feat: implement core initialization and local APIC setup

kernel/include/acpi/discovery.h

@@ -10,7 +10,6 @@ namespace acpi {
 struct discovery_result_t {
     optional<Rsdp> rsdp;
     optional<Madt> madt;
-    // TODO(wiring): capture the FADT in setupAcpi and store it here
     optional<Fadt> fadt;
 };
 

kernel/include/apic/core.h

@@ -2,6 +2,28 @@
 
 #include "memory/pointer.h"
 
+#include "acpi/discovery.h"
+#include "memory/pagetable.h"
+
 namespace apic {
-void startupCore(paddr_t code, paddr_t stack);
-}
+
+struct stack_t {
+    paddr_t ptr;
+    size_t size;
+};
+
+using core_id = uint8_t;
+
+struct Core {
+    core_id id;
+    stack_t stack;
+    PageTable kernelPageTable;
+};
+
+optional<Core> initCore(acpi::Madt madt, core_id id);
+
+void initCores(acpi::discovery_result_t);
+
+void transitionStartupCore();
+
+} // namespace apic

kernel/include/apic/lapic.h

@@ -0,0 +1,24 @@
+#pragma once
+
+#include "acpi/discovery.h"
+#include "util/number.h"
+
+namespace apic {
+
+using lapic_id_t = uint32_t;
+
+void initLocalApic(const acpi::discovery_result_t& discovery);
+
+lapic_id_t localApicId();
+
+void sendInit(lapic_id_t destination);
+
+// startup_page: physical address of the real-mode entry point divided by 4096;
+// must point below 1 MiB, so only values < 0x100 are meaningful.
+void sendStartup(lapic_id_t destination, uint8_t startup_page);
+
+void sendInterrupt(lapic_id_t destination, uint8_t vector);
+
+void signalEndOfInterrupt();
+
+} // namespace apic

kernel/include/apic/timer.h

@@ -33,6 +33,26 @@ constexpr duration_t operator*(uint64_t scalar, duration_t duration) {
     return duration_t{duration.timer_count * scalar};
 }
 
+constexpr bool operator==(duration_t left, duration_t right) {
+    return left.timer_count == right.timer_count;
+}
+
+constexpr bool operator<(duration_t left, duration_t right) {
+    return left.timer_count < right.timer_count;
+}
+
+constexpr bool operator<=(duration_t left, duration_t right) {
+    return left.timer_count <= right.timer_count;
+}
+
+constexpr bool operator>(duration_t left, duration_t right) {
+    return left.timer_count > right.timer_count;
+}
+
+constexpr bool operator>=(duration_t left, duration_t right) {
+    return left.timer_count >= right.timer_count;
+}
+
 constexpr timestamp_t operator+(timestamp_t timestamp, duration_t duration) {
     return timestamp_t{timestamp.timer_count + duration.timer_count};
 }
@@ -62,9 +82,4 @@ void busyWait(duration_t duration);
 
 timestamp_t now();
 
-// TODO: when dynamic allocation and lists are available
-// struct Timer {};
-//
-// Timer getCoreTimerInstance();
-
 } // namespace apic

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);

kernel/include/main.h

@@ -0,0 +1,3 @@
+#pragma once
+
+[[noreturn]] void main();

kernel/include/memory/allocator.h

@@ -12,10 +12,17 @@ struct PhysicalAllocator {
         size_t page_count;
     };
 
+    enum class address_range_t : uint8_t {
+        any,
+        below_4gib,
+    };
+
     static void init(const multiboot::tag_mmap* multiboot_info);
     static PhysicalAllocator& getInstance();
 
-    optional<allocation_t> allocPages(size_t page_count, size_t alignment_pages = 1);
+    optional<allocation_t> allocPages(
+        size_t page_count, size_t alignment_pages = 1, address_range_t range = address_range_t::any
+    );
     void free(allocation_t ptr);
     bool force(allocation_t ptr, bool free);
 

kernel/include/memory/pagetable.h

@@ -25,12 +25,12 @@ struct PageTable {
     };
 
     static PageTable fromCurrent();
+    static optional<PageTable> allocNew();
 
     void activate();
     bool map(void* vaddress, paddr_t paddress, flags_t flags);
     void unmap(void* vaddress);
     optional<info_t> get(void* vaddress);
 
