c - 使用 MinGW 的 GCC 内联汇编加载 IDT 结构失败

Question

尝试使用以下代码获取中断描述符表的数据:

/* SIDT returns IDT in following format */
#pragma pack(1)
typedef struct
{
    unsigned short IDTLimit;
    unsigned short LowIDTBase;
    unsigned short HighIDTBase

} s_idt_info;
#pragma pack()

.
.
.

    s_idt_info idt_info;        // returned by sidt
    s_idt_entry *idt_entries;   // obtained from idt_info
    unsigned long count;

    // load idt_info
    __asm ("sidt idt_info");

我收到以下错误:

||=== Build: Release in driver2 (compiler: gnu_64) ===|
obj\Release\driver.o:driver.c|| undefined reference to `idt_info'|
||error: ld returned 1 exit status|
||=== Build failed: 2 error(s), 0 warning(s) (0 minute(s), 0 second(s)) ===|

使用 MinGW-64bit 的 g++ 编译器

Answer 1

与 MSVC 不同，GCC 不支持内联汇编内的局部变量(在函数内)。如果变量 idt_info 是全局的，那么它应该对内联程序集可见，但是 GCC documentation不鼓励这样做:

Accessing data from C programs without using input/output operands (such as by using global symbols directly from the assembler template) may not work as expected. Similarly, calling functions directly from an assembler template requires a detailed understanding of the target assembler and ABI.

Since GCC does not parse the assembler template, it has no visibility of any symbols it references. This may result in GCC discarding those symbols as unreferenced unless they are also listed as input, output, or goto operands.

你想要做的是使用 GCC extended assembler template并使用输出约束以允许 GCC 传入将由 SIDT 更新的 idt_info 地址。这样的事情应该有效:

__asm__ __volatile__ ("sidt %[idtptr]" : [idtptr]"=m"(idt_info));

= 表示约束将用于输出，m 强制 GCC 传递内存操作数。 SIDT需要内存操作数而不是寄存器。 %[idtptr] 是给参数的名称，GCC 会将其替换为实际的内存引用。