Kernel/x86_64: Add GDT initialization under a portable cpu::platform_init interface

moon/src/arch/x86_64/cpu.rs

@@ -4,6 +4,8 @@
 use core::arch::x86_64::{__cpuid as do_cpuid, CpuidResult};
 use core::arch::asm;
 
+use super::gdt;
+
 fn safe_cpuid(leaf: u32) -> CpuidResult {
     return unsafe { do_cpuid(leaf) };
 }
@@ -22,10 +24,18 @@ pub fn halt() -> ! {
     }
 }
 
-pub fn wait_for_interrupt() -> () {
+pub fn wait_for_interrupt()
+{
     unsafe { asm!("hlt"); }
 }
 
-pub fn breakpoint() -> () {
+pub fn breakpoint()
+{
     unsafe { asm!("int3"); }
 }
+
+pub fn platform_init()
+{
+    gdt::init();
+    gdt::load_and_reload_segment_registers();
+}

moon/src/arch/x86_64/gdt.rs

@@ -0,0 +1,35 @@
+#![cfg(target_arch = "x86_64")]
+#![allow(dead_code)]
+
+use x86_64::structures::gdt::{Descriptor, GlobalDescriptorTable};
+use x86_64::registers::segmentation::{CS, SS, Segment, SegmentSelector};
+use x86_64::PrivilegeLevel;
+use spin::Mutex;
+
+static mut GDT: GlobalDescriptorTable = GlobalDescriptorTable::new();
+static GDT_LOCK: Mutex<u64> = Mutex::new(0); // we cannot lock the GDT directly as it must stay in the same place in memory and that behaves weirdly with mutexes
+
+fn add_entry(descriptor: Descriptor)
+{
+    GDT_LOCK.lock();
+    unsafe { GDT.add_entry(descriptor); }
+}
+
+pub fn init()
+{
+    add_entry(Descriptor::kernel_code_segment());
+    add_entry(Descriptor::kernel_data_segment());
+    add_entry(Descriptor::user_code_segment());
+    add_entry(Descriptor::user_data_segment());
+    // TODO: add a TSS.
+}
+
+pub fn load_and_reload_segment_registers()
+{
+    GDT_LOCK.lock();
+    unsafe { 
+        GDT.load();
+        CS::set_reg(SegmentSelector::new(1, PrivilegeLevel::Ring0));
+        SS::set_reg(SegmentSelector::new(2, PrivilegeLevel::Ring0));
+    }
+}

moon/src/arch/x86_64/mod.rs

@@ -2,3 +2,4 @@
 pub mod cpu;
 pub mod io;
 pub mod interrupts;
+mod gdt;

moon/src/main.rs

@@ -25,6 +25,8 @@ pub extern "C" fn _start() -> ! {
     init::halt_other_cores();
     init::check_magic();
 
+    cpu::platform_init();
+
     interrupts::load();
 
     video::clear(Color::White);