-private:
     paddr_t l4;
 };

kernel/include/util/msr.h

@@ -0,0 +1,16 @@
+#pragma once
+
+#include "util/number.h"
+
+inline uint64_t msr_read(uint32_t msr) {
+    uint32_t low = 0;
+    uint32_t high = 0;
+    __asm__ volatile("rdmsr" : "=a"(low), "=d"(high) : "c"(msr));
+    return (static_cast<uint64_t>(high) << 32) | low;
+}
+
+inline void msr_write(uint32_t msr, uint64_t value) {
+    uint32_t low = static_cast<uint32_t>(value);
+    uint32_t high = static_cast<uint32_t>(value >> 32);
+    __asm__ volatile("wrmsr" : : "c"(msr), "a"(low), "d"(high));
+}

kernel/include/util/optional.h

@@ -131,7 +131,7 @@ public:
         }
     }
 
-    operator bool() const {
+    explicit operator bool() const {
         return initialized;
     }
 

kernel/src/apic/core.cpp

@@ -0,0 +1,155 @@
+#include "apic/core.h"
+#include "apic/lapic.h"
+#include "apic/timer.h"
+#include "cpu/gdt.h"
+#include "memory/allocator.h"
+
+static volatile gdt_register_t secondary_gdt_register;
+
+static optional<Gdt> createSecondaryGdt() {
+    optional<Gdt> gdtOpt = Gdt::allocNew();
+    if (!gdtOpt.has_value()) {
+        return {};
+    }
+    Gdt gdt = gdtOpt.value();
+    Gdt::segment_t codeSegment{};
+    codeSegment.flags.type = Gdt::segment_type_t::code;
+    codeSegment.flags.privilege_level = 0;
+    codeSegment.flags.long_mode = true;
+    codeSegment.flags.readable_writeable = true;
+    Gdt::segment_t dataSegment{};
+    dataSegment.flags.type = Gdt::segment_type_t::data;
+    dataSegment.flags.privilege_level = 0;
+    dataSegment.flags.readable_writeable = true;
+    optional<Gdt::selector_t> codeSelector = gdt.add(codeSegment);
+    optional<Gdt::selector_t> dataSelector = gdt.add(dataSegment);
+    if (codeSelector.value_or(0) != 0x08 || dataSelector.value_or(0) != 0x10) {
+        return {};
+    }
+    return gdt;
+}
+
+static bool waitForStartedInfo(volatile uint64_t* started_info, uint64_t expected, apic::duration_t timeout) {
+    auto start = apic::now();
+    while (*started_info < expected) {
+        if (apic::now() - start > timeout) {
+            return false;
+        }
+        __asm__ volatile("pause");
+    }
+    return true;
+}
+
+bool sendStartupWithTimeout(uint8_t apicId, uint8_t page, volatile uint64_t* started_info) {
+    apic::sendStartup(apicId, page);
+    return waitForStartedInfo(started_info, 1, apic::milliseconds(10));
+}
+
+bool startupCore(uint8_t apicId, paddr_t code, paddr_t stack, PageTable pageTable, Gdt gdt) {
+    auto started_info_s = read_symbol("__started_info");
+    auto stack_ptr_s = read_symbol("__stack_ptr");
+    auto page_table_ptr_s = read_symbol("__page_table_ptr");
+    auto gdt_addr_ptr_s = read_symbol("__gdt_addr_ptr");
+    auto started_info = paddr_t{started_info_s}.access<volatile uint64_t>();
+    auto stack_ptr = paddr_t{stack_ptr_s}.access<volatile uint64_t>();
+    auto page_table_ptr = paddr_t{page_table_ptr_s}.access<volatile uint64_t>();
+    auto gdt_addr_ptr = paddr_t{gdt_addr_ptr_s}.access<volatile uint64_t>();
+    secondary_gdt_register.limit = gdt.limit;
+    secondary_gdt_register.base = gdt.base.address;
+    *started_info = 0;
+    *stack_ptr = stack.address;
+    *page_table_ptr = pageTable.l4.address;
