本文以Linux3.14版本源码为例分析其启动流程。各版本启动代码略有不同，但核心流程与思想万变不离
其宗。

内核映像被加载到内存并获得控制权之后，内核启动流程开始。通常，内核映像以压缩形式存储，并不是
一个可以执行的内核。因此，内核阶段的首要工作是自解压内核映像。

内核编译生成vmliunx后，通常会对其进行压缩，得到zImage（小内核，小于512KB）或bzImage（大内核，
大于512KB）。在它们的头部嵌有解压缩程序。

通过linux/arch/arm/boot/compressed目录下的Makefile寻找到vmlinux文件的链接脚本（vmlinux.lds），从中查找系统启动入口函数。

$(obj)/vmlinux: $(obj)/vmlinux.lds $(obj)/$(HEAD) $(obj)/piggy.$(suffix_y).o \
		$(addprefix $(obj)/, $(OBJS)) $(lib1funcs) $(ashldi3) \
		$(bswapsdi2) FORCE
	@$(check_for_multiple_zreladdr)
	$(call if_changed,ld)
	@$(check_for_bad_syms)
vmlinux.lds(linux/arch/arm/kernel/vmlinux.lds)链接脚本开头内容

OUTPUT_ARCH(arm)
ENTRY(stext)
jiffies = jiffies_64;
SECTIONS
{
	。
	。
	。
得到内核入口函数为 stext（linux/arch/arm/kernel/head.S）

内核引导阶段
ENTRY(stext)
	。
	。
	。
	bl	__lookup_processor_type	@ r5=procinfo r9=cpuid   //处理器是否支持
	movs	r10, r5				@ invalid processor (r5=0)?
 THUMB( it	eq )		@ force fixup-able long branch encoding
	beq	__error_p			@ yes, error 'p'           //不支持则打印错误信息
 
          。
	。
	。
	bl	__create_page_tables          //创建页表
 
	/*
	 * The following calls CPU specific code in a position independent
	 * manner.  See arch/arm/mm/proc-*.S for details.  r10 = base of
	 * xxx_proc_info structure selected by __lookup_processor_type
	 * above.  On return, the CPU will be ready for the MMU to be
	 * turned on, and r0 will hold the CPU control register value.
	 */
	ldr	r13, =__mmap_switched		@ address to jump to after   //保存MMU使能后跳转地址
						@ mmu has been enabled
	adr	lr, BSYM(1f)			@ return (PIC) address
	mov	r8, r4				@ set TTBR1 to swapper_pg_dir
 ARM(	add	pc, r10, #PROCINFO_INITFUNC	)
 THUMB(	add	r12, r10, #PROCINFO_INITFUNC	)
 THUMB(	mov	pc, r12				)
1:	b	__enable_mmu                                                                           //使能MMU后跳转到__mmap_switched
查找标签__mmap_switched所在位置：/linux/arch/arm/kernel/head-common.S
__mmap_switched:
	/*
	 * The following fragment of code is executed with the MMU on in MMU mode,
	 * and uses absolute addresses; this is not position independent.
	 *
	 *  r0  = cp#15 control register
	 *  r1  = machine ID
	 *  r2  = atags/dtb pointer
	 *  r9  = processor ID
	 */
	//保存设备信息、设备树及启动参数存储地址
	。
	。
	。
	b	start_kernel
 

内核初始化阶段
从start_kernel函数开始，内核进入C语言部分，完成内核的大部分初始化工作。

函数所在位置：/linux/init/Main.c

start_kernel涉及大量初始化工作，只例举重要的初始化工作。

asmlinkage void __init start_kernel(void)
{
	……                 //类型判断
	smp_setup_processor_id();                                                         //smp相关，返回启动CPU号
	……
	local_irq_disable();                                                                   //关闭当前CPU中断
	early_boot_irqs_disabled = true;
/*
 * Interrupts are still disabled. Do necessary setups, then
 * enable them
 */
	boot_cpu_init();
	page_address_init();             //初始化页地址
	pr_notice("%s", linux_banner);         //显示内核版本信息
	setup_arch(&command_line);
	mm_init_owner(&init_mm, &init_task);
	mm_init_cpumask(&init_mm);
	setup_command_line(command_line);
	setup_nr_cpu_ids();
	setup_per_cpu_areas();
	smp_prepare_boot_cpu();	/* arch-specific boot-cpu hooks */
 
	build_all_zonelists(NULL, NULL);
	page_alloc_init();                                                                            //页内存申请初始化
 
	pr_notice("Kernel command line: %s\n", boot_command_line);                                     //打印内核启动命令行参数
	parse_early_param();
	parse_args("Booting kernel", static_command_line, __start___param,
		   __stop___param - __start___param,
		   -1, -1, &unknown_bootoption);
 
