1. 问题引入

https://awakening-fong.github.io/posts/arm/arm_qemu_02 中说到 startkernel前设置sp为 initthreadunion + THREADSTART_SP

#define THREAD_START_SP (THREAD_SIZE - 8)

为何要-8?

2. 解

static inline struct thread_info *current_thread_info(void)
{
    register unsigned long sp asm ("sp");
    return (struct thread_info *)(sp & ~(THREAD_SIZE - 1));
}

(图示中A点到B点 大小是8字节)
如果sp是SP0, 那么,(sp & ~(THREADSIZE - 1)) 后还是指向SP0 (A点);
如果sp是SP1, 那么, (sp & ~(THREADSIZE - 1)) 后指向的是thread_info (C点).
所以, 需要-8.

          SP0 ---->  +.------------+   --.--        高地址  
                |    | A           |     |                  
                8    |             |     |            ^     
                |    |             |                  |     
          SP1 ---->  |.B           |  THREAD_SIZE     |     
                     |             |     .            |     
                     |             |     |            |     
                     |             |     |            |     
                     |             |     |            |     
  thread_info --->   |.C           |  ------          |     
                     |             |                  |     
                     |             |                  |     
                     |             |                  |      
                     |             |                        
                     +-------------+                 低地址 

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