set_cpu_boot_mode_flag

使用 el2_setup 的返回值，填充 __boot_cpu_mode 这个全局数组 ，

1、__boot_cpu_mode 也在 head.S 里面定义的。初始值如下：

/*
 * We need to find out the CPU boot mode long after boot, so we need to
 * store it in a writable variable.
 *
 * This is not in .bss, because we set it sufficiently early that the boot-time
 * zeroing of .bss would clobber it.
 */
SYM_DATA_START(__boot_cpu_mode)
    .long    BOOT_CPU_MODE_EL2
    .long    BOOT_CPU_MODE_EL1
SYM_DATA_END(__boot_cpu_mode)

2、set_cpu_boot_mode_flag 代码

/*
 * Sets the __boot_cpu_mode flag depending on the CPU boot mode passed
 * in w0. See arch/arm64/include/asm/virt.h for more info.
 */
SYM_FUNC_START_LOCAL(set_cpu_boot_mode_flag)
    adr_l    x1, __boot_cpu_mode
    cmp    w0, #BOOT_CPU_MODE_EL2
    b.ne    1f
    add    x1, x1, #4
1:    str    w0, [x1]            // This CPU has booted in EL1
    dmb    sy
    dc    ivac, x1            // Invalidate potentially stale cache line
    ret
SYM_FUNC_END(set_cpu_boot_mode_flag)

3、set_cpu_boot_mode_flag 代码逻辑

 1 running_mode = el2_setup();
 2 
 3 set_cpu_boot_mode_flag(running_mode){
 4 
 5     if(running_mode == #BOOT_CPU_MODE_EL2){
 6         __boot_cpu_mode.field2 = #BOOT_CPU_MODE_EL2
 7     }else{
 8         __boot_cpu_mode.field1 = #BOOT_CPU_MODE_EL1
 9     }
10 }

如果running_mode 为EL2， 则 __boot_cpu_mode 的两个field 都为   #BOOT_CPU_MODE_EL2 

如果 running_mode 为EL1， 则 __boot_cpu_mode 的两个field 都为   #BOOT_CPU_MODE_EL1