//! GDT (Global Descriptor Table) + TSS (Task State Segment).
//!
//! In 64-bit mode segmentation is mostly dead, but we still need a GDT for one
//! important reason: it is how the CPU finds the TSS, and the TSS holds the
//! Interrupt Stack Table (IST) -- a list of known-good stacks the CPU can
//! switch to when an exception happens.
//!
//! Why it matters: if the kernel stack overflows, the CPU hits the guard page
//! and raises a page fault. Pushing the exception frame onto the (already
//! overflowed) stack faults again -> double fault -> same problem -> TRIPLE
//! fault, which resets the machine. Giving the double-fault handler its own
//! IST stack breaks that chain.

use lazy_static::lazy_static;
use x86_64::VirtAddr;
use x86_64::structures::gdt::{Descriptor, GlobalDescriptorTable, SegmentSelector};
use x86_64::structures::tss::TaskStateSegment;

/// Which IST slot (0..=6) the double-fault handler uses.
pub const DOUBLE_FAULT_IST_INDEX: u16 = 0;

lazy_static! {
    static ref TSS: TaskStateSegment = {
        let mut tss = TaskStateSegment::new();
        tss.interrupt_stack_table[DOUBLE_FAULT_IST_INDEX as usize] = {
            // No memory allocator yet, so the stack is just a static array.
            // (It has no guard page, so don't do anything recursive in the
            // double-fault handler!)
            const STACK_SIZE: usize = 4096 * 5;
            static mut STACK: [u8; STACK_SIZE] = [0; STACK_SIZE];

            // Stacks grow downwards on x86, so hand the CPU the *end* address.
            let stack_start = VirtAddr::from_ptr(&raw const STACK);
            stack_start + STACK_SIZE as u64
        };
        tss
    };
}

lazy_static! {
    /// The GDT, plus the selectors (indexes) of the entries we added.
    static ref GDT: (GlobalDescriptorTable, Selectors) = {
        let mut gdt = GlobalDescriptorTable::new();
        let code_selector = gdt.append(Descriptor::kernel_code_segment());
        let tss_selector = gdt.append(Descriptor::tss_segment(&TSS));
        (gdt, Selectors { code_selector, tss_selector })
    };
}

struct Selectors {
    code_selector: SegmentSelector,
    tss_selector: SegmentSelector,
}

/// Load our GDT, point CS at our code segment and load the TSS.
pub fn init() {
    use x86_64::instructions::segmentation::{CS, Segment};
    use x86_64::instructions::tables::load_tss;

    GDT.0.load();
    unsafe {
        // The old CS value points into the bootloader's GDT, which we just
        // replaced, so reload it. Then tell the CPU where our TSS is.
        CS::set_reg(GDT.1.code_selector);
        load_tss(GDT.1.tss_selector);
    }
}