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前言 1、内容总结 汇编阶段#xff0c;或者说内核引导阶段#xff0c;主要是arch/arm/kernel/head.S文件#xff0c;主要完成以下内容#xff1a; #xff08;1#xff09;校验启动合法…以下内容源于朱有鹏嵌入式课程的学习如有侵权请告知删除。
前言 1、内容总结 汇编阶段或者说内核引导阶段主要是arch/arm/kernel/head.S文件主要完成以下内容 1校验启动合法性CPU ID机器码uboot给内核的传参格式等。 2建立段式映射的页表并开启MMU以方便使用内存。 3构建C运行环境跳入C阶段。 2、head.S文件代码 /** linux/arch/arm/kernel/head.S** Copyright (C) 1994-2002 Russell King* Copyright (c) 2003 ARM Limited* All Rights Reserved** This program is free software; you can redistribute it and/or modify* it under the terms of the GNU General Public License version 2 as* published by the Free Software Foundation.** Kernel startup code for all 32-bit CPUs*/
#include linux/linkage.h
#include linux/init.h#include asm/assembler.h
#include asm/domain.h
#include asm/ptrace.h
#include asm/asm-offsets.h
#include asm/memory.h
#include asm/thread_info.h
#include asm/system.h#if (PHYS_OFFSET 0x001fffff)
#error PHYS_OFFSET must be at an even 2MiB boundary!
#endif#define KERNEL_RAM_VADDR (PAGE_OFFSET TEXT_OFFSET)
#define KERNEL_RAM_PADDR (PHYS_OFFSET TEXT_OFFSET)/** swapper_pg_dir is the virtual address of the initial page table.* We place the page tables 16K below KERNEL_RAM_VADDR. Therefore, we must* make sure that KERNEL_RAM_VADDR is correctly set. Currently, we expect* the least significant 16 bits to be 0x8000, but we could probably* relax this restriction to KERNEL_RAM_VADDR PAGE_OFFSET 0x4000.*/
#if (KERNEL_RAM_VADDR 0xffff) ! 0x8000
#error KERNEL_RAM_VADDR must start at 0xXXXX8000
#endif.globl swapper_pg_dir.equ swapper_pg_dir, KERNEL_RAM_VADDR - 0x4000.macro pgtbl, rdldr \rd, (KERNEL_RAM_PADDR - 0x4000).endm#ifdef CONFIG_XIP_KERNEL
#define KERNEL_START XIP_VIRT_ADDR(CONFIG_XIP_PHYS_ADDR)
#define KERNEL_END _edata_loc
#else
#define KERNEL_START KERNEL_RAM_VADDR
#define KERNEL_END _end
#endif/** Kernel startup entry point.* ---------------------------** This is normally called from the decompressor code. The requirements* are: MMU off, D-cache off, I-cache dont care, r0 0,* r1 machine nr, r2 atags pointer.** This code is mostly position independent, so if you link the kernel at* 0xc0008000, you call this at __pa(0xc0008000).** See linux/arch/arm/tools/mach-types for the complete list of machine* numbers for r1.** Were trying to keep crap to a minimum; DO NOT add any machine specific* crap here - thats what the boot loader (or in extreme, well justified* circumstances, zImage) is for.*/__HEAD