+    *gdt_addr_ptr = reinterpret_cast<uint64_t>(&secondary_gdt_register) - high_base;
+    uint8_t page = code.address / page_size;
+    apic::sendInit(apicId);
+    apic::busyWait(apic::milliseconds(10));
+    if (!(sendStartupWithTimeout(apicId, page, started_info) //
+          || sendStartupWithTimeout(apicId, page, started_info))) {
+        return false;
+    }
+    return waitForStartedInfo(started_info, 2, apic::milliseconds(100));
+}
+
+PageTable createPageTable() {
+    PageTable pageTable = PageTable::allocNew().value();
+    PageTable::flags_t cachedFlags = {
+        .writeable = 1,
+        .user_accessible = 0,
+        .write_through = 0,
+        .cache_disabled = 0,
+        .execute_disabled = 0,
+        .global = 0,
+        .granularity = PageTable::flags_t::granularity_t::page_1GiB
+    };
+    for (uint64_t i = 0; i < 256; i++) {
+        auto vaddr = i * (1ull << 30);
+        auto paddr = i * (1ull << 30);
+        pageTable.map(reinterpret_cast<void*>(vaddr), paddr_t{paddr}, cachedFlags);
+    }
+    for (uint64_t i = 0; i < 256; i++) {
+        auto vaddr = i * (1ull << 30) + high_base;
+        auto paddr = i * (1ull << 30);
+        pageTable.map(reinterpret_cast<void*>(vaddr), paddr_t{paddr}, cachedFlags);
+    }
+    PageTable::flags_t uncachedFlags = {
+        .writeable = 1,
+        .user_accessible = 0,
+        .write_through = 1,
+        .cache_disabled = 1,
+        .execute_disabled = 0,
+        .global = 0,
+        .granularity = PageTable::flags_t::granularity_t::page_1GiB
+    };
+    for (uint64_t i = 0; i < 256; i++) {
+        auto vaddr = i * (1ull << 30) + uncached_base;
+        auto paddr = i * (1ull << 30);
+        pageTable.map(reinterpret_cast<void*>(vaddr), paddr_t{paddr}, uncachedFlags);
+    }
+    return pageTable;
+}
+
+optional<apic::Core> apic::initCore(acpi::Madt madt, core_id id) {
+    optional<uint8_t> apicId;
+    madt.for_each_entry([&](const acpi::madt_local_apic_t* apic) {
+        if (apic->acpi_processor_uid == id) {
+            apicId = apic->apic_id;
+        }
+    });
+    if (!apicId.has_value()) {
+        return {};
+    }
+    if (apicId.value() == apic::localApicId()) {
+        return {};
+    }
+    apic::Core core{};
+    size_t stackPageCount = 256;
+    paddr_t stackBase = PhysicalAllocator::getInstance().allocPages(stackPageCount).value().ptr;
+    paddr_t stackPtr = {stackBase.address + stackPageCount * page_size - 8};
+    paddr_t code = {read_symbol("__16bitStartup")};
+    auto pageTable = createPageTable();
+    optional<Gdt> gdt = createSecondaryGdt();
+    if (!gdt.has_value()) {
+        return {};
+    }
+    if (!startupCore(apicId.value(), code, stackPtr, pageTable, gdt.value())) {
+        return {};
+    }
+    return core;
+}
+
+void apic::initCores(acpi::discovery_result_t discovery) {
+    if (!discovery.madt.has_value()) {
+        return;
+    }
+    discovery.madt->for_each_entry([&](const acpi::madt_local_apic_t* apic) {
+        apic::initCore(discovery.madt.value(), apic->acpi_processor_uid);
+    });
+}
+
+void apic::transitionStartupCore() {}
+
+extern "C" [[noreturn]] void __secondary_init() {
+    while (true) {
+        __asm__ volatile("hlt");
+    }
+}

