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
movsr10, r5@ invalid processor (r5=0)?
THUMB( iteq )@ 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.
*/
ldrr13, =__mmap_switched@ address to jump to after //保存MMU使能后跳转地址
@ mmu has been enabled
adrlr, BSYM(1f)@ return (PIC) address
movr8, r4@ set TTBR1 to swapper_pg_dir
ARM(addpc, r10, #PROCINFO_INITFUNC)
THUMB(addr12, r10, #PROCINFO_INITFUNC)
THUMB(movpc, 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
。
。
。
bstart_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)函数最后调用rest_init()函数
{
…… //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();
}
/*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.");
}
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：
