rust-kernel

//! Interrupt handling: the IDT, CPU exception handlers and hardware IRQs.
//!
//! The IDT (Interrupt Descriptor Table) maps each interrupt vector number to
//! a handler function:
//!   * vectors 0..=31  are CPU exceptions (divide error, page fault, ...)
//!   * vectors 32..=47 are where we remap the 8259 PIC's 16 hardware IRQs
//!
//! Handlers use the special `extern "x86-interrupt"` calling convention: the
//! compiler saves/restores all registers and returns with `iretq`.

use crate::{gdt, println, serial_println};
use core::sync::atomic::{AtomicU64, Ordering};
use lazy_static::lazy_static;
use pic8259::ChainedPics;
use x86_64::structures::idt::{InterruptDescriptorTable, InterruptStackFrame, PageFaultErrorCode};

// ---------------------------------------------------------------------------
// 8259 PIC (Programmable Interrupt Controller)
// ---------------------------------------------------------------------------

/// The PICs deliver IRQ 0..15 on vectors 0..15 by default, which clash with
/// CPU exceptions. We remap them to 32..47.
pub const PIC_1_OFFSET: u8 = 32;
pub const PIC_2_OFFSET: u8 = PIC_1_OFFSET + 8;

/// Two PICs: primary handles IRQ 0-7, secondary IRQ 8-15 (chained via IRQ 2).
pub static PICS: spin::Mutex<ChainedPics> =
    spin::Mutex::new(unsafe { ChainedPics::new(PIC_1_OFFSET, PIC_2_OFFSET) });

/// Vector numbers of the hardware interrupts we handle.
#[derive(Debug, Clone, Copy)]
#[repr(u8)]
pub enum InterruptIndex {
    Timer = PIC_1_OFFSET, // IRQ 0: Programmable Interval Timer, ~18.2 Hz by default
    Keyboard,             // IRQ 1: PS/2 keyboard
}

impl InterruptIndex {
    fn as_u8(self) -> u8 {
        self as u8
    }
}

// ---------------------------------------------------------------------------
// IDT setup
// ---------------------------------------------------------------------------

lazy_static! {
    /// The IDT must live forever (`'static`): the CPU keeps a pointer to it.
    static ref IDT: InterruptDescriptorTable = {
        let mut idt = InterruptDescriptorTable::new();
        idt.breakpoint.set_handler_fn(breakpoint_handler);
        idt.page_fault.set_handler_fn(page_fault_handler);
        idt.general_protection_fault.set_handler_fn(general_protection_fault_handler);
        idt.invalid_opcode.set_handler_fn(invalid_opcode_handler);
        unsafe {
            // Run the double-fault handler on its own known-good stack (IST).
            idt.double_fault
                .set_handler_fn(double_fault_handler)
                .set_stack_index(gdt::DOUBLE_FAULT_IST_INDEX);
        }
        idt[InterruptIndex::Timer.as_u8()].set_handler_fn(timer_interrupt_handler);
        idt[InterruptIndex::Keyboard.as_u8()].set_handler_fn(keyboard_interrupt_handler);
        idt
    };
}

/// Load the IDT into the CPU (`lidt`).
pub fn init_idt() {
    IDT.load();
}

// ---------------------------------------------------------------------------
// CPU exception handlers
// ---------------------------------------------------------------------------

/// `int3` (what debuggers use). Harmless: print and carry on.
extern "x86-interrupt" fn breakpoint_handler(stack_frame: InterruptStackFrame) {
    println!("EXCEPTION: BREAKPOINT");
    serial_println!("EXCEPTION: BREAKPOINT\n{:#?}", stack_frame);
}

/// Did the exception happen in user mode (ring 3)? Then it's the process's
/// fault, not the kernel's: kill the process and keep the kernel running.
/// The CPU puts the interrupted code's CS in the frame; its low 2 bits are
/// the privilege level the code ran at.
fn kill_if_user(stack_frame: &InterruptStackFrame, what: &'static str) {
    use x86_64::PrivilegeLevel;
    if stack_frame.code_segment.rpl() == PrivilegeLevel::Ring3 {
        serial_println!(
            "[kernel] {} in user mode at rip {:#x}",
            what,
            stack_frame.instruction_pointer.as_u64()
        );
        crate::process::exit_current(crate::process::ExitStatus::Killed(what)); // never returns
    }
}

/// General protection fault: e.g. a privileged instruction (`hlt`, `cli`)
/// in ring 3, or a bad segment selector.
extern "x86-interrupt" fn general_protection_fault_handler(
    stack_frame: InterruptStackFrame,
    error_code: u64,
) {
    kill_if_user(&stack_frame, "general protection fault");
    panic!("EXCEPTION: GENERAL PROTECTION FAULT (error {:#x})\n{:#?}", error_code, stack_frame);
}

