x86_64 swapgs 初始化

在中断或异常处理的entry代码处, 会执行swapgs 切换到kernel GS, GS.base 是存储了中断stack 的地址。也就是the first percpu variable 存放了irq stack 的底。

arch/x86/include/asm/processor.h

union irq_stack_union {                                                                                                                                                     
    char irq_stack[IRQ_STACK_SIZE];
    /*
     * GCC hardcodes the stack canary as %gs:40.  Since the
     * irq_stack is the object at %gs:0, we reserve the bottom
     * 48 bytes of the irq stack for the canary.
     */
    struct {
        char gs_base[40];
        unsigned long stack_canary;
    };
};

调用的路径：
cpu_init->
       switch_to_new_gdt->
           load_percpu_segment:

void cpu_init(void)
{
    ...
    switch_to_new_gdt(cpu);
    ...
    wrmsrl(MSR_FS_BASE, 0);
    wrmsrl(MSR_KERNEL_GS_BASE, 0);
    ...
}

void load_percpu_segment(int cpu)
{

    __loadsegment_simple(gs, 0);
    wrmsrl(MSR_GS_BASE, (unsigned long)per_cpu(irq_stack_union.gs_base, cpu));                                                                                              

    load_stack_canary_segment();
}