TSS setup

src/arch/x86/gdt.c

@@ -1,6 +1,6 @@
 /*
  * @author  xamidev <xamidev@riseup.net>
- * @brief   Global Descriptor Table (for legacy reasons)
+ * @brief   Global Descriptor Table and Task State Segment setup
  * @license GPL-3.0-only
  */
 
@@ -14,6 +14,8 @@
 uint64_t gdt_entries[NUM_GDT_ENTRIES];
 struct GDTR gdtr;
 
+struct tss tss = {0};
+
 /*
  * gdt_load - Loads Global Descriptor Table
  */
@@ -85,7 +87,22 @@ static void gdt_set_entry(int num, uint8_t flags, uint8_t access)
  */
 static void gdt_set_tss(int num)
 {
+    uint64_t tss_base = (uint64_t)&tss;
+    uint64_t tss_limit = sizeof(struct tss) - 1;
 
+    tss.iopb = sizeof(struct tss);
+
+    uint64_t tss_low = 0;
+    tss_low |= (tss_limit & 0xFFFFULL);
+    tss_low |= (tss_base & 0xFFFFFFULL) << 16;
+    tss_low |= 0x89ULL << 40;
+    tss_low |= ((tss_limit >> 16) & 0xFULL) << 48;
+    tss_low |= ((tss_base >> 24) & 0xFFULL) << 56;
+
+    uint64_t tss_high = (tss_base >> 32) & 0xFFFFFFFFULL;
+
+    gdt_entries[num] = tss_low;
+    gdt_entries[num + 1] = tss_high;
 }
 
 /*
@@ -104,6 +121,7 @@ void gdt_init()
     gdt_set_entry(2, 0xC, 0x93); // Kernel data (0x10)
     gdt_set_entry(3, 0xA, 0xFB); // User code   (0x18)
     gdt_set_entry(4, 0xC, 0xF3); // User data   (0x20)
+    gdt_set_tss(5);                            // TSS         (0x28)
 
     // The -1 subtraction is some wizardry explained in the OSDev wiki -> GDT
     gdtr.limit = NUM_GDT_ENTRIES * sizeof(uint64_t) - 1;
@@ -113,7 +131,8 @@ void gdt_init()
     gdt_load();
     gdt_flush();
 
-    // Here we should load the task register with TSS selector
+    // Load task register with new TSS
+    asm volatile("ltr %%ax" : : "a"(TSS_SEGMENT) : "memory");
 
     DEBUG("GDT initialized");
 }