polka/src/Descriptors.cc

/* Descriptors.cc
 * vim: set tw=80:
 * Eryn Wells <eryn@erynwells.me>
 */
/**
 * Oh god oh god. Descriptor tables.
 */

#include <stddef.h>
#include <stdint.h>
#include "Descriptors.hh"

namespace {

/**
 * Six byte field containing the length and a linear address where a descriptor
 * table livs.
 */
struct PseudoDescriptor
{
    uint16_t limit;
    uint32_t base;
} __attribute((__packed__));

}

namespace kernel {

/*
 * Static
 */

GDT&
GDT::systemGDT()
{
    static GDT sGDT;
    return sGDT;
}

/*
 * Public
 */

GDT::DescriptorSpec
GDT::DescriptorSpec::null()
{
    // Specify ring 0 and RO data segment here because their values are 0.
    return {0, 0, 0, 0, 0, 0, 0, DPL::Ring0, 0, Type::DataRO};
}


GDT::DescriptorSpec
GDT::DescriptorSpec::kernelSegment(uint32_t base,
                                   uint32_t limit,
                                   GDT::Type type)
{
    return {base, limit, true, true, false, false, true, DPL::Ring0, true, type};
}


GDT::Descriptor
GDT::DescriptorSpec::descriptor()
    const
{
    Descriptor descriptor = 0;

    uint8_t g   = static_cast<uint8_t>(hasCoarseGranularity);
    uint8_t db  = static_cast<uint8_t>(has32BitOperations);
    uint8_t l   = static_cast<uint8_t>(hasNative64BitCode);
    uint8_t avl = static_cast<uint8_t>(hasNative64BitCode);
    uint8_t p   = static_cast<uint8_t>(isPresent);
    uint8_t dpl = static_cast<uint8_t>(privilegeLevel);
    uint8_t s   = static_cast<uint8_t>(isCodeDataSegment);
    uint8_t typ = static_cast<uint8_t>(type);

    descriptor  = base         & 0xFF000000;   // Bits 31:24 of the base address.
    descriptor |= (g << 23);                   // Granularity field
    descriptor |= (db << 22);                  // D/B field
    descriptor |= (l << 21);                   // L field
    descriptor |= (avl << 20);                 // AVL field
    descriptor |= limit        & 0x000F0000;   // Bits 19:16 of the segment limit.
    descriptor |= (p << 15);                   // P field
    descriptor |= (dpl << 13)  & 0x00006000;   // DPL field
    descriptor |= (s << 12);                   // S field
    descriptor |= (typ << 8)   & 0x00000F00;   // Type field: see Type
    descriptor |= (base >> 16) & 0x000000FF;   // Bits 23:16 of the base address.

    // Shift everything up by 32 to make room for the lower 4 bytes
    descriptor <<= 32;

    descriptor |= base << 16;                  // Bits 15:00 of the base address.
    descriptor |= limit        & 0x0000FFFF;   // Bits 15:00 of the segment limit.

    return descriptor;
}


GDT::GDT()
    : table{0}
{ }


void
GDT::setDescriptor(size_t index,
                   const GDT::DescriptorSpec& spec)
{
    table[index] = spec.descriptor();
}


void
GDT::setNullDescriptor(size_t index)
{
    table[index] = 0;
}


void
GDT::load()
{
    PseudoDescriptor gdt {Size * sizeof(Descriptor) - 1, uint32_t(&table)};

    /*
     * Load the new GDT with the pointer defined above. The GDT isn't actually
     * used until the segment registers are reladed. Below, CS is reloaded by
     * a long jump into the new code segment. The rest of the segment registers
     * can be loaded directly.
     */
    asm volatile(
        "lgdt %0\n"
        "ljmpl $0x08, $reloadSegments\n"
        "reloadSegments:\n"
        "movl $0x10, %%eax\n"
        "movl %%eax, %%ds\n"
        "movl %%eax, %%es\n"
        "movl %%eax, %%fs\n"
        "movl %%eax, %%gs\n"
        "movl %%eax, %%ss\n"
        : : "m" (gdt)
        : "%eax");
}

} /* namespace kernel */