被CPU0唤醒的CPUn可以在运行过程中进行热插拔。譬如运行如下命令即可卸载CPU1并且将CPU1上的任务全部迁移到其他CPU：
# echo 0
之后CPU1会主动参与系统中各个CPU之间要运行任务的负载均衡工作。
CPU0唤醒其他 CPU的动作在内核中被封装为一个smp_operations的结构体，该结构体的成员如下：
[cpp]view plaincopy
1. 83struct smp_operations { 2.
3. 84#ifdef CONFIG_SMP 4.
5. 85 /* 6.
7. 86 * Setup the set of possible CPUs (via set_cpu_possible) 8.
9. 87 */ 10.
11. 88 void (*smp_init_cpus)(void); 12.
13. 89 /*
14.
15. 90 * Initialize cpu_possible map, and enable coherency 16.
17. 91 */ 18.
19. 92 void (*smp_prepare_cpus)(unsigned int max_cpus); 20. 21. 93 22.
23. 94 /* 24.
25. 95 * Perform platform specific initialisation of the specified CPU. 26.
27. 96 */ 28.
29. 97 void (*smp_secondary_init)(unsigned int cpu); 30.
31. 98 /* 32.
33. 99 * Boot a secondary CPU, and assign it thespecified idle task. 34.
35. 100 * This also gives us the initial stack to use for this CPU. 36.
37. 101 */ 38.
39. 102 int (*smp_boot_secondary)(unsigned int cpu, struct task_struct *idle); 40.
41. 103#ifdef CONFIG_HOTPLUG_CPU 42.
43. 104 int (*cpu_kill)(unsigned intcpu); 44.
45. 105 void (*cpu_die)(unsigned int cpu); 46.
47. 106 int (*cpu_disable)(unsigned intcpu); 48.
49. 107#endif 50.
51. 108#endif 52. 53. 109};
我们从arch/arm/mach-vexpress/v2m.c看到VEXPRESS电路板用到的smp_ops为vexpress_smp_ops： 666DT_MACHINE_START(VEXPRESS_DT,\667 .dt_compat = v2m_dt_match,
668 .smp =smp_ops(vexpress_smp_ops), 669 .map_io = v2m_dt_map_io, 670 .init_early =v2m_dt_init_early, 671 .init_irq = v2m_dt_init_irq, 672 .timer =&v2m_dt_timer, 673 .init_machine = v2m_dt_init, 674 .handle_irq = gic_handle_irq, 675 .restart = v2m_restart, 676MACHINE_END
透过arch/arm/mach-vexpress/platsmp.c的实现代码可以看出，smp_operations的成员函数smp_init_cpus() 即vexpress_smp_init_cpus()会探测SoC内CPU核的个数，并设置了核间通信的方式为gic_raise_softirq()。可见于vexpress_smp_init_cpus()中调用的vexpress_dt_smp_init_cpus()： 103staticvoid __init vexpress_dt_smp_init_cpus(void) 104{ …
128 for (i = 0; i < ncores ip> 129 set_cpu_possible(i, true); 130
131 set_smp_cross_call(gic_raise_softirq); 132}
而smp_operations的成员函数smp_prepare_cpus()即vexpress_smp_prepare_cpus()则会透过v2m_flags_set(virt_to_phys(versatile_secondary_startup))设置其他CPU的启动地址为versatile_secondary_startup：
179staticvoid __init vexpress_smp_prepare_cpus(unsigned int max_cpus) 180{ 181 … 189 190 /*
191 * Write the address of secondary startup into the 192 * system-wide flags register. The boot monitor waits 193 * until it receives a soft interrupt, and then the 194 * secondary CPU branches to this address. 195 */
196 v2m_flags_set(virt_to_phys(versatile_secondary_startup)); 197}
注意这部分的具体实现方法是SoC相关的，由芯片的设计以及芯片内部的Bootrom决定。对于VEXPRESS来讲，设置方法如下：
139void__init v2m_flags_set(u32 data)
140{
141 writel(~0, v2m_sysreg_base + V2M_SYS_FLAGSCLR); 142 writel(data, v2m_sysreg_base + V2M_SYS_FLAGSSET); 143}
即填充v2m_sysreg_base +V2M_SYS_FLAGSCLR地址为0xFFFFFFFF,将其他CPU初始启动执行的指令地址填入v2m_sysreg_base +V2M_SYS_FLAGSSET。这2个地址属于芯片实现时候设定的。填入的CPUn的起始地址都透过virt_to_phys()转化为物理地址，因为此时CPUn的MMU尚未开启。
比较关键的是smp_operations的成员函数smp_boot_secondary()，它完成最终的CPUn的唤醒工作： 27static void __cpuinit write_pen_release(intval) 28{
29 pen_release = val; 30 smp_wmb();
31 __cpuc_flush_dcache_area((void *)&pen_release, sizeof(pen_release)); 32 outer_clean_range(__pa(&pen_release), __pa(&pen_release + 1)); 33}
59int __cpuinitversatile_boot_secondary(unsigned int cpu, struct task_struct *idle) 60{
61 unsigned long timeout; 62 63 /*
64 * Set synchronisation state between this boot processor 65 * and the secondary one 66 */
67 spin_lock(&boot_lock); 68 69 /*
70 * This is really belt and braces; we hold unintended secondary 71 * CPUs in the holding pen until we're ready for them. However, 72 * since we haven't sent them a soft interrupt, they shouldn't 73 * be there. 74 */
75 write_pen_release(cpu_logical_map(cpu)); 76 77 /*
78 * Send the secondary CPU a soft interrupt, thereby causing 79 * the boot monitor to read the system wide flags register, 80 * and branch to the address found there. 81 */
82 gic_raise_softirq(cpumask_of(cpu), 0); 83
84 timeout = jiffies + (1 * HZ);
85 while (time_before(jiffies, timeout)) { 86 smp_rmb();
87 if (pen_release == -1) 88 break; 89
90 udelay(10); 91 } 92 93 /*
94 * now the secondary core is starting up let it run its 95 * calibrations, then wait for it to finish 96 */
97 spin_unlock(&boot_lock); 98
99 return pen_release != -1 ? -ENOSYS : 0; 100}
上述代码中高亮的部分首先会将pen_release变量设置为要唤醒的CPU核的CPU号cpu_logical_map(cpu)，而后透过gic_raise_softirq(cpumask_of(cpu), 0)给CPUcpu发起0号IPI，这个时候，CPUcpu核会从前面smp_operations中的smp_prepare_cpus()成员函数即vexpress_smp_prepare_cpus()透过v2m_flags_set()设置的其他CPU核的起始地址versatile_secondary_startup开始执行，如果顺利的话，该CPU会将原先为正数的pen_release写为-1，以便CPU0从等待pen_release成为-1的循环中跳出。
versatile_secondary_startup实现于arch/arm/plat-versatile/headsmp.S，是一段汇编： 21ENTRY(versatile_secondary_startup) 22 mrc p15, 0, r0, c0, c0, 5 23 and r0, r0, #15 24 adr r4, 1f 25 ldmia r4, {r5, r6} 26 sub r4, r4, r5 27 add r6, r6, r4 28pen: ldr r7, [r6] 29 cmp r7, r0 30 bne pen 31 32 /*
33 * we've been released from the holding pen: secondary_stack