本文转自xboot.org
linux的启动框架如下：
linux的内核映像文件zImage本身是一个压缩的文件，在arch/arm/boot/Makefile中，我们可以找到如下语句： - $(obj)/zImage: $(obj)/compressed/vmlinux FORCE
- $(call if_changed,objcopy)
- @echo ' Kernel: $@ is ready'
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可见，zImage是由arch\arm\boot\compressed\vmlinux二进制文件转化而来。在arch/arm/boot/compressed/Makefile中，有如下语句： - $(obj)/vmlinux: $(obj)/vmlinux.lds $(obj)/$(HEAD) $(obj)/piggy.$(suffix_y).o \
- $(addprefix $(obj)/, $(OBJS)) $(lib1funcs) FORCE
- $(call if_changed,ld)
- $(obj)/piggy.$(suffix_y): $(obj)/../Image FORCE
- $(call if_changed,$(suffix_y))
- $(obj)/piggy.$(suffix_y).o: $(obj)/piggy.$(suffix_y) FORCE
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由此可以分析得知，zImage是由vmlinux.lds，head.o，misc.o以及压缩的内核piggy.gzip.o组成。也就是说，zImage文件即包含了未压缩部分，如head.o,misc.o，也包含了压缩的部分，如piggy.gzip.o。那么，zImage的程序入口在哪里呢？ 在arch\arm\boot\compressed\vmlinux.lds中，可以看到其框架如下： - OUTPUT_ARCH(arm)
- ENTRY(_start)
- SECTIONS
- {
- ……
- . = 0;
- _text = .;
- .text : {
- _start = .;
- *(.start)
- *(.text)
- *(.text.*)
- *(.fixup)
- *(.gnu.warning)
- *(.rodata)
- *(.rodata.*)
- *(.glue_7)
- *(.glue_7t)
- *(.piggydata)
- . = ALIGN(4);
- }
- ……
- }
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可以看出，其入口点为_start，程序arch\arm\boot\compressed\head.S会首先被执行，在head.S中，程序执行了一堆准备工作后，开始调用misc.c中的decompress_kernel函数，开始解压内核。其函数如下： - void
- decompress_kernel(unsigned long output_start, unsigned long free_mem_ptr_p,
- unsigned long free_mem_ptr_end_p,
- int arch_id)
- {
- int ret;
- output_data = (unsigned char *)output_start;
- free_mem_ptr = free_mem_ptr_p;
- free_mem_end_ptr = free_mem_ptr_end_p;
- __machine_arch_type = arch_id;
- arch_decomp_setup();
- putstr("Uncompressing Linux...");
- ret = do_decompress(input_data, input_data_end - input_data,
- output_data, error);
- if (ret)
- error("decompressor returned an error");
- else
- putstr(" done, booting the kernel.\n");
- }
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这里有几句经典的打印信息： - Uncompressing Linux... done, booting the kernel.