kernel/src/apic/lapic.cpp

@@ -0,0 +1,81 @@
+#include "apic/lapic.h"
+
+#include "memory/pointer.h"
+#include "util/msr.h"
+
+namespace {
+
+constexpr uint32_t apic_base_msr = 0x1B;
+constexpr uint64_t apic_base_enable = uint64_t{1} << 11;
+constexpr uint64_t apic_base_address_mask = 0x000FFFFFFFFFF000;
+
+constexpr size_t id_register = 0x20;
+constexpr size_t eoi_register = 0xB0;
+constexpr size_t spurious_vector_register = 0xF0;
+constexpr size_t icr_low_register = 0x300;
+constexpr size_t icr_high_register = 0x310;
+
+constexpr uint32_t software_enable = 1u << 8;
+constexpr uint32_t spurious_vector = 0xFF;
+
+constexpr uint32_t delivery_mode_fixed = 0b000u << 8;
+constexpr uint32_t delivery_mode_init = 0b101u << 8;
+constexpr uint32_t delivery_mode_startup = 0b110u << 8;
+constexpr uint32_t delivery_status_pending = 1u << 12;
+constexpr uint32_t level_assert = 1u << 14;
+
+volatile uint32_t* lapic_registers = nullptr;
+
+volatile uint32_t& lapicRegister(size_t offset) {
+    return lapic_registers[offset / sizeof(uint32_t)];
+}
+
+void waitWhileDeliveryPending() {
+    while ((lapicRegister(icr_low_register) & delivery_status_pending) != 0) {
+        __asm__ volatile("pause");
+    }
+}
+
+void sendIpi(apic::lapic_id_t destination, uint32_t command) {
+    waitWhileDeliveryPending();
+    lapicRegister(icr_high_register) = destination << 24;
+    lapicRegister(icr_low_register) = command;
+    waitWhileDeliveryPending();
+}
+
+paddr_t localApicPhysicalBase(const acpi::discovery_result_t& discovery) {
+    paddr_t msr_base{msr_read(apic_base_msr) & apic_base_address_mask};
+    return discovery.madt
+        .map([](acpi::Madt madt) {
+            return madt.local_apic_address();
+        })
+        .value_or(msr_base);
+}
+
+} // namespace
+
+void apic::initLocalApic(const acpi::discovery_result_t& discovery) {
+    msr_write(apic_base_msr, msr_read(apic_base_msr) | apic_base_enable);
+    lapic_registers = localApicPhysicalBase(discovery).access_uc<volatile uint32_t>();
+    lapicRegister(spurious_vector_register) = spurious_vector | software_enable;
+}
+
+apic::lapic_id_t apic::localApicId() {
+    return lapicRegister(id_register) >> 24;
+}
+
+void apic::sendInit(lapic_id_t destination) {
+    sendIpi(destination, delivery_mode_init | level_assert);
+}
+
+void apic::sendStartup(lapic_id_t destination, uint8_t startup_page) {
+    sendIpi(destination, delivery_mode_startup | level_assert | startup_page);
+}
+
+void apic::sendInterrupt(lapic_id_t destination, uint8_t vector) {
+    sendIpi(destination, delivery_mode_fixed | level_assert | vector);
+}
+
+void apic::signalEndOfInterrupt() {
+    lapicRegister(eoi_register) = 0;
+}

kernel/src/apic/timer.cpp

@@ -11,11 +11,11 @@ constexpr uint32_t processor_frequency_leaf = 0x16;
 
 constexpr uint32_t feature_edx_tsc = 1u << 4;
 
-struct lapic_timer_calibration_t {
+struct timer_calibration_t {
     uint64_t tsc_frequency_hz;
 };
 
-lapic_timer_calibration_t timer_calibration{};
+timer_calibration_t timer_calibration{};
 
 bool isTscSupported() {
     return cpuid(feature_info_leaf)

