jsix/src/kernel/gdt.cpp

#include <stdint.h>

#include "kutil/assert.h"
#include "kutil/memory.h"
#include "kutil/no_construct.h"
#include "console.h"
#include "cpu.h"
#include "gdt.h"
#include "log.h"
#include "tss.h"

extern "C" void gdt_write(const void *gdt_ptr, uint16_t cs, uint16_t ds, uint16_t tr);

static constexpr uint8_t kern_cs_index   = 1;
static constexpr uint8_t kern_ss_index   = 2;
static constexpr uint8_t user_cs32_index = 3;
static constexpr uint8_t user_ss_index   = 4;
static constexpr uint8_t user_cs64_index = 5;
static constexpr uint8_t tss_index       = 6; // Note that this takes TWO GDT entries

// The BSP's GDT is initialized _before_ global constructors are called,
// so we don't want it to have a global constructor, lest it overwrite
// the previous initialization.
static kutil::no_construct<GDT> __g_bsp_gdt_storage;
GDT &g_bsp_gdt = __g_bsp_gdt_storage.value;


GDT::GDT(TSS *tss) :
    m_tss(tss)
{
    kutil::memset(this, 0, sizeof(GDT));

    m_ptr.limit = sizeof(m_entries) - 1;
    m_ptr.base = &m_entries[0];

    // Kernel CS/SS - always 64bit
    set(kern_cs_index, 0, 0xfffff, true, type::read_write | type::execute);
    set(kern_ss_index, 0, 0xfffff, true, type::read_write);

    // User CS32/SS/CS64 - layout expected by SYSRET
    set(user_cs32_index, 0, 0xfffff, false, type::ring3 | type::read_write | type::execute);
    set(user_ss_index,   0, 0xfffff, true,  type::ring3 | type::read_write);
    set(user_cs64_index, 0, 0xfffff, true,  type::ring3 | type::read_write | type::execute);

    set_tss(tss);
}

GDT &
GDT::current()
{
    cpu_data &cpu = current_cpu();
    return *cpu.gdt;
}

void
GDT::install() const
{
    gdt_write(
        static_cast<const void*>(&m_ptr),
        kern_cs_index << 3,
        kern_ss_index << 3,
        tss_index << 3);
}

void
GDT::set(uint8_t i, uint32_t base, uint64_t limit, bool is64, type t)
{
    m_entries[i].limit_low = limit & 0xffff;
    m_entries[i].size = (limit >> 16) & 0xf;
    m_entries[i].size |= (is64 ? 0xa0 : 0xc0);

    m_entries[i].base_low = base & 0xffff;
    m_entries[i].base_mid = (base >> 16) & 0xff;
    m_entries[i].base_high = (base >> 24) & 0xff;

    m_entries[i].type = t | type::system | type::present;
}

struct tss_descriptor
{
    uint16_t limit_low;
    uint16_t base_00;
    uint8_t base_16;
    GDT::type type;
    uint8_t size;
    uint8_t base_24;
    uint32_t base_32;
    uint32_t reserved;
} __attribute__ ((packed));

void
GDT::set_tss(TSS *tss)
{
    tss_descriptor tssd;

    size_t limit = sizeof(TSS);
    tssd.limit_low = limit & 0xffff;
    tssd.size = (limit >> 16) & 0xf;

    uintptr_t base = reinterpret_cast<uintptr_t>(tss);
    tssd.base_00 = base & 0xffff;
    tssd.base_16 = (base >> 16) & 0xff;
    tssd.base_24 = (base >> 24) & 0xff;
    tssd.base_32 = (base >> 32) & 0xffffffff;
    tssd.reserved = 0;

    tssd.type =
        type::accessed |
        type::execute |
        type::ring3 |
        type::present;

    kutil::memcpy(&m_entries[tss_index], &tssd, sizeof(tss_descriptor));
}

void
GDT::dump(unsigned index) const
{
    console *cons = console::get();

    unsigned start = 0;
    unsigned count = (m_ptr.limit + 1) / sizeof(descriptor);
    if (index != -1) {
        start = index;
        count = 1;
    } else {
        cons->printf("         GDT: loc:%lx size:%d\n", m_ptr.base, m_ptr.limit+1);
    }

    const descriptor *gdt =
        reinterpret_cast<const descriptor *>(m_ptr.base);

    for (int i = start; i < start+count; ++i) {
        uint32_t base =
            (gdt[i].base_high << 24) |
            (gdt[i].base_mid << 16) |
            gdt[i].base_low;

        uint32_t limit =
            static_cast<uint32_t>(gdt[i].size & 0x0f) << 16 |
            gdt[i].limit_low;

        cons->printf("          %02d:", i);
        if (! (gdt[i].type && type::present)) {
            cons->puts(" Not Present\n");
            continue;
        }

        cons->printf(" Base %08x  limit %05x ", base, limit);

        switch (gdt[i].type & type::ring3) {
            case type::ring3: cons->puts("ring3"); break;
            case type::ring2: cons->puts("ring2"); break;
            case type::ring1: cons->puts("ring1"); break;
            default: cons->puts("ring0"); break;
        }

        cons->printf(" %s %s %s %s %s %s %s\n",
            (gdt[i].type && type::accessed) ? "A" : " ",
            (gdt[i].type && type::read_write) ? "RW" : "  ",
            (gdt[i].type && type::conforming) ? "C" : " ",
            (gdt[i].type && type::execute) ? "EX" : "  ",
            (gdt[i].type && type::system) ? "S" : " ",
            (gdt[i].size & 0x80) ? "KB" : " B",
            (gdt[i].size & 0x60) == 0x20 ? "64" :
                (gdt[i].size & 0x60) == 0x40 ? "32" : "16");
    }
}