pt_index = PML1_GET_INDEX((uint64_t) virtual_address);
 
    pml_t* pdpr_table = nullptr;
    pml_t* pd_table = nullptr;
    pml_t* pt_table = nullptr;
 
    // If it is before initialization then cant rely on the direct map
    if(!m_initialised) {
        pdpr_table = get_and_create_table(pml4_table, pml4_index, get_higher_half_table(pml4_index));
        pd_table = get_and_create_table(pdpr_table, pdpr_index, get_higher_half_table(pdpr_index, pml4_index));
        pt_table = get_and_create_table(pd_table, pd_index, get_higher_half_table(pd_index, pdpr_index, pml4_index));
 
    } else {
        pdpr_table = get_or_create_table(pml4_table, pml4_index, flags);
        pd_table = get_or_create_table(pdpr_table, pdpr_index, flags);
        pt_table = get_or_create_table(pd_table, pd_index, flags);
    }
 
    // Get the entry
    return &pt_table->entries[pt_index];
}
 
 
uint64_t PhysicalMemoryManager::physical_address_of_entry(pte_t* entry) {
 
    return entry->physical_address << 12;
}
 
 
virtual_address_t* PhysicalMemoryManager::map(physical_address_t* physical_address, virtual_address_t* virtual_address, size_t flags) {
 
    // Map using the kernel's pml4 table
    return map(physical_address, virtual_address, flags, m_pml4_root_address);
}
 
virtual_address_t* PhysicalMemoryManager::map(physical_address_t* physical_address, virtual_address_t* virtual_address, size_t flags, uint64_t* pml4_table) {
 
    // If it is in a lower region then assume it is the user space
    if(!in_higher_region((uint64_t) virtual_address))
        flags |= (USER | GLOBAL);
 
    // If the entry already exists then the mapping is already done
    pte_t* pte = get_entry(virtual_address, (pml_t*) pml4_table);
    if(pte->present)
        return virtual_address;
 
    // Map the physical address to the virtual address
    *pte = create_page_table_entry((uint64_t) physical_address, flags);
 
    // Flush the TLB (cache)
    asm volatile("invlpg (%0)"::"r" (virtual_address) : "memory");
 
    return virtual_address;
}
 
virtual_address_t* PhysicalMemoryManager::map(virtual_address_t* virtual_address, size_t flags) {
 
    // Map a new physical address to the requested virtual address
    return map(allocate_frame(), virtual_address, flags);
 
}
 
void PhysicalMemoryManager::map_area(virtual_address_t* virtual_address_start, size_t length, size_t flags) {
 
    // Map the required frames
    for(size_t i = 0; i < size_to_frames(length); ++i)
        map((virtual_address_t*) ((uintptr_t) virtual_address_start + (i * PAGE_SIZE)), flags);
 
}
 
void PhysicalMemoryManager::map_area(physical_address_t* physical_address_start, virtual_address_t* virtual_address_start, size_t length, size_t flags) {
 
    // Map the required frames
    for(size_t i = 0; i < size_to_frames(length); ++i)
        map((virtual_address_t*) ((uintptr_t) physical_address_start + (i * PAGE_SIZE)),
            (virtual_address_t*) ((uintptr_t) virtual_address_start + (i * PAGE_SIZE)), flags);
}
 
void PhysicalMemoryManager::identity_map(physical_address_t* physical_address, size_t flags) {
 
    // Map the physical address to its virtual address counter-part
    map(physical_address, physical_address, flags);
}
 
void PhysicalMemoryManager::unmap(virtual_address_t* virtual_address) {
 
    // Pass the kernel's pml4 table
    unmap(virtual_address, m_pml4_root_address);
}
 
void PhysicalMemoryManager::unmap(virtual_address_t* virtual_address, uint64_t* pml4_root) {
 
    // Get the entry
    pte_t* pte = get_entry(virtual_address, (pml_t*) pml4_root);
 
    // Make sure the address is actually mapped
    if(!pte->present)
        return;
 
    // Unmap the entry
    pte->present = false;
 
    // Flush the TLB (cache)
    asm volatile("invlpg (%0)"::"r" (virtual_address) : "memory");
}
 
void PhysicalMemoryManager::unmap_area(virtual_address_t* virtual_address_start, size_t length) {
 
