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// MIT License, Copyright (c) 2021 Marvin Borner
#include <assert.h>
#include <def.h>
#include <gdt.h>
#include <mem.h>
static struct gdt_entry gdt[6] = { 0 };
static struct tss_entry tss = { 0 };
PROTECTED static struct gdt_ptr gp = { 0 };
CLEAR static void gdt_set_gate(u32 num, u32 base, u32 limit, u8 access, u8 gran)
{
// Set descriptor base address
gdt[num].base_low = (u16)(base & 0xffff);
gdt[num].base_middle = (u8)((base >> 16) & 0xff);
gdt[num].base_high = (u8)((base >> 24) & 0xff);
gdt[num].limit_low = (u16)(limit & 0xffff);
// Set granularity and access flags
gdt[num].granularity = (u8)((limit >> 16) & 0x0f);
gdt[num].granularity |= (gran & 0xf0);
gdt[num].access = access;
}
CLEAR static void tss_write(u32 num, u16 ss0, u32 esp0)
{
u32 base = (u32)&tss;
u32 limit = base + sizeof(tss);
gdt_set_gate(num, base, limit, 0xe9, 0x00);
memset(&tss, 0, sizeof(tss));
tss.ss0 = ss0;
tss.esp0 = esp0;
tss.cs = 0x0b;
tss.ss = tss.ds = tss.es = tss.fs = tss.gs = 0x13;
}
CLEAR static void tss_flush(void)
{
__asm__ volatile("ltr %0" ::"r"((u16)0x2b));
}
CLEAR static void gdt_flush(void)
{
__asm__ volatile("lgdt %0" ::"m"(gp) : "memory");
}
void tss_set_stack(u32 ss, u32 esp)
{
assert(ss && esp);
tss.esp0 = esp;
tss.ss0 = ss;
}
CLEAR void gdt_install(u32 esp)
{
// Set GDT pointer and limit
gp.limit = (sizeof(struct gdt_entry) * 6) - 1;
gp.base = &gdt;
// NULL descriptor
gdt_set_gate(0, 0, 0, 0, 0);
// Code segment
gdt_set_gate(1, 0, 0xffffffff, 0x9a, 0xcf);
// Data segment
gdt_set_gate(2, 0, 0xffffffff, 0x92, 0xcf);
// User mode code segment
gdt_set_gate(3, 0, 0xffffffff, 0xfa, 0xcf);
// User mode data segment
gdt_set_gate(4, 0, 0xffffffff, 0xf2, 0xcf);
// Write TSS
tss_write(5, GDT_SUPER_DATA_OFFSET, esp);
// Remove old GDT and install the new changes!
gdt_flush();
tss_flush();
__asm__ volatile("mov %%ax, %%ds\n"
"mov %%ax, %%es\n"
"mov %%ax, %%fs\n"
"mov %%ax, %%gs\n"
"mov %%ax, %%ss\n" ::"a"(GDT_SUPER_DATA_OFFSET)
: "memory");
__asm__ volatile("ljmpl $" STRINGIFY(GDT_SUPER_CODE_OFFSET) ", $code\n"
"code:\n");
}
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