/* 
 * Copyright 2010 Stefan Lankes, Chair for Operating Systems,
 *                               RWTH Aachen University
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 *
 * This file is part of MetalSVM.
 */

#include <metalsvm/string.h>
#include <metalsvm/stdlib.h>
#include <metalsvm/tasks.h>
#include <asm/gdt.h>
#include <asm/tss.h>

gdt_ptr_t		gp;
static tss_t		task_state_segments[MAX_TASKS];
static gdt_entry_t	gdt[GDT_ENTRIES] = {[0 ... GDT_ENTRIES-1] = {0, 0, 0, 0, 0, 0}};
static unsigned char	kstacks[MAX_TASKS][KERNEL_STACK_SIZE];

/* 
 * This is in start.asm. We use this to properly reload
 * the new segment registers
 */
extern void gdt_flush(void);

int register_task(task_t* task) {
        uint16_t sel = (task->id+5) << 3;

	asm volatile ("mov %0, %%ax; ltr %%ax" : : "ir"(sel));

	return 0;
}

int create_default_frame(task_t* task, entry_point_t ep, void* arg, int user)
{
	uint16_t cs = user ? 0x1B : 0x08;
	uint16_t ds = user ? 0x23 : 0x10;
	uint32_t id;

	if (BUILTIN_EXPECT(!task, 0))
		return -1;	
	if (BUILTIN_EXPECT(user && !task->ustack, 0))
		return -1;
	if (BUILTIN_EXPECT(user && !task->stack_size, 0))
		return -1;
	id = task->id;

	/* reset buffers */
	memset(task_state_segments+id, 0x00, sizeof(tss_t));
	memset(kstacks[id], 0xCD, KERNEL_STACK_SIZE);
	if (user)
		memset(task->ustack, 0XCD, task->stack_size);

	/* set default values of all regsiters */
	task_state_segments[id].cs = cs;
	task_state_segments[id].ss = ds;
	task_state_segments[id].ds = ds;
	task_state_segments[id].fs = ds;
	task_state_segments[id].gs = ds;
	task_state_segments[id].es = ds;
	task_state_segments[id].eflags = 0x1202;
	task_state_segments[id].eip = (uint32_t) ep;
	if (user)
		task_state_segments[id].esp = (uint32_t) task->ustack + task->stack_size - sizeof(size_t);
	else
		task_state_segments[id].esp = (uint32_t) kstacks[id] + KERNEL_STACK_SIZE - sizeof(size_t);
	
	/* build default stack frame */
	*((size_t*)task_state_segments[id].esp) = 0xDEADBEAF;   /* dead-end */
	task_state_segments[id].ebp = task_state_segments[id].esp;
	task_state_segments[id].esp -= sizeof(size_t);
	*((size_t*)task_state_segments[id].esp) = (size_t) arg; 
	task_state_segments[id].esp -= sizeof(size_t);
	if (user)
		*((size_t*)task_state_segments[id].esp) = (size_t) leave_user_task;
	else
		*((size_t*)task_state_segments[id].esp) = (size_t) leave_kernel_task;

	/* setup for the kernel stack frame */
	task_state_segments[id].ss0 = 0x10;
	if (user)
		task_state_segments[id].esp0 = (uint32_t) kstacks[id] + KERNEL_STACK_SIZE - sizeof(size_t);
	else 
		task_state_segments[id].esp0 = task_state_segments[id].esp;

	return 0;
}

/* Setup a descriptor in the Global Descriptor Table */
static void gdt_set_gate(int num, unsigned long base, unsigned long limit,
			  unsigned char access, unsigned char gran)
{

	/* Setup the descriptor base address */
	gdt[num].base_low = (base & 0xFFFF);
	gdt[num].base_middle = (base >> 16) & 0xFF;
	gdt[num].base_high = (base >> 24) & 0xFF;

	/* Setup the descriptor limits */
	gdt[num].limit_low = (limit & 0xFFFF);
	gdt[num].granularity = ((limit >> 16) & 0x0F);

	/* Finally, set up the granularity and access flags */
	gdt[num].granularity |= (gran & 0xF0);
	gdt[num].access = access;
}

/* 
 * This will setup the special GDT
 * pointer, set up the entries in our GDT, and then
 * finally call gdt_flush() in our assembler file in order
 * to tell the processor where the new GDT is and update the
 * new segment registers 
 */
void gdt_install(void)
{
	unsigned int i;

	memset(task_state_segments, 0x00, MAX_TASKS*sizeof(tss_t));

	/* Setup the GDT pointer and limit */
	gp.limit = (sizeof(gdt_entry_t) * GDT_ENTRIES) - 1;
	gp.base = (unsigned int) &gdt;

	/* Our NULL descriptor */
	gdt_set_gate(0, 0, 0, 0, 0);

	/* 
	 * The second entry is our Code Segment. The base address
	 * is 0, the limit is 4 GByte, it uses 4KByte granularity,
	 * uses 32-bit opcodes, and is a Code Segment descriptor.
	 */
	gdt_set_gate(1, 0, 0xFFFFFFFF,
		GDT_FLAG_RING0 | GDT_FLAG_SEGMENT | GDT_FLAG_CODESEG | GDT_FLAG_PRESENT,
		GDT_FLAG_4K_GRAN | GDT_FLAG_32_BIT);

	/* 
	 * The third entry is our Data Segment. It's EXACTLY the
	 * same as our code segment, but the descriptor type in
	 * this entry's access byte says it's a Data Segment 
	 */
	gdt_set_gate(2, 0, 0xFFFFFFFF,
		GDT_FLAG_RING0 | GDT_FLAG_SEGMENT | GDT_FLAG_DATASEG | GDT_FLAG_PRESENT,
		GDT_FLAG_4K_GRAN | GDT_FLAG_32_BIT);
		
	/*
	 * Create code segement for userspace applications (ring 3)
	 */
	gdt_set_gate(3, 0, 0xFFFFFFFF,
		GDT_FLAG_RING3 | GDT_FLAG_SEGMENT | GDT_FLAG_CODESEG | GDT_FLAG_PRESENT,
		GDT_FLAG_4K_GRAN | GDT_FLAG_32_BIT);

	/*
	 * Create data segement for userspace applications (ring 3)
	 */
	gdt_set_gate(4, 0, 0xFFFFFFFF,
                GDT_FLAG_RING3 | GDT_FLAG_SEGMENT | GDT_FLAG_DATASEG | GDT_FLAG_PRESENT,
                GDT_FLAG_4K_GRAN | GDT_FLAG_32_BIT);

	/*
	 * Create a TSS for each task (we use these segments for task switching)
	 */
	for(i=0; i<MAX_TASKS; i++) {
		gdt_set_gate(5+i, (unsigned long) (task_state_segments+i), sizeof(tss_t)-1,
			GDT_FLAG_PRESENT | GDT_FLAG_TSS | GDT_FLAG_RING3,
			GDT_FLAG_32_BIT);
	}


	/* Flush out the old GDT and install the new changes! */
	gdt_flush();
}