/// Invalid opcode (`ud2`, or garbage executed as code).
extern "x86-interrupt" fn invalid_opcode_handler(stack_frame: InterruptStackFrame) {
    kill_if_user(&stack_frame, "invalid opcode");
    panic!("EXCEPTION: INVALID OPCODE\n{:#?}", stack_frame);
}

/// Page fault: accessing unmapped/protected memory. CR2 holds the bad address,
/// the error code says why (not present? write? user mode?). A real OS would
/// load the page from disk / swap here; we kill a faulting user process, and
/// stop if the kernel itself faulted.
extern "x86-interrupt" fn page_fault_handler(
    stack_frame: InterruptStackFrame,
    error_code: PageFaultErrorCode,
) {
    use x86_64::registers::control::Cr2;
    if error_code.contains(PageFaultErrorCode::USER_MODE) {
        serial_println!("[kernel] user page fault: address {:?}, error {:?}", Cr2::read(), error_code);
        kill_if_user(&stack_frame, "page fault");
    }
    println!("EXCEPTION: PAGE FAULT at {:?}", Cr2::read());
    serial_println!(
        "EXCEPTION: PAGE FAULT\nAccessed address: {:?}\nError code: {:?}\n{:#?}",
        Cr2::read(),
        error_code,
        stack_frame
    );

    // In the deliberate page-fault demo, reaching here means success.
    #[cfg(all(feature = "headless", feature = "page-fault"))]
    {
        serial_println!("[ok] page fault was caught");
        crate::exit_qemu(crate::QemuExitCode::Success);
    }

    crate::hlt_loop();
}

/// Double fault: an exception happened while handling another one. Must not
/// return (hence `-> !`). Runs on the IST stack set up in gdt.rs.
extern "x86-interrupt" fn double_fault_handler(
    stack_frame: InterruptStackFrame,
    _error_code: u64, // always 0 for double faults
) -> ! {
    serial_println!("EXCEPTION: DOUBLE FAULT\n{:#?}", stack_frame);

    // In the deliberate stack-overflow test, reaching here means success.
    #[cfg(all(feature = "headless", feature = "stack-overflow"))]
    {
        serial_println!("[ok] stack overflow was caught by the double fault handler");
        crate::exit_qemu(crate::QemuExitCode::Success);
    }

    panic!("EXCEPTION: DOUBLE FAULT");
}

// ---------------------------------------------------------------------------
// Hardware interrupt handlers
// ---------------------------------------------------------------------------

/// Timer ticks since boot. Atomic, so it's safe to touch from an interrupt.
pub static TICKS: AtomicU64 = AtomicU64::new(0);

/// IRQ 0. Fires ~18.2 times per second (the PIT's power-on default).
extern "x86-interrupt" fn timer_interrupt_handler(_stack_frame: InterruptStackFrame) {
    let ticks = TICKS.fetch_add(1, Ordering::Relaxed) + 1;

    // Show uptime in the top-right corner of the screen, updated each second.
    if ticks % 18 == 0 {
        crate::vga_buffer::show_uptime(ticks / 18);
    }

    // Headless runs: print a dot per tick to serial (proof the timer fires),
    // then stop after ~5 seconds so scripts finish on their own. (Not done in
    // normal runs, so the dots don't get mixed into your keyboard echo.)
    #[cfg(feature = "headless")]
    {
        crate::serial_print!(".");
        if ticks % 18 == 0 {
            serial_println!(" {} ticks", ticks);
        }
        #[cfg(not(any(feature = "stack-overflow", feature = "page-fault")))]
        if ticks >= 90 {
            serial_println!("[ok] timer works, exiting");
            crate::exit_qemu(crate::QemuExitCode::Success);
        }
    }

    // Tell the PIC we're done, or it will never send another timer interrupt.
    // Must happen *before* switching threads: the thread we switch to might
    // be brand new and never return through this handler.
    unsafe {
        PICS.lock().notify_end_of_interrupt(InterruptIndex::Timer.as_u8());
    }

    // Preemption: maybe switch to another kernel thread (see thread.rs).
    crate::thread::schedule();
}

/// IRQ 1. The keyboard controller has a scancode waiting at I/O port 0x60.
/// We must read it (or no more keyboard interrupts arrive), then just queue
/// it for the async keyboard task (task/keyboard.rs) to decode and print.
extern "x86-interrupt" fn keyboard_interrupt_handler(_stack_frame: InterruptStackFrame) {
    use x86_64::instructions::port::Port;

    let mut port = Port::new(0x60);
    let scancode: u8 = unsafe { port.read() };
    crate::task::keyboard::add_scancode(scancode);

    unsafe {
        PICS.lock().notify_end_of_interrupt(InterruptIndex::Keyboard.as_u8());
    }
}

// Runs under `cargo test`: if the handler were missing, this would fault.
#[test_case]
fn test_breakpoint_exception() {
    x86_64::instructions::interrupts::int3();
}