From f581e6b6fefc64f99e4261e17f30a6486166a5e7 Mon Sep 17 00:00:00 2001
From: Xnoe <xnoe@xnoe.moe>
Date: Sun, 21 Nov 2021 16:39:31 +0000
Subject: [PATCH] Completely overhauled how GDT is defined to make it much
 easier to add more entries to the GDT in the future

---
 src/kernel/gdt.cpp | 138 +++++++++++++++++++++++++++------------------
 src/kernel/gdt.h   |  45 +++++++++++++++
 2 files changed, 128 insertions(+), 55 deletions(-)

diff --git a/src/kernel/gdt.cpp b/src/kernel/gdt.cpp
index 3ffd823..0c062fa 100644
--- a/src/kernel/gdt.cpp
+++ b/src/kernel/gdt.cpp
@@ -1,57 +1,89 @@
 #include "gdt.h"
 
+constexpr tss_struct::tss_struct() :
+  link(0),
+  _reserved0(0),
+  esp0(0),
+  ss0(0),
+  _reserved1(0),
+  esp1(0),
+  ss1(0),
+  _reserved2(0),
+  esp2(0),
+  ss2(0),
+  _reserved3(0),
+  cr3(0),
+  eip(0),
+  eflags(0),
+  eax(0),
+  ebx(0),
+  ecx(0),
+  edx(0),
+  esp(0),
+  ebp(0),
+  esi(0),
+  edi(0),
+  es(0),
+  _reserved4(0),
+  cs(0),
+  _reserved5(0),
+  ss(0),
+  _reserved6(0),
+  ds(0),
+  _reserved7(0),
+  fs(0),
+  _reserved8(0),
+  gs(0),
+  _reserved9(0),
+  ldtr(0),
+  _reserved10(0),
+  _reserved11(0),
+  iopb(0)
+{}
+
+tss_struct tss = tss_struct();
+
+constexpr gdt_entry::gdt_entry(uint32_t limit, uint32_t base, bool rw, bool exec, bool system, uint8_t ring) :
+  limit_lo(limit & 0xffff),
+  limit_hi((limit & 0xf0000) >> 16),
+  base_lo(base & 0xffff),
+  base_mid((base & 0xff0000) >> 16),
+  base_hi((base & 0xff000000) >> 24),
+  __ignored__(0),
+  accessed(0),
+  granularity(1),
+  size(1),
+  present(1),
+  read_write(rw),
+  executable(exec),
+  system_segment(system),
+  privilege(ring),
+  direction(0)
+{}
+
+constexpr gdt_entry::gdt_entry() :
+  limit_lo(0),
+  base_lo(0),
+  base_mid(0),
+  accessed(0),
+  read_write(0),
+  direction(0),
+  executable(0),
+  system_segment(0),
+  privilege(0),
+  present(0),
+  limit_hi(0),
+  __ignored__(0),
+  size(0),
+  granularity(0),
+  base_hi(0)
+{}
+
 gdt_entry gdt[] = {
-  (gdt_entry){ // Null Segment
-    .limit_lo = 0,
-    .base_lo = 0,
-    .base_mid = 0,
-    .accessed = 0,
-    .read_write = 0,
-    .direction = 0,
-    .executable = 0,
-    .system_segment = 0,
-    .privilege = 0,
-    .present = 0,
-    .limit_hi = 0,
-    .__ignored__ = 0,
-    .size = 0,
-    .granularity = 0,
-    .base_hi = 0,
-  },
-  (gdt_entry){ // Code Segment
-    .limit_lo = 0xffff,
-    .base_lo = 0,
-    .base_mid = 0,
-    .accessed = 0,
-    .read_write = 1,
-    .direction = 0,
-    .executable = 1,
-    .system_segment = 1,
-    .privilege = 0,
-    .present = 1,
-    .limit_hi = 0xf,
-    .__ignored__ = 0,
-    .size = 1,
-    .granularity = 1,
-    .base_hi = 0
-  },
-  (gdt_entry){ // Data Segment
-    .limit_lo = 0xffff,
-    .base_lo = 0,
-    .base_mid = 0,
-    .accessed = 0,
-    .read_write = 1,
-    .direction = 0,
-    .executable = 0,
-    .system_segment = 1,
-    .privilege = 0,
-    .present = 1,
-    .limit_hi = 0xf,
-    .__ignored__ = 0,
-    .size = 1,
-    .granularity = 1,
-    .base_hi = 0
-  }
+  gdt_entry(), // Null Segment
+  gdt_entry(0xfffff, 0, 1, 1, 1, 0), // Code Segment
+  gdt_entry(0xfffff, 0, 1, 0, 1, 0), // Data Segment
+  gdt_entry(sizeof(tss), &tss, 0, 1, 0, 0) // Task State Segment 1
 };
 
 gdt_descr descr = (gdt_descr){
@@ -59,14 +91,10 @@ gdt_descr descr = (gdt_descr){
   .offset = gdt,
 };
 
-__attribute__((far)) void far_call() {}
-
 void init_gdt() {
   asm volatile("lgdt %0;"
                "mov $0x10, %%eax;"
                "mov %%eax, %%ss;"
                "mov $0x10, %%eax;"
                "mov %%eax, %%ds" : : "m" (descr));
-  
-  far_call();
 }
\ No newline at end of file
diff --git a/src/kernel/gdt.h b/src/kernel/gdt.h
index b300106..2309896 100644
--- a/src/kernel/gdt.h
+++ b/src/kernel/gdt.h
@@ -3,6 +3,48 @@
 
 #include "types.h"
 
+struct __attribute__((packed)) tss_struct {
+  uint16_t link;
+  uint16_t _reserved0;
+  uint32_t esp0;
+  uint16_t ss0;
+  uint16_t _reserved1;
+  uint32_t esp1;
+  uint16_t ss1;
+  uint16_t _reserved2;
+  uint32_t esp2;
+  uint16_t ss2;
+  uint16_t _reserved3;
+  uint32_t cr3;
+  uint32_t eip;
+  uint32_t eflags;
+  uint32_t eax;
+  uint32_t ebx;
+  uint32_t ecx;
+  uint32_t edx;
+  uint32_t esp;
+  uint32_t ebp;
+  uint32_t esi;
+  uint32_t edi;
+  uint16_t es;
+  uint16_t _reserved4;
+  uint16_t cs;
+  uint16_t _reserved5;
+  uint16_t ss;
+  uint16_t _reserved6;
+  uint16_t ds;
+  uint16_t _reserved7;
+  uint16_t fs;
+  uint16_t _reserved8;
+  uint16_t gs;
+  uint16_t _reserved9;
+  uint16_t ldtr;
+  uint16_t _reserved10;
+  uint16_t _reserved11;
+  uint16_t iopb;
+  constexpr tss_struct();
+};
+
 struct __attribute__((packed)) gdt_entry {
   uint32_t limit_lo : 16;
   uint32_t base_lo : 16;
@@ -22,6 +64,9 @@ struct __attribute__((packed)) gdt_entry {
   uint32_t granularity : 1;
   
   uint32_t base_hi : 8;
+
+  constexpr gdt_entry(uint32_t limit, uint32_t base, bool rw, bool exec, bool system, uint8_t ring);
+  constexpr gdt_entry();
 };
 
 struct __attribute__((packed)) gdt_descr {