Legacy-Sync/paging.c

 #include "headers/paging.h"
#include "headers/kheap.h"
#include "headers/screen.h"
#include "headers/serial.h"
#include <stdint.h>

// The kernel's page directory
page_directory_t *kernel_directory=0;

// The current page directory;
page_directory_t *current_directory=0;


// a bitset of frames - used or free
uint32_t *frames;
uint32_t nframes;

// defined in kheap.c
extern uint32_t placement_address;

// Macros for bitset algo's
#define INDEX_FROM_BIT(a) (a/(8*4))
#define OFFSET_FROM_BIT(a) (a%(8*4))

// set a bit in the frames 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 frame in a 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);
}

/*// test if a bit is set
static u32int test_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);
    return (frames[idx] & (0x1 << off));
} */

// find the first free frame
static uint32_t first_frame()
{
    uint32_t i, j;
    for(i = 0; i < INDEX_FROM_BIT(nframes); i++)
    {
        if(frames[i] != 0xFFFFFFFF) // nothing free exit early
        {
            // atleast one bit is free
            for(j = 0; j < 32; j++)
            {
                uint32_t toTest = 0x1 << j;
                if(!(frames[i]&toTest))
                {
                    return i*4*8+j;
                }
            }
        }
    }
	return -1;
}

// alloc a frame
void alloc_frame(page_t *page, int is_kernel, int is_writeable)
{
    if(page->frame != 0)
    {
        return; // frame allready alloced
    }
    else
    {
        uint32_t idx = first_frame(); // idx now index for first free frame
        if(idx == (uint32_t)-1)
        {
            PANIC("No free frames!");
        }
        set_frame(idx*0x1000); // frame is now ours
        page->present = 1; // mark as present
        page->rw = (is_writeable)?1:0;  // should page be w
        page->user = (is_kernel)?0:1; // should page be user-mode
        page->frame = idx;
    }
}

// function to dealloc a frame
void free_frame(page_t * page)
{
    uint32_t frame;
    if(!(frame=page->frame))
    {
        return; // page didnt actually have a alloced frame
    }
    else
    {
        clear_frame(frame); // free the frame
        page->frame = 0x0; // page has no frame
    }
}

void initialize_paging()
{
    //size of physcial mem for now we assume it is 16MB
    uint32_t mem_end_page = 0x1000000;

    nframes = mem_end_page / 0x1000;
    frames = (uint32_t*)kmalloc(INDEX_FROM_BIT(nframes),0,0);
    memset(frames,0,INDEX_FROM_BIT(nframes));

    // lets make a page directory <--- cont here
	kernel_directory = (page_directory_t*)kmalloc_a(sizeof(page_directory_t));
	memset(kernel_directory, 0, sizeof(page_directory_t));
	current_directory = kernel_directory;


	// my need to identify map (phys addr = virt addr from
	// 0x0 to end of used mem so this can be accessed transparently
	// as if paging isnt enabled
	uint32_t i = 0;
	while(i < placement_address)
	{
		// read but not writeable from user space
		alloc_frame(get_page(i,1,kernel_directory),0,0);
		i += 0x1000;
	}

	// before we enable paging set our page fault handler
	register_interrupt_handler(14, &page_fault);


	// enable paging! <-- should have a seperate function
	// for the first time this is done and not constantly renable it
	switch_page_directory(kernel_directory);
	print("Paging enabled!\n");
}


void switch_page_directory(page_directory_t *dir)
{
	current_directory = dir;
	asm volatile("mov cr3, %0":: "r"(&dir->tablesPhysical));
	uint32_t cr0;
	asm volatile("mov %0, cr0": "=r"(cr0));
	serial_printf("cr0 = %x\n",cr0);
	cr0 |= 0x80000000; // Enable paging!
	asm volatile("mov cr0, %0":: "r"(cr0));
}


page_t *get_page(uint32_t address, int make, page_directory_t *dir)
{
	// turn address into an index
	address /= 0x1000;

	// find the page table that contains this address
	uint32_t table_idx = address / 1024;
	if(dir->tables[table_idx]) // if this page is allready assigned
	{
		return &dir->tables[table_idx]->pages[address%1024];
	}
	else if(make)
	{
		uint32_t tmp;
		dir->tables[table_idx] = (page_table_t*)kmalloc_ap(sizeof(page_table_t), &tmp);
		memset(dir->tables[table_idx], 0, 0x1000);
		dir->tablesPhysical[table_idx] = tmp | 0x7; // PRESENT, RW, US.
		return &dir->tables[table_idx]->pages[address%1024];
	}
	else
	{
		return 0;
	}
}


// fix issue with page fault passing see
// Problem: Interrupt handlers corrupt interrupted state 
// & Problem 3: regs var must called by reference instead of by value in the irq and isr handlers 
void page_fault(registers_t *regs)
{
	// a page fault has occured
	// faulting address is in cr2
	uint32_t faulting_address;
	asm volatile("mov %0, cr2" : "=r" (faulting_address));

	// the error codes gives us details of what happened
   int present   = !(regs->err_code & 0x1); // Page not present
   int rw = regs->err_code & 0x2;           // Write operation?
   int us = regs->err_code & 0x4;           // Processor was in user-mode?
   int reserved = regs->err_code & 0x8;     // Overwritten CPU-reserved bits of page entry?
   int id = regs->err_code & 0x10;          // Caused by an instruction fetch?


	// output an error message
	print("Page fault! ( ");
	if(present) { print("present "); }
	else if(rw) { print("read-only "); }
	else if(us) { print("user-mode "); }
	else if(id) { print("instr-fetch "); }
	else if(reserved) {print("reserved "); }
	kprintf(") at 0x%x\n",faulting_address);
	PANIC("Page fault");
}