Kernel: Initialize GDT in kernel code

We dont use the boot GDT only while booting

kernel/arch/x86_64/GDT.cpp

@@ -0,0 +1,122 @@
+#include <BAN/Assert.h>
+#include <kernel/GDT.h>
+
+#include <string.h>
+
+extern "C" uintptr_t g_boot_stack_top[0];
+
+namespace Kernel::GDT
+{
+
+	struct TaskStateSegment
+	{
+		uint32_t reserved1;
+		uint64_t rsp0;
+		uint64_t rsp1;
+		uint64_t rsp2;
+		uint64_t reserved2;
+		uint64_t ist1;
+		uint64_t ist2;
+		uint64_t ist3;
+		uint64_t ist4;
+		uint64_t ist5;
+		uint64_t ist6;
+		uint64_t ist7;
+		uint64_t reserved3;
+		uint16_t reserved4;
+		uint16_t iopb;
+	} __attribute__((packed));
+
+	union SegmentDescriptor
+	{
+		struct
+		{
+			uint16_t	limit1;
+			uint16_t	base1;
+			uint8_t		base2;
+			uint8_t		access;
+			uint8_t		limit2 : 4;
+			uint8_t		flags  : 4;
+			uint8_t		base3;
+		} __attribute__((packed));
+		
+		struct
+		{
+			uint32_t low;
+			uint32_t high;
+		} __attribute__((packed));
+
+	} __attribute__((packed));
+
+	struct GDTR
+	{
+		uint16_t size;
+		uint64_t address;
+	} __attribute__((packed));
+
+	static TaskStateSegment* s_tss = nullptr;
+	static SegmentDescriptor* s_gdt = nullptr;
+	static GDTR s_gdtr;
+
+	static void write_entry(uint8_t offset, uint32_t base, uint32_t limit, uint8_t access, uint8_t flags)
+	{
+		SegmentDescriptor& desc = *(SegmentDescriptor*)((uintptr_t)s_gdt + offset);
+		desc.base1 = base;
+		desc.base2 = base >> 16;
+		desc.base3 = base >> 24;
+
+		desc.limit1 = limit;
+		desc.limit2 = limit >> 16;
+
+		desc.access = access;
+
+		desc.flags = flags;
+	}
+
+	static void write_tss(uint8_t offset)
+	{
+		s_tss = new TaskStateSegment();
+		ASSERT(s_tss);
+		
+		memset(s_tss, 0x00, sizeof(TaskStateSegment));
+		s_tss->rsp0 = (uintptr_t)g_boot_stack_top;
+		
+		uintptr_t base = (uintptr_t)s_tss;
+
+		write_entry(offset, (uint32_t)base, sizeof(TaskStateSegment), 0x89, 0x0);
+		SegmentDescriptor& desc = *(SegmentDescriptor*)((uintptr_t)s_gdt + offset + 0x08);
+		desc.low = base >> 32;
+		desc.high = 0;
+	}
+
+	static void flush_gdt()
+	{
+		asm volatile("lgdt %0" :: "m"(s_gdtr));
+	}
+
+	extern "C" void flush_tss(uint16_t offset)
+	{
+		asm volatile("ltr %0" :: "m"(offset));
+	}
+
+	void initialize()
+	{
+		constexpr uint32_t descriptor_count = 6 + 1; // tss takes 2
+		s_gdt = new SegmentDescriptor[descriptor_count]; 
+		ASSERT(s_gdt);
+
+		s_gdtr.address = (uint64_t)s_gdt;
+		s_gdtr.size = descriptor_count * sizeof(SegmentDescriptor) - 1;
+
+		write_entry(0x00, 0x00000000, 0x00000, 0x00, 0x0); // null
+		write_entry(0x08, 0x00000000, 0xFFFFF, 0x9A, 0xA); // kernel code
+		write_entry(0x10, 0x00000000, 0xFFFFF, 0x92, 0xC); // kernel data
+		write_entry(0x18, 0x00000000, 0xFFFFF, 0xFA, 0xA); // user code
+		write_entry(0x20, 0x00000000, 0xFFFFF, 0xF2, 0xC); // user data
+		write_tss(0x28);
+
+		flush_gdt();
+		flush_tss(0x28);
+	}
+
+}

kernel/arch/x86_64/boot.S

@@ -43,7 +43,7 @@
 	# 0 MiB -> 1 MiB: bootloader stuff
 	# 1 MiB -> 2 MiB: kernel
 	# 2 MiB -> 3 MiB: kmalloc
-	# 3 MiB -> 4 MiB: kmalloc_eternal
+	# 3 MiB -> 4 MiB: kmalloc_fixed
 	.align 4096
 	boot_pml4:
 		.skip 512 * 8
@@ -52,8 +52,6 @@
 	boot_pd1:
 		.skip 512 * 8
 
-.section .text, "a"
-
 .global g_multiboot_info
 g_multiboot_info:
 	.skip 8
@@ -61,6 +59,8 @@ g_multiboot_info:
 g_multiboot_magic:
 	.skip 8
 
+.section .text, "a"
+
 boot_gdt:
 	.quad 0x0000000000000000 # null descriptor
 	.quad 0x00AF9A000000FFFF # kernel code

kernel/arch/x86_64/make.config

@@ -7,6 +7,7 @@ ELF_FORMAT=elf64-x86-64
 
 KERNEL_ARCH_OBJS=		\
 $(ARCHDIR)/boot.o		\
+$(ARCHDIR)/GDT.o		\
 $(ARCHDIR)/IDT.o		\
 $(ARCHDIR)/interrupts.o	\
 $(ARCHDIR)/MMU.o		\