banan-os/kernel/arch/x86_64/GDT.cpp

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#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 constexpr uint16_t s_tss_offset = 0x28;
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()
{
s_tss = new TaskStateSegment();
ASSERT(s_tss);
memset(s_tss, 0x00, sizeof(TaskStateSegment));
s_tss->rsp0 = 0;
uintptr_t base = (uintptr_t)s_tss;
write_entry(s_tss_offset, (uint32_t)base, sizeof(TaskStateSegment), 0x89, 0x0);
SegmentDescriptor& desc = *(SegmentDescriptor*)((uintptr_t)s_gdt + s_tss_offset + 0x08);
desc.low = base >> 32;
desc.high = 0;
}
void set_tss_stack(uintptr_t rsp)
{
s_tss->rsp0 = rsp;
}
static void flush_gdt()
{
asm volatile("lgdt %0" :: "m"(s_gdtr));
}
static void flush_tss()
{
asm volatile("ltr %0" :: "m"(s_tss_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();
flush_gdt();
flush_tss();
}
}