From 94ac0609d84e05fd3334a2280ed8c950bc948274 Mon Sep 17 00:00:00 2001
From: Baptiste Wicht <baptiste.wicht@gmail.com>
Date: Sat, 18 Jan 2014 23:39:27 +0100
Subject: [PATCH] Prepare Task State Segment

---
 kernel/include/gdt.hpp      | 44 +++++++++++++++++++++++++++++++++++++
 kernel/src/boot/boot_16.cpp | 40 +++++++++++++++++++++++++--------
 2 files changed, 75 insertions(+), 9 deletions(-)

diff --git a/kernel/include/gdt.hpp b/kernel/include/gdt.hpp
index 550f139e..5162e8c5 100644
--- a/kernel/include/gdt.hpp
+++ b/kernel/include/gdt.hpp
@@ -15,6 +15,50 @@ constexpr const uint16_t DATA_SELECTOR = 0x10;
 constexpr const uint16_t LONG_SELECTOR = 0x18;
 constexpr const uint16_t USER_DATA_SELECTOR = 0x20;
 constexpr const uint16_t USER_CODE_SELECTOR = 0x28;
+constexpr const uint16_t TSS_SELECTOR = 0x30;
+
+struct gdt_ptr {
+    uint16_t length;
+    uint32_t pointer;
+} __attribute__ ((packed));
+
+struct tss_descriptor_t {
+    uint16_t limit_low      : 16;
+    uint32_t base_low       : 24;
+    uint8_t type            : 4;
+    uint8_t always_0_1      : 1;
+    uint8_t dpl             : 2;
+    uint8_t present         : 1;
+    uint8_t limit_high      : 4;
+    uint8_t avl             : 1;
+    uint8_t always_0_2      : 2;
+    uint8_t granularity     : 1;
+    uint8_t base_middle     : 8;
+    uint32_t base_high      : 32;
+    uint8_t reserved_1      : 8;
+    uint8_t always_0_3      : 5;
+    uint32_t reserved_2     : 19;
+} __attribute__((packed));
+
+static_assert(sizeof(tss_descriptor_t) == 16, "TSS descriptor in long mode is 128 bits long");
+
+struct task_state_segment_t {
+    uint32_t reserved_0;
+    uint64_t rsp0;
+    uint64_t rsp1;
+    uint64_t rsp2;
+    uint64_t reserved_1;
+    uint64_t ist1;
+    uint64_t ist2;
+    uint64_t ist3;
+    uint64_t ist4;
+    uint64_t ist5;
+    uint64_t ist6;
+    uint64_t ist7;
+    uint64_t reserved_2;
+    uint16_t reserved_3;
+    uint16_t io_map_base_address;
+};
 
 } //end of namespace gdt
 
diff --git a/kernel/src/boot/boot_16.cpp b/kernel/src/boot/boot_16.cpp
index 10ef9553..2bff0236 100644
--- a/kernel/src/boot/boot_16.cpp
+++ b/kernel/src/boot/boot_16.cpp
@@ -32,6 +32,8 @@ constexpr const uint16_t DEFAULT_BPP = 32;
 
 } //end of anonymous namespace
 
+#include "gdt.hpp"
+
 #define CODE_16
 #include "e820.hpp" //Just for the address of the e820 map
 
@@ -46,11 +48,6 @@ bool vesa::vesa_enabled = false;
 
 namespace {
 
-struct gdt_ptr {
-    uint16_t length;
-    uint32_t pointer;
-} __attribute__ ((packed));
-
 void out_byte(uint8_t value, uint16_t port){
     __asm__ __volatile__("out %1, %0" : : "a" (value), "dN" (port));
 }
@@ -238,7 +235,7 @@ void enable_a20_gate(){
 }
 
 void setup_idt(){
-    static const gdt_ptr null_idt = {0, 0};
+    static const gdt::gdt_ptr null_idt = {0, 0};
     asm volatile("lidt %0" : : "m" (null_idt));
 }
 
@@ -338,17 +335,42 @@ constexpr uint16_t user_code_64_selector(){
 }
 
 void setup_gdt(){
+    //The Task State Segment
+    static gdt::task_state_segment_t tss;
+
     //TODO On some machines, this should be aligned to 16 bits
-    static const uint64_t gdt[] = {
+    static uint64_t gdt[] = {
         0,                                      //Null Selector
         gdt_entry(0, 0xFFFFF, code_32_selector()),
         gdt_entry(0, 0xFFFFF, data_selector()),
         gdt_entry(0, 0xFFFFF, code_64_selector()),
         gdt_entry(0, 0xFFFFF, user_data_selector()),
-        gdt_entry(0, 0xFFFFF, user_code_64_selector())
+        gdt_entry(0, 0xFFFFF, user_code_64_selector()),
+        0, //Make space for TSS selector
+        0  //Make space for TSS selector
     };
 
-    static gdt_ptr gdtr;
+    uint32_t base = reinterpret_cast<uint32_t>(&tss);
+    uint32_t limit = base + sizeof(gdt::task_state_segment_t);
+
+    auto tss_selector = reinterpret_cast<gdt::tss_descriptor_t*>(&gdt[6]);
+    tss_selector->type = 9; //64bit TSS (Available)
+    tss_selector->always_0_1 = 0;
+    tss_selector->always_0_2 = 0;
+    tss_selector->always_0_3 = 0;
+    tss_selector->dpl = 3;
+    tss_selector->present = 1;
+    tss_selector->avl = 0;
+    tss_selector->granularity = 0;
+
+    tss_selector->base_low = base & 0xFFFFFF;              //Bottom 24 bits
+    tss_selector->base_middle = (base & 0xFF000000) >> 24; //Top 8 bits
+    tss_selector->base_high = 0;                           //Top 32 bits are clear
+
+    tss_selector->limit_low = limit & 0xFFFF;              //Low 16 bits
+    tss_selector->limit_high = (limit&0xF0000) >> 16;      //Top 4 bits
+
+    static gdt::gdt_ptr gdtr;
     gdtr.length  = sizeof(gdt) - 1;
     gdtr.pointer = reinterpret_cast<uint32_t>(&gdt);