kernel/src/cpu/gdt.cpp

@@ -0,0 +1,218 @@
+#include "cpu/gdt.h"
+#include "memory/allocator.h"
+
+struct segment_descriptor_t {
+    uint64_t limit_low : 16;
+    uint64_t base_low : 24;
+    uint64_t accessed : 1;
+    uint64_t readable_writeable : 1;
+    uint64_t conforming_expand_down : 1;
+    uint64_t executable : 1;
+    uint64_t user_segment : 1;
+    uint64_t privilege_level : 2;
+    uint64_t present : 1;
+    uint64_t limit_high : 4;
+    uint64_t available : 1;
+    uint64_t long_mode : 1;
+    uint64_t default_operand_32bit : 1;
+    uint64_t granularity_4KiB : 1;
+    uint64_t base_high : 8;
+};
+static_assert(sizeof(segment_descriptor_t) == sizeof(uint64_t));
+
+struct system_descriptor_t {
+    uint64_t limit_low : 16;
+    uint64_t base_low : 24;
+    uint64_t type : 4;
+    uint64_t user_segment : 1;
+    uint64_t privilege_level : 2;
+    uint64_t present : 1;
+    uint64_t limit_high : 4;
+    uint64_t available : 1;
+    uint64_t reserved_long_mode : 1;
+    uint64_t reserved_default_operand_32bit : 1;
+    uint64_t granularity_4KiB : 1;
+    uint64_t base_high : 8;
+    uint64_t base_upper : 32;
+    uint64_t reserved : 32;
+};
+static_assert(sizeof(system_descriptor_t) == 2 * sizeof(uint64_t));
+
+constexpr uint64_t task_state_available_type = 0b1001;
+constexpr uint64_t task_state_busy_type = 0b1011;
+
+static size_t entry_count_of(uint16_t limit) {
+    return (static_cast<size_t>(limit) + 1) / sizeof(uint64_t);
+}
+
+static bool is_present(uint64_t entry) {
+    const auto* descriptor = reinterpret_cast<const segment_descriptor_t*>(&entry);
+    return descriptor->present;
+}
+
+static bool is_task_state_low_half(uint64_t entry) {
+    const auto* descriptor = reinterpret_cast<const system_descriptor_t*>(&entry);
+    if (!descriptor->present) {
+        return false;
+    }
+    if (descriptor->user_segment) {
+        return false;
+    }
+    return descriptor->type == task_state_available_type || descriptor->type == task_state_busy_type;
+}
+
+static segment_descriptor_t encode_segment(Gdt::segment_t segment) {
+    segment_descriptor_t descriptor{};
+    descriptor.limit_low = segment.limit & 0xFFFF;
+    descriptor.limit_high = (segment.limit >> 16) & 0xF;
+    descriptor.base_low = segment.base & 0xFFFFFF;
+    descriptor.base_high = (segment.base >> 24) & 0xFF;
+    descriptor.readable_writeable = segment.flags.readable_writeable;
+    descriptor.executable = segment.flags.type == Gdt::segment_type_t::code;
+    descriptor.user_segment = 1;
+    descriptor.privilege_level = segment.flags.privilege_level;
+    descriptor.present = 1;
+    descriptor.long_mode = segment.flags.long_mode;
+    descriptor.default_operand_32bit = segment.flags.default_operand_32bit;
+    descriptor.granularity_4KiB = segment.flags.granularity_4KiB;
+    return descriptor;
+}
+
+static system_descriptor_t encode_task_state(Gdt::segment_t segment) {
+    system_descriptor_t descriptor{};
+    descriptor.limit_low = segment.limit & 0xFFFF;
+    descriptor.limit_high = (segment.limit >> 16) & 0xF;
+    descriptor.base_low = segment.base & 0xFFFFFF;
+    descriptor.base_high = (segment.base >> 24) & 0xFF;
+    descriptor.base_upper = segment.base >> 32;
+    descriptor.type = task_state_available_type;
+    descriptor.user_segment = 0;
+    descriptor.privilege_level = segment.flags.privilege_level;
+    descriptor.present = 1;
+    descriptor.granularity_4KiB = segment.flags.granularity_4KiB;
+    return descriptor;
+}
+
+static Gdt::segment_t decode_task_state(const system_descriptor_t* descriptor) {
+    Gdt::segment_t segment{};
+    segment.flags.type = Gdt::segment_type_t::task_state;
+    segment.flags.privilege_level = descriptor->privilege_level;
+    segment.flags.granularity_4KiB = descriptor->granularity_4KiB;
+    segment.base = static_cast<uint64_t>(descriptor->base_low) | (static_cast<uint64_t>(descriptor->base_high) << 24) |
+                   (static_cast<uint64_t>(descriptor->base_upper) << 32);
+    segment.limit = static_cast<uint32_t>(descriptor->limit_low | (descriptor->limit_high << 16));
