Luna/kernel/src/MemoryManager.cpp

#include "MemoryManager.h"
#include "arch/MMU.h"
#include "arch/Serial.h"
#include "bootboot.h"
#include <String.h>
#include <Types.h>

extern BOOTBOOT bootboot;

static bool page_bitmap_read(u64 index);
static void page_bitmap_set(u64 index, bool value);

static u64 free_mem = 0;
static u64 used_mem = 0;
static u64 reserved_mem = 0;

static char* page_bitmap_addr = nullptr;
static char* page_virtual_bitmap_addr = nullptr;
static u64 page_bitmap_size;

static u64 start_index = 0;

static bool page_bitmap_read(u64 index)
{
    return (page_virtual_bitmap_addr[index / 8] & (0b10000000 >> (index % 8))) > 0;
}

static void page_bitmap_set(u64 index, bool value)
{
    uint64_t byteIndex = index / 8;
    uint8_t bitIndexer = 0b10000000 >> (index % 8);
    page_virtual_bitmap_addr[byteIndex] &= (uint8_t)(~bitIndexer);
    if (value) { page_virtual_bitmap_addr[byteIndex] |= bitIndexer; }
}

namespace MemoryManager
{
    void init_physical_allocator()
    {
        u64 total_mem = 0;

        void* biggest_memory_block = nullptr;
        u64 biggest_memory_block_size = 0;

        // walk the memory map
        MMapEnt* ptr = &bootboot.mmap;
        uint64_t mmap_entries = (bootboot.size - 128) / 16;
        for (u64 i = 0; i < mmap_entries; i++)
        {
            u64 size = MMapEnt_Size(ptr);
            total_mem += size;
            if (!MMapEnt_IsFree(ptr))
            {
                ptr++;
                continue;
            }
            if (size > biggest_memory_block_size)
            {
                biggest_memory_block = (void*)MMapEnt_Ptr(ptr);
                biggest_memory_block_size = MMapEnt_Size(ptr);
            }
            ptr++;
        }

        page_bitmap_addr = (char*)biggest_memory_block;
        page_virtual_bitmap_addr = page_bitmap_addr; // we'll map this to virtual memory as soon as the MMU is ready
        if ((total_mem / MMU::page_size() / 8) >= biggest_memory_block_size)
        {
            Serial::println("ERROR: No single memory block is enough to hold the page bitmap");
            for (;;)
                ;
        }

        page_bitmap_size = total_mem / MMU::page_size() / 8 + 1;
        memset(page_bitmap_addr, 0xFF, page_bitmap_size);

        ptr = &bootboot.mmap;
        for (uint64_t i = 0; i < mmap_entries; i++)
        {
            uint64_t index = MMapEnt_Ptr(ptr) / MMU::page_size();
            if (!MMapEnt_IsFree(ptr)) { reserved_mem += MMapEnt_Size(ptr); }
            else
            {
                free_mem += MMapEnt_Size(ptr);
                for (uint64_t j = 0; j < (MMapEnt_Size(ptr) / MMU::page_size()); j++)
                {
                    page_bitmap_set(index + j, false);
                }
            }
            ptr++;
        }

        lock_pages((u64)page_bitmap_addr, page_bitmap_size / MMU::page_size() + 1);
    }

    void init()
    {
        init_physical_allocator();
        MMU::setup_initial_page_directory();
    }

    void lock_page(u64 page)
    {
        uint64_t index = ((uint64_t)page) / MMU::page_size();
        if (page_bitmap_read(index)) return;
        page_bitmap_set(index, true);
        used_mem += MMU::page_size();
        free_mem -= MMU::page_size();
    }

    void lock_pages(u64 pages, u64 count)
    {
        for (u64 index = 0; index < count; index++) { lock_page(pages + (index * MMU::page_size())); }
    }

    Result<u64> alloc_physical_page()
    {
        for (u64 index = start_index; index < (page_bitmap_size * 8); index++)
        {
            if (page_bitmap_read(index)) continue;
            page_bitmap_set(index, true);
            start_index = index + 1;
            free_mem -= MMU::page_size();
            used_mem += MMU::page_size();
            return index * MMU::page_size();
        }

        return err; // FIXME: ENOMEM.
    }

    Result<void> free_physical_page(u64 page)
    {
        u64 index = page / MMU::page_size();
        if (index > (page_bitmap_size * 8)) return err;
        if (!page_bitmap_read(index)) return err;
        page_bitmap_set(index, false);
        used_mem -= MMU::page_size();
        free_mem += MMU::page_size();
        if (start_index > index) start_index = index;
        return {};
    }
}