    // Unmap the required frames
    for(size_t i = 0; i < size_to_frames(length); ++i)
        unmap((virtual_address_t*) ((uintptr_t) virtual_address_start + (i * PAGE_SIZE)));
}
 
void PhysicalMemoryManager::clean_page_table(uint64_t* table) {
 
    // Null the table (prevents false mappings when re-using frames)
    for(int i = 0; i < 512; i++)
        table[i] = 0x00l;
}
 
pte_t PhysicalMemoryManager::create_page_table_entry(uintptr_t address, size_t flags) const {
 
    pte_t page = (pte_t) {
    .present            = (flags & PRESENT) != 0,
    .write              = (flags & WRITE) != 0,
    .user               = (flags & USER) != 0,
    .write_through      = (flags & WRITE_THROUGH) != 0,
    .cache_disabled     = (flags & CACHE_DISABLED) != 0,
    .accessed           = (flags & ACCESSED) != 0,
    .dirty              = (flags & DIRTY) != 0,
    .huge_page          = (flags & HUGE_PAGE) != 0,
    .global             = (flags & GLOBAL) != 0,
    .available          = 0,
    .physical_address   = address >> 12,
    };
 
    // Set the NX bit if it is allowed
    if(m_nx_allowed && (flags & NO_EXECUTE)) {
        auto page_raw = (uint64_t) &page;
        page_raw |= NO_EXECUTE;
        page = *(pte_t*) page_raw;
    }
 
    return page;
}
 
void PhysicalMemoryManager::initialise_bit_map() {
 
    // Earliest address to place the bitmap (after the kernel and hh direct map)
    uint64_t limit = m_kernel_start_page;
 
    // Find a region for the bitmap to handle
    for(multiboot_mmap_entry* entry = m_mmap_tag->entries; (multiboot_uint8_t*) entry <
                                                           (multiboot_uint8_t*) m_mmap_tag +
                                                           m_mmap_tag->size; entry = (multiboot_mmap_entry*) (
    (unsigned long) entry + m_mmap_tag->entry_size)) {
 
        // Cant use a non-free entry or an entry past limit (ie don't user higher addresses that will be overridden later)
        if(entry->type != MULTIBOOT_MEMORY_AVAILABLE || entry->len > limit)
            continue;
 
        size_t space = entry->len;
        size_t offset = 0;
 
        // Determine how much of the region is below the limit and adjust the starting point to there
        if(entry->addr < limit) {
            offset = limit - entry->addr;
            space -= offset;
        }
 
        // Make sure there is enough space
        ASSERT(space >= (m_bitmap_size / 8 + 1), "Not enough space for the bitmap (too big)\n");
 
        // Return the address (ensuring that it is in the safe region)
        m_bit_map = (uint64_t*) to_dm_region(entry->addr + offset);
        break;
    }
 
    // Error no suitable space for the bitmap found
    ASSERT(m_bit_map != nullptr, "No space for the bitmap (no region)\n");
 
    // Clear the bitmap (mark all as free)
    for(uint32_t i = 0; i < m_total_entries; ++i)
        m_bit_map[i] = 0;
 
    Logger::DEBUG() << "Bitmap: location: 0x" << (uint64_t) m_bit_map << " - 0x"
                    << (uint64_t) (m_bit_map + m_bitmap_size / 8) << " (range of 0x" << (uint64_t) m_bitmap_size / 8
                    << ")\n";
 
}
 
uint64_t* PhysicalMemoryManager::pml4_root_address() {
 
    return m_pml4_root_address;
}
 
uint64_t PhysicalMemoryManager::memory_size() const {
 
    return m_memory_size;
}
 
uint64_t PhysicalMemoryManager::memory_used() const {
 
    return m_used_frames * PAGE_SIZE;
}
 
size_t PhysicalMemoryManager::align_direct_to_page(size_t size) {
 
    return (size & (~(PAGE_SIZE - 1)));
}
 
void PhysicalMemoryManager::reserve(uint64_t address) {
 
    reserve(address, PAGE_SIZE);
}
 
void PhysicalMemoryManager::reserve(uint64_t address, size_t size, const char* type) {
 
    // Cant reserve virtual addresses (ensure the address is physical)
    ASSERT(address < m_memory_size, "Attempt to reserve address bigger then the memory can contain: 0x%x\n", address);
    ASSERT(address + size < m_memory_size, "Attempt to reserve region bigger then the memory can contain: 0x%x-0x%x\n",
           address, address + size);
 