+    return segment;
+}
+
+static Gdt::segment_t decode_segment(const segment_descriptor_t* descriptor) {
+    Gdt::segment_t segment{};
+    segment.flags.type = descriptor->executable ? Gdt::segment_type_t::code : Gdt::segment_type_t::data;
+    segment.flags.privilege_level = descriptor->privilege_level;
+    segment.flags.long_mode = descriptor->long_mode;
+    segment.flags.default_operand_32bit = descriptor->default_operand_32bit;
+    segment.flags.granularity_4KiB = descriptor->granularity_4KiB;
+    segment.flags.readable_writeable = descriptor->readable_writeable;
+    segment.base = static_cast<uint64_t>(descriptor->base_low) | (static_cast<uint64_t>(descriptor->base_high) << 24);
+    segment.limit = static_cast<uint32_t>(descriptor->limit_low | (descriptor->limit_high << 16));
+    return segment;
+}
+
+Gdt Gdt::fromCurrent() {
+    gdt_register_t gdt_register{};
+    asm volatile("sgdt %0" : "=m"(gdt_register));
+    Gdt gdt{};
+    gdt.limit = gdt_register.limit;
+    if (gdt_register.base >= high_base) {
+        gdt.base = paddr_t{gdt_register.base - high_base};
+    } else {
+        gdt.base = paddr_t{gdt_register.base};
+    }
+    return gdt;
+}
+
+optional<Gdt> Gdt::allocNew() {
+    auto allocation =
+        PhysicalAllocator::getInstance().allocPages(1, 1, PhysicalAllocator::address_range_t::below_4gib);
+    if (!allocation.has_value()) {
+        return {};
+    }
+    memset(allocation->ptr.access<uint8_t>(), 0, page_size);
+    Gdt gdt{};
+    gdt.base = allocation->ptr;
+    gdt.limit = page_size - 1;
+    return gdt;
+}
+
+void Gdt::activate() {
+    gdt_register_t gdt_register{};
+    gdt_register.limit = limit;
+    gdt_register.base = reinterpret_cast<uint64_t>(base.access<uint8_t>());
+    asm volatile("lgdt %0" : : "m"(gdt_register));
+}
+
+optional<Gdt::selector_t> Gdt::add(segment_t segment) {
+    uint64_t* entries = base.access<uint64_t>();
+    size_t entry_count = entry_count_of(limit);
+    size_t needed_slots = segment.flags.type == segment_type_t::task_state ? 2 : 1;
+    size_t index = 1;
+    while (index + needed_slots <= entry_count) {
+        if (is_task_state_low_half(entries[index])) {
+            index += 2;
+            continue;
+        }
+        if (is_present(entries[index])) {
+            index += 1;
+            continue;
+        }
+        if (needed_slots == 2 && is_present(entries[index + 1])) {
+            index += 1;
+            continue;
+        }
+        if (segment.flags.type == segment_type_t::task_state) {
+            *reinterpret_cast<system_descriptor_t*>(&entries[index]) = encode_task_state(segment);
+        } else {
+            *reinterpret_cast<segment_descriptor_t*>(&entries[index]) = encode_segment(segment);
+        }
+        return static_cast<selector_t>(index * sizeof(uint64_t));
+    }
+    return {};
+}
+
+optional<Gdt::segment_t> Gdt::get(selector_t selector) {
+    size_t index = selector / sizeof(uint64_t);
+    size_t entry_count = entry_count_of(limit);
+    if (index == 0 || index >= entry_count) {
+        return {};
+    }
+    uint64_t* entries = base.access<uint64_t>();
+    if (is_task_state_low_half(entries[index])) {
+        if (index + 2 > entry_count) {
+            return {};
+        }
+        return decode_task_state(reinterpret_cast<const system_descriptor_t*>(&entries[index]));
+    }
+    if (!is_present(entries[index])) {
+        return {};
+    }
+    return decode_segment(reinterpret_cast<const segment_descriptor_t*>(&entries[index]));
+}
+
+void reloadSegments(Gdt::selector_t code_selector, Gdt::selector_t data_selector) {
+    uint64_t code = code_selector;
+    uint16_t data = data_selector;
+    asm volatile(
+        "pushq %[code]\n"
+        "leaq 1f(%%rip), %%rax\n"
+        "pushq %%rax\n"
+        "lretq\n"
+        "1:\n"
+        "mov %[data], %%ds\n"
+        "mov %[data], %%es\n"
+        "mov %[data], %%fs\n"
+        "mov %[data], %%gs\n"
+        "mov %[data], %%ss\n"
+        :
+        : [code] "r"(code), [data] "r"(data)
+        : "rax", "memory"
+    );
+}

