os_kernel/userprog/tss.c

/**
 * @brief 安装 TSS 到 GDT，安装用户代码段、数据段到 GDT
 * @note 安装后系统 GDT 显示如下：
 *      gdt (base=0x00000000c0000900, limit=55):
 *      gdt[0x0000]=<null entry>
 *      gdt[0x0008]=Code segment, base=0x00000000, limit=0xffffffff, Execute-Only, Non-Conforming, Accessed, 32-bit
 *      gdt[0x0010]=Data segment, base=0x00000000, limit=0xffffffff, Read/Write, Accessed
 *      gdt[0x0018]=Data segment, base=0xc00b8000, limit=0x00007fff, Read/Write, Accessed
 *      gdt[0x0020]=32-Bit TSS (Busy) at 0xc0005560, length 0x0006b
 *      gdt[0x0028]=Code segment, base=0x00000000, limit=0xffffffff, Execute-Only, Non-Conforming, 32-bit
 *      gdt[0x0030]=Data segment, base=0x00000000, limit=0xffffffff, Read/Write
 */
#include "tss.h"
#include "../kernel/global.h"
#include "../lib/kernel/print.h"
#include "../lib/string.h"

/// @brief TSS implementation
static Tss tss;
/// @brief 创建 GDT 描述符
/// @param desc_addr 基址
/// @param limit 界限
/// @param attr_low 属性（低位）
/// @param attr_high 属性（低位）
/// @return GdtDescriptor
static GdtDesc make_gdt_desc(uint32_t *desc_addr, uint32_t limit, uint8_t attr_low, uint8_t attr_high);

void update_tss_esp(struct task_struct *pthread)
{
    tss.esp0 = (uint32_t *)((uintptr_t)pthread + PG_SIZE); // 0 级线程的 esp0 指向 pthread 的 0 级线程的 ESP
}

static GdtDesc make_gdt_desc(uint32_t *desc_addr, uint32_t limit, uint8_t attr_low, uint8_t attr_high)
{
    uint32_t desc_base = (uint32_t)(uintptr_t)desc_addr;
    struct gdt_desc desc;
    desc.limit_low_word = limit & 0xFFFF;
    desc.base_low_word = desc_base & 0xFFFF;
    desc.base_middle_byte = ((desc_base & 0x00ff0000) >> 16);
    desc.attr_low_byte = (uint8_t)attr_low;
    desc.limit_high_attr_high = ((uint8_t)(attr_high) + ((limit & 0x000f0000) >> 16));
    desc.base_high_byte = desc_base >> 24;

    return desc;
}
/* 在 GDT 中创建 tss 并加载 gdt */
void tss_init(void)
{
    put_str("tss_init start\n");
    uint32_t tss_size = sizeof(tss);
    _memset(&tss, 0, tss_size);

    tss.ss0 = SELECTOR_K_STACK;
    tss.iomap = tss_size;

    /* gdt 段基址为 0x900，把 tss 放到第 4 个位置，也就是 0x900+0x20 的位置 */

    // 在 gdt 中添加 dpl 为 0 的 TSS 描述符
    *((GdtDesc *)0xc0000920) = make_gdt_desc((uint32_t *)&tss, tss_size - 1, TSS_ATTR_LOW, TSS_ATTR_HIGH);
    // 在 gdt 中添加 dpl 为 3 的数据段和代码段描述符
    *((GdtDesc *)0xc0000928) = make_gdt_desc((uint32_t *)0, 0xfffff, GDT_CODE_ATTR_LOW_DPL3, GDT_ATTR_HIGH);
    *((GdtDesc *)0xc0000930) = make_gdt_desc((uint32_t *)0, 0xfffff, GDT_DATA_ATTR_LOW_DPL3, GDT_ATTR_HIGH);

    // gdt 的指针，前 2 个字节(16 位)是 gdt 界限 limit，后 4 字节(32位)是 gdt 起始地址 Base
    uint64_t gdt_operand_ptr = (8 * 7 - 1) | ((uint64_t)(uint32_t)0xc0000900 << 16); // 7 个描述符大小

    asm volatile("lgdt %0" ::"m"(gdt_operand_ptr));
    asm volatile("ltr %w0" ::"r"(SELECTOR_TSS));

    put_str("tss_init and ltr done\n");
}