    // Wait to be able to reserve
    m_lock.lock();
 
    // Align to a page, if rounding down need to correct the range
    size_t aligned_address = align_direct_to_page(address);
    if(aligned_address < address)
        size += address - aligned_address;
 
    // Convert in to amount of pages
    size = align_up_to_page(size, PAGE_SIZE);
    size_t page_count = size / PAGE_SIZE;
    uint64_t frame_index = aligned_address / PAGE_SIZE;
 
    // Mark all as used
    for(size_t i = 0; i < page_count; ++i)
        m_bit_map[(frame_index + i) / ROW_BITS] |= (1ULL << ((frame_index + i) % ROW_BITS));
 
    // Update the used frames
    m_used_frames += page_count;
 
    // Clear the lock
    m_lock.unlock();
    Logger::DEBUG() << "Reserved Address for " << type << ": 0x" << aligned_address << " - 0x" << aligned_address + size
                    << " (length of 0x" << size << ")\n";
}
 
physical_address_t* PhysicalMemoryManager::get_physical_address(virtual_address_t* virtual_address, uint64_t* pml4_root) {
 
    pte_t* entry = get_entry(virtual_address, (pml_t*) pml4_root);
 
    // Cant get a physical address if its inst free
    if(!entry->present)
        return nullptr;
 
    return (physical_address_t*) physical_address_of_entry(entry);
}
 
void PhysicalMemoryManager::change_page_flags(virtual_address_t* virtual_address, size_t flags, uint64_t* pml4_root) {
 
    pte_t* entry = get_entry(virtual_address, (pml_t*) pml4_root);
 
    // Cant edit a non-present entry (will page fault)
    if(!entry->present)
        return;
 
    *entry = create_page_table_entry(physical_address_of_entry(entry), flags);
 
    // Flush the TLB (cache)
    asm volatile("invlpg (%0)"::"r" (virtual_address) : "memory");
 
}
 
bool PhysicalMemoryManager::is_mapped(uintptr_t physical_address, uintptr_t virtual_address, uint64_t* pml4_root) {
 
    return get_physical_address((virtual_address_t*) virtual_address, pml4_root) ==
           (physical_address_t*) physical_address;
 
}
 
void* PhysicalMemoryManager::to_higher_region(uintptr_t physical_address) {
 
    // If it's in the lower half then add the offset
    if(physical_address < HIGHER_HALF_KERNEL_OFFSET)
        return (void*) (physical_address + HIGHER_HALF_KERNEL_OFFSET);
 
    // Must be in the higher half
    return (void*) physical_address;
}
 
void* PhysicalMemoryManager::to_lower_region(uintptr_t virtual_address) {
 
    // If it's in the lower half then add the offset
    if(virtual_address > HIGHER_HALF_KERNEL_OFFSET)
        return (void*) (virtual_address - HIGHER_HALF_KERNEL_OFFSET);
 
    // Must be in the lower half
    return (void*) virtual_address;
}
 
void* PhysicalMemoryManager::to_io_region(uintptr_t physical_address) {
 
    if(physical_address < HIGHER_HALF_MEM_OFFSET)
        return (void*) (physical_address + HIGHER_HALF_MEM_OFFSET);
 
    // Must be in the higher half
    return (void*) physical_address;
}
 
void* PhysicalMemoryManager::to_dm_region(uintptr_t physical_address) {
 
    if(physical_address < HIGHER_HALF_OFFSET)
        return (void*) (physical_address + HIGHER_HALF_DIRECT_MAP);
 
    // Must be in the higher half
    return (void*) physical_address;
}
 
void* PhysicalMemoryManager::from_dm_region(uintptr_t virtual_address) {
 
    if(virtual_address > HIGHER_HALF_DIRECT_MAP)
        return (void*) (virtual_address - HIGHER_HALF_DIRECT_MAP);
 
    // Must be in the lower half
    return (void*) virtual_address;
}
 
 
bool PhysicalMemoryManager::in_higher_region(uintptr_t virtual_address) {
 
    return virtual_address & (1l << 62);
}
 
 
void PhysicalMemoryManager::unmap_lower_kernel() {
 
    p4_table[0] = 0;
}