ENTRY(stext)setmode PSR_F_BIT | PSR_I_BIT | SVC_MODE, r9 ensure svc mode and irqs disabledmrc p15, 0, r9, c0, c0 get processor idbl __lookup_processor_type r5procinfo r9cpuidmovs r10, r5 invalid processor (r50)?beq __error_p yes, error pbl __lookup_machine_type r5machinfomovs r8, r5 invalid machine (r50)?beq __error_a yes, error abl __vet_atagsbl __create_page_tables/** 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_machine_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, __switch_data address to jump to after mmu has been enabledadr lr, BSYM(__enable_mmu) return (PIC) addressARM( add pc, r10, #PROCINFO_INITFUNC )THUMB( add r12, r10, #PROCINFO_INITFUNC )THUMB( mov pc, r12 )
ENDPROC(stext)#if defined(CONFIG_SMP)
ENTRY(secondary_startup)/** Common entry point for secondary CPUs.** Ensure that were in SVC mode, and IRQs are disabled. Lookup* the processor type - there is no need to check the machine type* as it has already been validated by the primary processor.*/setmode PSR_F_BIT | PSR_I_BIT | SVC_MODE, r9mrc p15, 0, r9, c0, c0 get processor idbl __lookup_processor_typemovs r10, r5 invalid processor?moveq r0, #p yes, error pbeq __error/** Use the page tables supplied from __cpu_up.*/adr r4, __secondary_dataldmia r4, {r5, r7, r12} address to jump to aftersub r4, r4, r5 mmu has been enabledldr r4, [r7, r4] get secondary_data.pgdiradr lr, BSYM(__enable_mmu) return addressmov r13, r12 __secondary_switched addressARM( add pc, r10, #PROCINFO_INITFUNC ) initialise processor (return control reg)THUMB( add r12, r10, #PROCINFO_INITFUNC )THUMB( mov pc, r12 )
ENDPROC(secondary_startup)/** r6 secondary_data*/
ENTRY(__secondary_switched)ldr sp, [r7, #4] get secondary_data.stackmov fp, #0b secondary_start_kernel
ENDPROC(__secondary_switched).type __secondary_data, %object
__secondary_data:.long ..long secondary_data.long __secondary_switched
#endif /* defined(CONFIG_SMP) *//** Setup common bits before finally enabling the MMU. Essentially* this is just loading the page table pointer and domain access* registers.*/
__enable_mmu:
#ifdef CONFIG_ALIGNMENT_TRAPorr r0, r0, #CR_A
#elsebic r0, r0, #CR_A
#endif
#ifdef CONFIG_CPU_DCACHE_DISABLEbic r0, r0, #CR_C
#endif
#ifdef CONFIG_CPU_BPREDICT_DISABLEbic r0, r0, #CR_Z
#endif
#ifdef CONFIG_CPU_ICACHE_DISABLEbic r0, r0, #CR_I
#endifmov r5, #(domain_val(DOMAIN_USER, DOMAIN_MANAGER) | \domain_val(DOMAIN_KERNEL, DOMAIN_MANAGER) | \domain_val(DOMAIN_TABLE, DOMAIN_MANAGER) | \domain_val(DOMAIN_IO, DOMAIN_CLIENT))mcr p15, 0, r5, c3, c0, 0 load domain access registermcr p15, 0, r4, c2, c0, 0 load page table pointerb __turn_mmu_on
ENDPROC(__enable_mmu)/** Enable the MMU. This completely changes the structure of the visible* memory space. You will not be able to trace execution through this.* If you have an enquiry about this, *please* check the linux-arm-kernel* mailing list archives BEFORE sending another post to the list.** r0 cp#15 control register* r13 *virtual* address to jump to upon completion** other registers depend on the function called upon completion*/.align 5