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执行完这个后，程序将会跳到init/main.c中，执行经典的start_kernel函数。start_kernel()会调用一系列初始化函数来设置中断，执行进一步的内存配置等，其函数原型如下： - asmlinkage void __init start_kernel(void)
- {
- char * command_line;
- extern const struct kernel_param __start___param[], __stop___param[];
- smp_setup_processor_id();//返回启动的CPU的ID号，如为单核则什么也不做
- /*
- * Need to run as early as possible, to initialize the
- * lockdep hash:
- */
- lockdep_init();
- debug_objects_early_init();
- /*
- * Set up the the initial canary ASAP:
- */
- boot_init_stack_canary();
- cgroup_init_early();
- local_irq_disable();//关闭当前CPU的中断
- early_boot_irqs_disabled = true;
- /*
- * Interrupts are still disabled. Do necessary setups, then
- * enable them
- */
- tick_init();
- boot_cpu_init();
- page_address_init();//初始化页地址，使用链表将其链接起来
- printk(KERN_NOTICE "%s", linux_banner);//打印内核版本信息
- setup_arch(&command_line);//设置体系结构，由内核根目录的Makefile决定
- 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);
- page_alloc_init();
- printk(KERN_NOTICE "Kernel command line: %s\n", boot_command_line);//打印命令行信息
- parse_early_param();//解析内核选项
- parse_args("Booting kernel", static_command_line, __start___param,
- __stop___param - __start___param,
- &unknown_bootoption);
- /*
- * These use large bootmem allocations and must precede
- * kmem_cache_init()
- */
- setup_log_buf(0);
- pidhash_init();
- vfs_caches_init_early();
- sort_main_extable();
- trap_init();
- mm_init();
- /*
- * 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 (!irqs_disabled()) {//检查中断是否已经打开，如果已经打开，则关闭中断
- printk(KERN_WARNING "start_kernel(): bug: interrupts were "
- "enabled *very* early, fixing it\n");
- local_irq_disable();
- }
- idr_init_cache();
- perf_event_init();
- rcu_init();//初始化RCU(Read-Copy Update)机制
- radix_tree_init();
- /* init some links before init_ISA_irqs() */
- early_irq_init();
- init_IRQ();//中断向量初始化
- prio_tree_init();
- init_timers();//初始化定时器相关的数据结构
- hrtimers_init();//对高精度时钟进行初始化
- softirq_init();//初始化tasklet_softirq和hi_softirq
- timekeeping_init();
- time_init();//初始化系统时钟源
- profile_init();//对内核的profile（一个内核性能调式工具）功能进行初始化
- call_function_init();
- if (!irqs_disabled())
- printk(KERN_CRIT "start_kernel(): bug: interrupts were "
- "enabled early\n");
- early_boot_irqs_disabled = false;
- local_irq_enable();
- /* Interrupts are enabled now so all GFP allocations are safe. */
- gfp_allowed_mask = __GFP_BITS_MASK;
- 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();// 初始化控制台以显示printk的内容，在此之前调用的printk只是把数据存到缓冲区里
- if (panic_later)
- panic(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) {
- printk(KERN_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();
- enable_debug_pagealloc();
- 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();
- #ifdef CONFIG_X86
- if (efi_enabled)
- efi_enter_virtual_mode();
- #endif
- thread_info_cache_init();
- cred_init();
- fork_init(totalram_pages);
- proc_caches_init();
- buffer_init();
- key_init();
- security_init();
- dbg_late_init();
- vfs_caches_init(totalram_pages);//虚拟文件系统的初始化
- 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();
- acpi_early_init(); /* before LAPIC and SMP init */
- sfi_init_late();
- ftrace_init();
- /* Do the rest non-__init'ed, we're now alive */
- rest_init();
- }
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start_kernel函数大致执行任务如下： 输出Linux版本信息，设置与体系结构相关的环境，页表结构初始化，初始化系统IRQ，核心进程调度器初始化，时间、定时器初始化，提取并分析核心启动参数，控制台初始化，剖析器数据结构初始化，核心Cache初始化，延迟校准，内存初始化，创建文件，目录cache，创建与虚拟内存相关的cache，块设备读写缓冲区初始化，创建页cache，创建信号队列cache，初始化内存inode表，创建内存文件描述符表，检查体系结构，SMP机器除引导CPU之外的CPU初始化，创建第一个核心线程，调用init函数，调用cpu_idle()等待调度。 作为核心线程的init()函数完成外设及其驱动程序的加载和初始化，挂接根文件系统。init()打开/dev/console设备，重定向stdin、stdout和stderr到控制台。之后，它搜索文件系统中的init程序（也可以由“init=”命令行参数指定init程序），并使用execve()系统调用执行init程序。搜索init程序的顺序为/sbin/init、/etc/init、/bin/init 和/bin/sh。在嵌入式系统中，多数情况下，可以给内核传入一个简单的shell脚本来启动必需的嵌入式应用程序。 至此，漫长的 Linux 内核引导和启动过程就结束了，而init()对应的由start_kernel()创建的第一个线程也进入用户模式。
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