include/arch/gdt.h

@@ -13,19 +13,32 @@
 // we'll only use this as a requirement for paging, not more.
 // This means base 0 and no limit (whole address space)
 
-#define NUM_GDT_ENTRIES 5
+#define NUM_GDT_ENTRIES 7
 
 #define NULL_SELECTOR 0x00
 #define KERNEL_CODE_SEGMENT 0x08
 #define KERNEL_DATA_SEGMENT 0x10
 #define USER_CODE_SEGMENT 0x18
 #define USER_DATA_SEGMENT 0x20
+#define TSS_SEGMENT 0x28
 
 struct GDTR {
     uint16_t limit;
     uint64_t address;
 } __attribute__((packed));
 
+struct tss {
+    uint32_t reserved0;
+    uint64_t rsp0;
+    uint64_t rsp1;
+    uint64_t rsp2;
+    uint64_t reserved1;
+    uint64_t ist[7];
+    uint64_t reserved2;
+    uint16_t reserved3;
+    uint16_t iopb;
+} __attribute__((packed));
+
 void gdt_init(void);
 
 #endif

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");
 }