__turn_mmu_on:mov r0, r0mcr p15, 0, r0, c1, c0, 0 write control regmrc p15, 0, r3, c0, c0, 0 read id regmov r3, r3mov r3, r13mov pc, r3
ENDPROC(__turn_mmu_on)/** Setup the initial page tables. We only setup the barest* amount which are required to get the kernel running, which* generally means mapping in the kernel code.** r8 machinfo* r9 cpuid* r10 procinfo** Returns:* r0, r3, r6, r7 corrupted* r4 physical page table address*/
__create_page_tables:pgtbl r4 page table address/** Clear the 16K level 1 swapper page table*/mov r0, r4mov r3, #0add r6, r0, #0x4000
1: str r3, [r0], #4str r3, [r0], #4str r3, [r0], #4str r3, [r0], #4teq r0, r6bne 1bldr r7, [r10, #PROCINFO_MM_MMUFLAGS] mm_mmuflags/** Create identity mapping for first MB of kernel to* cater for the MMU enable. This identity mapping* will be removed by paging_init(). We use our current program* counter to determine corresponding section base address.*/mov r6, pcmov r6, r6, lsr #20 start of kernel sectionorr r3, r7, r6, lsl #20 flags kernel basestr r3, [r4, r6, lsl #2] identity mapping/** Now setup the pagetables for our kernel direct* mapped region.*/add r0, r4, #(KERNEL_START 0xff000000) 18str r3, [r0, #(KERNEL_START 0x00f00000) 18]!ldr r6, (KERNEL_END - 1)add r0, r0, #4add r6, r4, r6, lsr #18
1: cmp r0, r6add r3, r3, #1 20strls r3, [r0], #4bls 1b#ifdef CONFIG_XIP_KERNEL/** Map some ram to cover our .data and .bss areas.*/orr r3, r7, #(KERNEL_RAM_PADDR 0xff000000).if (KERNEL_RAM_PADDR 0x00f00000)orr r3, r3, #(KERNEL_RAM_PADDR 0x00f00000).endifadd r0, r4, #(KERNEL_RAM_VADDR 0xff000000) 18str r3, [r0, #(KERNEL_RAM_VADDR 0x00f00000) 18]!ldr r6, (_end - 1)add r0, r0, #4add r6, r4, r6, lsr #18
1: cmp r0, r6add r3, r3, #1 20strls r3, [r0], #4bls 1b
#endif/** Then map first 1MB of ram in case it contains our boot params.*/add r0, r4, #PAGE_OFFSET 18orr r6, r7, #(PHYS_OFFSET 0xff000000).if (PHYS_OFFSET 0x00f00000)orr r6, r6, #(PHYS_OFFSET 0x00f00000).endifstr r6, [r0]#ifdef CONFIG_DEBUG_LLldr r7, [r10, #PROCINFO_IO_MMUFLAGS] io_mmuflags/** Map in IO space for serial debugging.* This allows debug messages to be output* via a serial console before paging_init.*/ldr r3, [r8, #MACHINFO_PGOFFIO]add r0, r4, r3rsb r3, r3, #0x4000 PTRS_PER_PGD*sizeof(long)cmp r3, #0x0800 limit to 512MBmovhi r3, #0x0800add r6, r0, r3ldr r3, [r8, #MACHINFO_PHYSIO]orr r3, r3, r7
1: str r3, [r0], #4add r3, r3, #1 20teq r0, r6bne 1b
#if defined(CONFIG_ARCH_NETWINDER) || defined(CONFIG_ARCH_CATS)/** If were using the NetWinder or CATS, we also need to map* in the 16550-type serial port for the debug messages*/add r0, r4, #0xff000000 18orr r3, r7, #0x7c000000str r3, [r0]