	……
	/*
	 * Set up the scheduler prior starting any interrupts (such as the
	 * timer interrupt). Full topology setup happens at smp_init()
	 * time - but meanwhile we still have a functioning scheduler.
	 */
	sched_init();                                                                                    //进程调度器初始化

	/*
	 * Disable preemption - early bootup scheduling is extremely
	 * fragile until we cpu_idle() for the first time.
	 */
	preempt_disable();                                                                                    //禁止内核抢占

	if (WARN(!irqs_disabled(), "Interrupts were enabled *very* early, fixing it\n"))
		local_irq_disable();                                                                      //检查关闭CPU中断	
	
          /*大量初始化内容 见名知意*/
	idr_init_cache();
	rcu_init();
	tick_nohz_init();
	context_tracking_init();
	radix_tree_init();
	/* init some links before init_ISA_irqs() */
	early_irq_init();
	init_IRQ();
	tick_init();
	init_timers();
	hrtimers_init();
	softirq_init();
	timekeeping_init();
	time_init();
	sched_clock_postinit();
	perf_event_init();
	profile_init();
	call_function_init();
	WARN(!irqs_disabled(), "Interrupts were enabled early\n");
	early_boot_irqs_disabled = false;
	local_irq_enable();                                                                            //本地中断可以使用了
 
	kmem_cache_init_late();
 
	/*
	 * HACK ALERT! This is early. We're enabling the console before
	 * we've done PCI setups etc, and console_init() must be aware of
	 * this. But we do want output early, in case something goes wrong.
	 */
	console_init();                                                                            //初始化控制台，可以使用printk了

	if (panic_later)
		panic("Too many boot %s vars at `%s'", panic_later,
		      panic_param);
 
	lockdep_info();
 
	/*
	 * Need to run this when irqs are enabled, because it wants
	 * to self-test [hard/soft]-irqs on/off lock inversion bugs
	 * too:
	 */
	locking_selftest();
 
#ifdef CONFIG_BLK_DEV_INITRD
	if (initrd_start && !initrd_below_start_ok &&
	    page_to_pfn(virt_to_page((void *)initrd_start)) < min_low_pfn) {
		pr_crit("initrd overwritten (0x%08lx < 0x%08lx) - disabling it.\n",
		    page_to_pfn(virt_to_page((void *)initrd_start)),
		    min_low_pfn);
		initrd_start = 0;
	}
#endif
	page_cgroup_init();
	debug_objects_mem_init();
	kmemleak_init();
	setup_per_cpu_pageset();
	numa_policy_init();
	if (late_time_init)
		late_time_init();
	sched_clock_init();
	calibrate_delay();
	pidmap_init();
	anon_vma_init();
	acpi_early_init();
#ifdef CONFIG_X86
	if (efi_enabled(EFI_RUNTIME_SERVICES))
		efi_enter_virtual_mode();
#endif
#ifdef CONFIG_X86_ESPFIX64
	/* Should be run before the first non-init thread is created */
	init_espfix_bsp();
#endif
	thread_info_cache_init();
	cred_init();
	fork_init(totalram_pages);                                                             //初始化fork

	proc_caches_init();
	buffer_init();
	key_init();
	security_init();
	dbg_late_init();
	vfs_caches_init(totalram_pages);                                                      //虚拟文件系统初始化

	signals_init();
	/* rootfs populating might need page-writeback */
	page_writeback_init();
#ifdef CONFIG_PROC_FS
	proc_root_init();
#endif
	cgroup_init();
	cpuset_init();
	taskstats_init_early();
	delayacct_init();
 
	check_bugs();
 
	sfi_init_late();
 
	if (efi_enabled(EFI_RUNTIME_SERVICES)) {
		efi_late_init();
		efi_free_boot_services();
	}
 
	ftrace_init();
 
	/* Do the rest non-__init'ed, we're now alive */
	rest_init();  //函数最后调用rest_init()函数
}

/*最重要使命：创建kernel_init进程，并进行后续初始化*/
static noinline void __init_refok rest_init(void)
{
	int pid;
 
	rcu_scheduler_starting();
	/*
	 * We need to spawn init first so that it obtains pid 1, however
	 * the init task will end up wanting to create kthreads, which, if
	 * we schedule it before we create kthreadd, will OOPS.
	 */
	
	kernel_thread(kernel_init, NULL, CLONE_FS | CLONE_SIGHAND);                             //创建kernel_init进程
	
	numa_default_policy();
	pid = kernel_thread(kthreadd, NULL, CLONE_FS | CLONE_FILES);
	rcu_read_lock();
	kthreadd_task = find_task_by_pid_ns(pid, &init_pid_ns);
	rcu_read_unlock();
	complete(&kthreadd_done);
 
