gdt.cpp

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#include <system/gdt.h>
#include <common/logger.h>
 
using namespace MaxOS;
using namespace MaxOS::system;
 
GlobalDescriptorTable::GlobalDescriptorTable()
{
 
    // Null descriptor
    table[0] = 0;
 
    // Kernel Code Segment descriptor
    uint64_t kernel_cs = 0;
    kernel_cs |= (uint64_t) DescriptorFlags::Write;
    kernel_cs |= (uint64_t) DescriptorFlags::Execute;
    kernel_cs |= (uint64_t) DescriptorFlags::CodeOrDataSegment;
    kernel_cs |= (uint64_t) DescriptorFlags::Present;
    kernel_cs |= (uint64_t) DescriptorFlags::LongMode;
    table[1] = kernel_cs;
 
    // Kernel Data Segment descriptor
    uint64_t kernel_ds = 0;
    kernel_ds |= (uint64_t) DescriptorFlags::Write;
    kernel_ds |= (uint64_t) DescriptorFlags::CodeOrDataSegment;
    kernel_ds |= (uint64_t) DescriptorFlags::Present;
    table[2] = kernel_ds;
 
    // User code segment descriptor (Change the privilege level to 3)
    uint64_t user_cs = 0;
    user_cs |= (uint64_t) DescriptorFlags::Write;
    user_cs |= (uint64_t) DescriptorFlags::Execute;
    user_cs |= (uint64_t) DescriptorFlags::CodeOrDataSegment;
    user_cs |= (uint64_t) DescriptorFlags::Present;
    user_cs |= (uint64_t) DescriptorFlags::LongMode;
    user_cs |= (3ULL << 45);
    table[3] = user_cs;
 
    // User data segment descriptor (Change the privilege level to 3)
    uint64_t user_ds = 0;
    user_ds |= (uint64_t) DescriptorFlags::Write;
    user_ds |= (uint64_t) DescriptorFlags::CodeOrDataSegment;
    user_ds |= (uint64_t) DescriptorFlags::Present;
    user_ds |= (3ULL << 45);
    table[4] = user_ds;
 
    // Reserve space for the TSS
    table[5] = 0;
    table[6] = 0;
 
    // Load the GDT
    load();
 
    // Reload the segment registers
    asm volatile("mov %0, %%ds" : : "r"(0x10));
    asm volatile("mov %0, %%es" : : "r"(0x10));
    asm volatile("mov %0, %%fs" : : "r"(0x10));
    asm volatile("mov %0, %%gs" : : "r"(0x10));
    asm volatile("mov %0, %%ss" : : "r"(0x10));
    Logger::DEBUG() << "Reloaded segment registers\n";
}
 
void GlobalDescriptorTable::load() {
    gdtr = {
            .size    = sizeof(table) - 1,
            .address = (uint64_t) table,
    };
    asm volatile("lgdt %0" : : "m"(gdtr));
    Logger::DEBUG() << "Loaded GDT of limit 0x" << (uint64_t) gdtr.size << " and address 0x" << (uint64_t) gdtr.address << "\n";
 
}
 
GlobalDescriptorTable::~GlobalDescriptorTable() = default;