vulcanos/kernel/drivers/paging.c

#include "paging.h"
#include "kheap.h"
#include "../libc/string.h"

// Macros for bitset algorithms
#define INDEX_FROM_BIT(a) (a/(8*4))
#define OFFSET_FROM_BIT(a) (a%(8*4))

// Kernel's page directory
page_directory_t *kernel_directory = 0;
// Current page directory
page_directory_t *current_directory = 0;

// Bitset of frames, used or free
uint32_t *frames;
uint32_t nframes;

// From kheap.c
extern uint32_t placement_addr;
extern heap_t *kheap;

// Set a bit in the frame bitset
static void set_frame(uint32_t frame_addr) {
    uint32_t frame = frame_addr / 0x1000;
    uint32_t idx = INDEX_FROM_BIT(frame);
    uint32_t off = OFFSET_FROM_BIT(frame);
    frames[idx] |= (0x1 << off);
}

// clear a bit in the frame bitset
static void clear_frame(uint32_t frame_addr) {
    uint32_t frame = frame_addr / 0x1000;
    uint32_t idx = INDEX_FROM_BIT(frame);
    uint32_t off = OFFSET_FROM_BIT(frame);
    frames[idx] &= ~(0x1 << off);
}

// Find the first free frames
static uint32_t first_frame() {
    uint32_t nsections = nframes / FRAME_ALLOCATION_SECTION_SIZE;
    for(uint32_t section = 0; section < INDEX_FROM_BIT(nframes); section++)
        if(frames[section] != 0xFFFFFFFF) // If nothing is free, exit
            for(uint32_t idx = 0; idx < FRAME_ALLOCATION_SECTION_SIZE; idx++)
                if (!(frames[idx] & (0x1 << idx)))
                    return (section * FRAME_ALLOCATION_SECTION_SIZE) + idx;
    return nsections * FRAME_ALLOCATION_SECTION_SIZE;
}

void alloc_frame(page_t *page, int32_t is_supervisored, int32_t is_writeable) {
    if(page->frame != 0)
        return; // Frame already allocated
    else {
        uint32_t free_frame = first_frame();
        if(free_frame == (uint32_t)-1) {
            PANIC("No free frames found!");
        } else {
            // Set free frames to page
            page->present = PAGE_PRESENT;
            page->rw = (is_writeable) ? PAGE_READ_WRITE : PAGE_READ_ONLY;
            page->user = (is_supervisored) ? PAGE_SUPERVISOR : PAGE_USER;
            page->frame = free_frame;

            // Set new frames as used
            uint32_t physical_address = free_frame * FRAME_SIZE;
            set_frame(physical_address);
        }
    }
}

void free_frame(page_t *page) {
    uint32_t frame;
    if(!(frame=page->frame))
        return;
    else {
        clear_frame(frame);
        page->frame = 0x0;
    }
}

void init_paging() {
    uint32_t nframes = PHYSICAL_MEMORY_SIZE / FRAME_SIZE;
    frames = (uint32_t*)kmalloc(INDEX_FROM_BIT(nframes));
    
    // Create a page directory
    kernel_directory = (page_directory_t*)kmalloc_a(sizeof(page_directory_t));
    memset(frames, 0, INDEX_FROM_BIT(nframes));
    current_directory = kernel_directory;

    /*  Map pages in the kernel heap area.
     * We only call get_page and not alloc_frame to create a new page_table_t 
     * only where necessary.*/
    for(int32_t i = KHEAP_START; i < (int32_t)KHEAP_START+KHEAP_INITIAL_SIZE; i += 0x1000)
        get_page(i, 1, kernel_directory);

    /* We have eto identify map from 0x0 to the end of the use memory
     * so we can use this memory region as if paging was not enabled. */
    int32_t i = 0;
    while(i < (int32_t)placement_addr+0x1000) {
        // Kernel code is read only from userspace
        alloc_frame(get_page(i, 1, kernel_directory), 0, 0);
        i += 0x1000;
    }
    // Perform the real allocation of what we have done so far
    for(i = KHEAP_START; i < (int32_t)KHEAP_START+KHEAP_INITIAL_SIZE; i += 0x1000)
        alloc_frame(get_page(i, 1, kernel_directory), 0, 0);
    
    // Register a new ISR to handle page faults
    register_interrupt_handler(14, page_fault);

    // Enable paging
    enable_paging(kernel_directory);

    // Set up kernel heap
    kheap = create_heap(KHEAP_START, KHEAP_START+KHEAP_INITIAL_SIZE, 0xCFFFF000, 0, 0);
}

void enable_paging(page_directory_t *dir) {
    current_directory = dir;
    asm volatile("mov %0, %%cr3" :: "r"(&dir->page_table_physical));
    uint32_t cr0;
    asm volatile("mov %%cr0, %0": "=r"(cr0));
    cr0 |= 0x80000000; // code to enable paging
    asm volatile("mov %0, %%cr0":: "r"(cr0));
}

page_t *get_page(uint32_t address, int32_t make, page_directory_t *dir) {
    // Turn address into an index
    address /= 0x1000;
    // Find page table that contains this index
    uint32_t table_idx = address / 1024;
    if(dir->page_table_virtual[table_idx]) // If current table is already assigned
        return &dir->page_table_virtual[table_idx]->pages[address%1024];
    else if(make) {
        uint32_t tmp;
        dir->page_table_virtual[table_idx] = (page_table_t*)kmalloc_p(sizeof(page_table_t), &tmp);
        memset(dir->page_table_virtual[table_idx], 0, 0x1000);
        dir->page_table_physical[table_idx] = tmp | 0x7;
        return &dir->page_table_virtual[table_idx]->pages[address%1024];
    } else
        return 0;
}

void page_fault(registers_t regs) {
    // Retrieve faulted address from CR2 register
    uint32_t fault_addr;
    asm volatile("mov %%cr2, %0" : "=r"(fault_addr));

    // Retrieve other infos about the error
    int32_t present = !(regs.err_code & 0x1); // Page not present
    int32_t rw = regs.err_code & 0x2;  // Write operation
    int32_t us = regs.err_code & 0x4; // CPU mode(kernel or user mode)
    int32_t reserved = regs.err_code & 0x8;

    // Output of those informations
    kprint((uint8_t*)"Page fault! ( ");
    if(present)
        kprint((uint8_t*)"present ");
    if(rw)
        kprint((uint8_t*)"read-only");
    if(us)
        kprint((uint8_t*)"user-mode");
    if(reserved)
        kprint((uint8_t*)"reserved");
    kprint((uint8_t*)") at 0x");
    kprint_hex(fault_addr);
    kprint((uint8_t*)"\n");
    PANIC("Page fault");
}