	/*
	 * The boot idle thread must execute schedule()
	 * at least once to get things moving:
	 */
	init_idle_bootup_task(current);
	schedule_preempt_disabled();
	/* Call into cpu_idle with preempt disabled */
	//cpu_idle就是在系统闲置时用来降低电力的使用和减少热的产生的空转函数，函数至此不再返回，
其余工作从kernel_init进程处发起
	cpu_startup_entry(CPUHP_ONLINE);
}
kernel_init函数将完成设备驱动程序的初始化，并调用init_post函数启动用户进程

部分书籍介绍的内核启动流程基于经典的2.6版本，kernel_init函数还会调用init_post函数专门负责
_init进程的启动，现版本已经被整合到了一起。

static int __ref kernel_init(void *unused)
{
	int ret;
 
	kernel_init_freeable();     //该函数中完成smp开启  驱动初始化 共享内存初始化等工作
	/* need to finish all async __init code before freeing the memory */
	async_synchronize_full();
	free_initmem();         //初始化尾声，清除内存无用数据
	mark_rodata_ro();
	system_state = SYSTEM_RUNNING;
	numa_default_policy();
 
	flush_delayed_fput();
 
	if (ramdisk_execute_command) {
		ret = run_init_process(ramdisk_execute_command);
		if (!ret)
			return 0;
		pr_err("Failed to execute %s (error %d)\n",
		       ramdisk_execute_command, ret);
	}
 
	/*
	 * We try each of these until one succeeds.
	 *
	 * The Bourne shell can be used instead of init if we are
	 * trying to recover a really broken machine.
     * 寻找init函数，创建一号进程_init (第一个用户空间进程)*/
	if (execute_command) {
		ret = run_init_process(execute_command);
		if (!ret)
			return 0;
		pr_err("Failed to execute %s (error %d).  Attempting defaults...\n",
			execute_command, ret);
	}
	if (!try_to_run_init_process("/sbin/init") ||
	    !try_to_run_init_process("/etc/init") ||
	    !try_to_run_init_process("/bin/init") ||
	    !try_to_run_init_process("/bin/sh"))
		return 0;
 
	panic("No working init found.  Try passing init= option to kernel. "
	      "See Linux Documentation/init.txt for guidance.");
}
static int __ref kernel_init(void *unused)
{
	int ret;
 
	kernel_init_freeable(); //该函数中完成smp开启  驱动初始化 共享内存初始化等工作
	/* need to finish all async __init code before freeing the memory */
	async_synchronize_full();
	free_initmem();        //初始化尾声，清除内存无用数据
	mark_rodata_ro();
	system_state = SYSTEM_RUNNING;
	numa_default_policy();
 
	flush_delayed_fput();
 
	if (ramdisk_execute_command) {
		ret = run_init_process(ramdisk_execute_command);
		if (!ret)
			return 0;
		pr_err("Failed to execute %s (error %d)\n",
		       ramdisk_execute_command, ret);
	}
 
	/*
	 * We try each of these until one succeeds.
	 *
	 * The Bourne shell can be used instead of init if we are
	 * trying to recover a really broken machine.
	 * 寻找init函数，创建一号进程_init (第一个用户空间进程)*/
	if (execute_command) {
		ret = run_init_process(execute_command);
		if (!ret)
			return 0;
		pr_err("Failed to execute %s (error %d).  Attempting defaults...\n",
			execute_command, ret);
	}
	if (!try_to_run_init_process("/sbin/init") ||
	    !try_to_run_init_process("/etc/init") ||
	    !try_to_run_init_process("/bin/init") ||
	    !try_to_run_init_process("/bin/sh"))
		return 0;
 
	panic("No working init found.  Try passing init= option to kernel. "
	      "See Linux Documentation/init.txt for guidance.");
}
 

到此，内核初始化已经接近尾声，所有的初始化函数都已经调用，因此free_initmem函数可以舍弃内存的__init_begin至__init_end之间的数据。

当内核被引导并进行初始化后，内核启动了自己的第一个用户空间应用程序_init，这是调用的第一个使用
标准C库编译的程序，其进程编号时钟为1.

_init负责出发其他必须的进程，以使系统进入整体可用的状态。

以下为内核启动流程图：