#endif
#ifdef CONFIG_ARCH_RPC/** Map in screen at 0x02000000 SCREEN2_BASE* Similar reasons here - for debug. This is* only for Acorn RiscPC architectures.*/add r0, r4, #0x02000000 18orr r3, r7, #0x02000000str r3, [r0]add r0, r4, #0xd8000000 18str r3, [r0]
#endif
#endifmov pc, lr
ENDPROC(__create_page_tables).ltorg#include head-common.S一、分析kernel的链接脚本 由内核配置与编译——内核的链接脚本可知kernel的入口地址在arch/arm/kernel/head.S文件的ENTRY(stext)处。 二、分析head.S文件
1、内核运行的物理地址与虚拟地址 1KERNEL_RAM_VADDRVADDR就是virtual address这个宏定义了内核运行时的虚拟地址值为0xC0008000。 2KERNEL_RAM_PADDRPADDR就是physical address这个宏定义内核运行时的物理地址值为0x30008000。 3因此内核运行的物理地址是0x30008000对应的虚拟地址是0xC0008000。 2、内核的真正入口 1__HEAD定义了段名为.head.text的段。在/include/linux/init.h文件中有如下代码 /* For assembly routines */
#define __HEAD .section .head.text,ax //定义了段名为.head.text的段
#define __INIT .section .init.text,ax
#define __FINIT .previous2“ENTRY(stext)”表明内核的真正入口。 3uboot启动内核后实际调用zImage前面的那段未经压缩的解压代码解压代码运行时先将zImage后面的部分解压开然后再去调用运行真正的内核入口即这里。 4内核启动需要一定先决条件这个条件由启动内核的bootloader比如uboot来构建保证。 5ARM体系中函数调用时实际是通过寄存器传参的。 函数调用时传参有两种设计一种是寄存器传参另一种是栈内存传参。uboot中最后theKernel (0, machid, bd-bi_boot_params);执行内核时实际把0放入r0中machid放入到了r1中bd-bi_boot_params放入到了r2中。ARM的这种处理技巧刚好满足了kernel启动的条件和要求。6此时MMU是关闭的因此硬件上需要的是物理地址。但是内核是一个整体zImage只能被链接到一个地址不能分散加载这个链接地址肯定是虚拟地址。因此head.S文件中尚未开启MMU之前的代码必须是位置无关码而且其中涉及到操作硬件寄存器等时必须使用物理地址。 3、检验CPU_ID与机器码的合法性 分别通过__lookup_processor_type与__lookup_machine_type校验CPU_ID与机器码的合法性。这两个函数都在arch/arm/kernel/head-common.S文件中。 __lookup_processor_type函数内容如下 __lookup_machine_type函数内容如下 1cp15协处理器的c0寄存器中读取出硬件的CPU ID号然后调用__lookup_processor_type来进行合法性检验。如果合法则继续启动如果不合法则停止启动转向__error_p启动失败。 2__lookup_processor_type检验cpu id合法性的方法。内核会维护一个本内核支持的CPU ID号码的数组然后该函数将从硬件中读取到的cpu id号码和数组中存储的各个id号码依次对比如果没有一个相等则不合法如果有一个相等的则合法。 3内核启动时设计这个校验也是为了内核启动的安全性着想。 4__lookup_machine_type函数的设计理念和思路和上面校验cpu id的函数一样的不同之处是本函数校验的是机器码。 4、校验uboot给内核传参的格式 利用__vet_atags函数对uboot通过tag给内核传参的格式进行校验。 这函数在arch/arm/kernel/head-common.S文件中。 1该函数的设计思路和上面2个一样用来对uboot通过tag给内核传参的格式进行校验。参数包括板子的内存分布memtag、uboot的bootargs等等。 2如果uboot给内核传参的格式不对内核将启动不起来。比如uboot的bootargs设置不正确则内核可能就会不启动。 5、建立段式页表 利用__create_page_tables函数建立段式页表。 这函数在arch/arm/kernel/head.S文件中。 1linux内核本身被链接在虚拟地址处因此kernel希望尽快建立页表并且启动MMU进入虚拟地址工作状态。 2kernel建立页表分为2步。 第一步先建立一个段式页表1MB为单位的段页表。段式页表建立过程简单段式页表1MB一个映射4GB空间需要4096个页表项每个页表项4字节因此一共需要16KB内存来做页表但不能精细管理内存。上面的函数就是用来建立段式页表的。第二步然后建立一个细页表4kb为单位的细页表然后启用新的细页表并废除第一步建立的段式映射页表。3内核启动的早期建立段式页表并在内核启动早期使用内核启动的后期再次建立细页表并启用。等内核工作起来后就只有细页表了。 6、构建C语言运行环境 1建立段式页表后进入__switch_data部分它是一个函数指针数组。 2分析得知下一步要执行__mmap_switched函数。 复制数据段、清除bss段目的是构建C语言运行环境。保存起来cpu id号、机器码、tag传参的首地址。b start_kernel跳转到C语言运行阶段。
