feat: implement core initialization and local APIC setup

kernel/include/cpu/gdt.h

@@ -0,0 +1,58 @@
+#pragma once
+
+#include "memory/pointer.h"
+#include "util/optional.h"
+
+struct [[gnu::packed]] gdt_register_t {
+    uint16_t limit;
+    uint64_t base;
+};
+static_assert(sizeof(gdt_register_t) == 10);
+
+struct Gdt {
+    using selector_t = uint16_t;
+
+    enum class segment_type_t : uint8_t {
+        code,
+        data,
+        task_state,
+    };
+
+    struct flags_t {
+        segment_type_t type;
+        uint8_t privilege_level;
+        bool long_mode;
+        bool default_operand_32bit;
+        bool granularity_4KiB;
+        bool readable_writeable;
+    };
+
+    struct segment_t {
+        flags_t flags;
+        uint64_t base;
+        uint32_t limit;
+    };
+
+    static Gdt fromCurrent();
+    static optional<Gdt> allocNew();
+
+    void activate();
+    optional<selector_t> add(segment_t segment);
+    optional<segment_t> get(selector_t selector);
+
+    paddr_t base;
+    uint16_t limit;
+};
+
+struct [[gnu::packed]] tss_t {
+    uint32_t reserved0;
+    uint64_t rsp[3];
+    uint64_t reserved1;
+    uint64_t ist[7];
+    uint64_t reserved2;
+    uint16_t reserved3;
+    uint16_t iomap_base;
+};
+static_assert(sizeof(tss_t) == 104);
+
+void reloadSegments(Gdt::selector_t code_selector, Gdt::selector_t data_selector);