kernel/src/init/init.cpp

@@ -3,6 +3,8 @@
 #include "acpi/discovery.h"
 #include "acpi/fadt.h"
 #include "acpi/madt.h"
+#include "apic/core.h"
+#include "apic/lapic.h"
 #include "apic/timer.h"
 #include "init/multiboot2.h"
 #include "init/print.h"
@@ -211,31 +213,18 @@ static void calibrateTimer(acpi::discovery_result_t acpi) {
     }
 }
 
-static void demonstateTimer() {
-    auto time1 = apic::now();
-    print("Time 1: ");
-    print_hex(time1.timer_count);
-    print("\nTime 2: ");
-    auto time2 = apic::now();
-    print_hex(time2.timer_count);
-    print("\n");
-    auto duration = time2 - time1;
-    auto readableDuration = apic::toSecondsAndNanoseconds(duration);
-    print_dec(readableDuration.seconds);
-    print("s ");
-    print_dec(readableDuration.nanoseconds);
-    print("ns\n");
-    print("Wait for 5s!\n");
-    apic::busyWait(apic::seconds(5));
-    print("Waited for 5s!\n");
-}
-
 [[maybe_unused]] [[noreturn]] __attribute__((used)) void init() {
     initFromLow();
     print("Reached init\n");
     step("memory", setupMemory);
     auto acpi = step("acpi", setupAcpi);
+    step("lapic", [&]() {
+        apic::initLocalApic(acpi);
+    });
+    print("BSP local APIC id: ");
+    print_dec(apic::localApicId());
+    print("\n");
     step("timer", partial(calibrateTimer, acpi));
-    demonstateTimer();
+    step("cpu startup", partial(apic::initCores, acpi));
     halt();
 }

kernel/src/main.cpp

@@ -0,0 +1,7 @@
+#include "main.h"
+
+[[noreturn]] void main() {
+    while (true) {
+        __asm__ volatile("hlt");
+    }
+}

kernel/src/memory/allocator.cpp

@@ -286,7 +286,7 @@ struct PhysicalAllocatorImpl {
         return true;
     }
 
-    optional<allocation_t> allocPages(size_t page_count, size_t alignment_pages) {
+    optional<allocation_t> allocPages(size_t page_count, size_t alignment_pages, PhysicalAllocator::address_range_t range) {
         if (page_count == 0) {
             return {};
         }
@@ -297,6 +297,11 @@ struct PhysicalAllocatorImpl {
             return {};
         }
 
+        uint64_t page_limit = ~uint64_t{0};
+        if (range == PhysicalAllocator::address_range_t::below_4gib) {
+            page_limit = (uint64_t{1} << 32) / page_size;
+        }
+
         Cursor cursor = cursor_at(0);
         uint64_t run_start = 0;
         size_t i = 0;
@@ -306,9 +311,16 @@ struct PhysicalAllocatorImpl {
             if (length == 0 && prev_zero) {
                 break;
             }
+            if (run_start >= page_limit) {
+                break;
+            }
             if ((i & 1) && length > 0) {
+                uint64_t run_end = run_start + length;
+                if (run_end > page_limit) {
+                    run_end = page_limit;
+                }
                 uint64_t aligned_start = ((run_start + alignment_pages - 1) / alignment_pages) * alignment_pages;
-                if (aligned_start + page_count <= run_start + length) {
+                if (aligned_start + page_count <= run_end) {
                     set_range(aligned_start, page_count, false);
                     reclaim_nodes();
                     return allocation_t{paddr_t{aligned_start * page_size}, page_count};
@@ -331,8 +343,9 @@ bool PhysicalAllocator::force(allocation_t ptr, bool free) {
     return PhysicalAllocatorImpl{*this}.force(ptr, free);
 }
 
-optional<PhysicalAllocator::allocation_t> PhysicalAllocator::allocPages(size_t page_count, size_t alignment_pages) {
-    return PhysicalAllocatorImpl{*this}.allocPages(page_count, alignment_pages);
+optional<PhysicalAllocator::allocation_t>
+PhysicalAllocator::allocPages(size_t page_count, size_t alignment_pages, address_range_t range) {
+    return PhysicalAllocatorImpl{*this}.allocPages(page_count, alignment_pages, range);
 }
 
 void PhysicalAllocator::free(allocation_t ptr) {

kernel/src/memory/pagetable.cpp

@@ -129,7 +129,8 @@ static optional<frame_entry_t*> walk(level1_table_t* table, void* vaddress, Fn&&
 struct ascend_t {};
 
 template<typename NextEntryType, typename Fn>
-static optional<frame_entry_t*> walk(entry_table_t<table_or_huge_entry_t<NextEntryType>>* table, void* vaddress, Fn&& fn) {
+static optional<frame_entry_t*>
+walk(entry_table_t<table_or_huge_entry_t<NextEntryType>>* table, void* vaddress, Fn&& fn) {
     using entry_t = table_or_huge_entry_t<NextEntryType>;
     entry_t& entry = table->entries[table_index<entry_t::level>(vaddress)];
     if constexpr (requires { fn(entry); }) {
@@ -176,7 +177,8 @@ static void reclaim_if_empty(table_or_huge_entry_t<NextEntryType>& entry) {
     if (entry.subtable.huge_page) {
         return;
     }
-    entry_table_t<NextEntryType>* child_table = address_of(entry.subtable).template access<entry_table_t<NextEntryType>>();
+    entry_table_t<NextEntryType>* child_table =
+        address_of(entry.subtable).template access<entry_table_t<NextEntryType>>();
     if (!is_table_empty(child_table)) {
         return;
     }
@@ -269,6 +271,18 @@ PageTable PageTable::fromCurrent() {
     return table;
 }
 
+optional<PageTable> PageTable::allocNew() {
+    auto alloc = PhysicalAllocator::getInstance().allocPages(1, 1, PhysicalAllocator::address_range_t::below_4gib);
+    if (!alloc.has_value()) {
+        return {};
+    }
+    auto addr = alloc->ptr;
+    memset(addr.access<uint8_t>(), 0, page_size);
+    PageTable table{};
+    table.l4 = addr;
+    return table;
+}
+
 bool PageTable::map(void* vaddress, paddr_t paddress, flags_t flags) {
     size_t target_level = target_level_of(flags.granularity);
     bool mapped = false;

kernel/src/startup/secondaryStartup.cpp

@@ -1,5 +1,90 @@
 asm(R"(
-.section .trampoline_text
+.global __16bitStartup
+.global __started_info
+.global __stack_ptr
+.global __page_table_ptr
+.global __gdt_addr_ptr
+
+.extern __secondary_init
+
+.section .trampoline_text, "ax"
 .align 0x1000
 .code16
-)");
+__16bitStartup:
+    cli
+    movw $1, __started_info
+    mov $__tmp_gdt_ptr, %eax
+    lgdtl (%eax)
+    mov %cr0, %eax
+    or $1, %eax
+    mov %eax, %cr0
+    ljmp $0x08, $__tmp32BitStartup
+stop:
+    hlt
+    jmp stop
+
+.code32
+__tmp32BitStartup:
+    mov $0x10, %eax
+    mov %ax, %ds
+    mov %ax, %es
+    mov %ax, %fs
+    mov %ax, %gs
+    mov %ax, %ss
+
+    mov __page_table_ptr, %eax
+    mov %eax, %cr3
+
+    mov %cr4, %eax
+    or $(0x1 << 5), %eax
+    mov %eax, %cr4
+
+    mov $0xC0000080, %ecx
+    rdmsr
+    or $((0x1 << 8) | (0x1 << 11)), %eax
+    wrmsr
+
+    mov %cr0, %eax
+    or $(0x1 << 31), %eax
+    mov %eax, %cr0
+
+    mov $__gdt_addr_ptr, %eax
+    mov (%eax), %eax
+    lgdt (%eax)
+    jmp $0b1000, $__tmp64BitStartup
+.code64
+__tmp64BitStartup:
+    mov $0b10000, %ax
+    mov %ax, %ds
+    mov %ax, %es
+    mov %ax, %fs
+    mov %ax, %gs
+    mov %ax, %ss
+    mov __stack_ptr, %rsp
+    movq $2, __started_info
+    movabsq $__secondary_init, %rax
+    jmp* %rax
+
+.section .trampoline_data, "aw"
+.align 0x1000
+__started_info:
+.quad 0 # write here 1 as soon as started, 2 as soon as in 64bit mode and all tmp resources released
+__stack_ptr:
+.quad 0 # pointer to top of stack
+__page_table_ptr:
+.quad 0 # value of l4 page table
+__gdt_addr_ptr:
+.quad 0 # pointer to gdt descriptor (limit word + base) to load before entering 64 bit mode
+__tmp_stack:
+    .skip 0x1000
+    .align 8
+__tmp_stack_end:
+    .quad 0
+__tmp_gdt:
+    .quad 0 # zero
+    .quad (0xFFFF) | (0xF << 48) | (1 << 43) | (1 << 44) | (1 << 47) | (1 << 54) | (1 << 55) # 32 bit code
+    .quad (0xFFFF) | (0xF << 48) | (1 << 41) | (1 << 44) | (1 << 47) | (1 << 54) | (1 << 55) # 32 bit data
+__tmp_gdt_ptr:
+    .word __tmp_gdt_ptr - __tmp_gdt - 1
+    .long __tmp_gdt
+)");