1. kernel运行的史前时期和内存布局
在arm平
台下,zImage.bin压缩镜像是由bootloader加载到物理内存,然后跳到zImage.bin里一段程序,它专门于将被压缩的kernel
解压缩到KERNEL_RAM_PADDR开始的一段内存中,接着跳进真正的kernel去执行。该kernel的执行起点是stext函数,定义于
arch/arm/kernel/head.S。
在分析stext函数前,先介绍此时内存的布局如下图所示
在开发板
tqs3c2440中,SDRAM连接到内存控制器的Bank6中,它的开始内存地址是0x30000000,大小为64M,即0x20000000。
ARM Linux
kernel将SDRAM的开始地址定义为PHYS_OFFSET。经bootloader加载kernel并由自解压部分代码运行后,最终kernel
被放置到KERNEL_RAM_PADDR(=PHYS_OFFSET
+ TEXT_OFFSET,即0x30008000)地址上的一段内存,经此放置后,kernel代码以后均不会被移动。
在进入
kernel代码前,即bootloader和自解压缩阶段,ARM未开启MMU功能。因此kernel启动代码一个重要功能是设置好相应的页表,并开启
MMU功能。为了支持MMU功能,kernel镜像中的所有符号,包括代码段和数据段的符号,在链接时都生成了它在开启MMU时,所在物理内存地址映射到
的虚拟内存地址。
以arm
kernel第一个符号(函数)stext为例,在编译链接,它生成的虚拟地址是0xc0008000,而放置它的物理地址为0x30008000(还记
得这是PHYS_OFFSET+TEXT_OFFSET吗?)。实际上这个变换可以利用简单的公式进行表示:va = pa – PHYS_OFFSET
+ PAGE_OFFSET。Arm linux最终的kernel空间的页表,就是按照这个关系来建立。
之所以较早
提及arm linux
的内存映射,原因是在进入kernel代码,里面所有符号地址值为清一色的0xCXXXXXXX地址,而此时ARM未开启MMU功能,故在执行stext
函数第一条执行时,它的PC值就是stext所在的内存地址(即物理地址,0x30008000)。因此,下面有些代码,需要使用地址无关技术。
2.一览stext函数
stext函数定义在Arch/arm/kernel/head.S,它的功能是获取处理器类型和机器类型信息,并创建临时的页表,然后开启MMU功能,并跳进第一个C语言函数start_kernel。
stext函数的在前置条件是:MMU, D-cache, 关闭; r0 = 0, r1 = machine nr, r2 = atags prointer.
代码如下:
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.section ".text.head", "ax"
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(stext)
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msr cpsr_c, #PSR_F_BIT | PSR_I_BIT | SVC_MODE @ ensure svc mode
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@ and irqs disabled
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mrc p15, 0, r9, c0, c0 @ get processor id
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bl __lookup_processor_type @ r5=procinfo r9=cupid
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movs r10, r5 @ invalid processor (r5=0)?
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beq __error_p @ yes, error 'p'
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bl __lookup_machine_type @ r5=machinfo
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movs r8, r5 @ invalid machine (r5=0)?
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beq __error_a @ yes, error 'a'
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bl __vet_atags
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bl __create_page_tables
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ldr r13, __switch_data @ address to jump to after
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@ mmu has been enabled
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adr lr, __enable_mmu @ return (PIC) address
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add pc, r10, #PROCINFO_INITFUNC
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OC(stext)
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.section ".text.head", "ax"
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(stext)
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msr cpsr_c, #PSR_F_BIT | PSR_I_BIT | SVC_MODE @ ensure svc mode
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@ and irqs disabled
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mrc p15, 0, r9, c0, c0 @ get processor id
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bl __lookup_processor_type @ r5=procinfo r9=cupid
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movs r10, r5 @ invalid processor (r5=0)?
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beq __error_p @ yes, error 'p'
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bl __lookup_machine_type @ r5=machinfo
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movs r8, r5 @ invalid machine (r5=0)?
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beq __error_a @ yes, error 'a'
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bl __vet_atags
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bl __create_page_tables
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ldr r13, __switch_data @ address to jump to after
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@ mmu has been enabled
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adr lr, __enable_mmu @ return (PIC) address
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add pc, r10, #PROCINFO_INITFUNC
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OC(stext)
3 __lookup_processor_type 函数
__lookup_processor_type 函数是一个非常讲究技巧的函数,如果你将它领会,也将领会kernel了一些魔法。
Kernel
代码将所有CPU信息的定义都放到.proc.info.init段中,因此可以认为.proc.info.init段就是一个数组,每个元素都定义了一
个或一种CPU的信息。目前__lookup_processor_type使用该元素的前两个字段cpuid和mask来匹配当前CPUID,如果满足
CPUID & mask ==
cpuid,则找到当前cpu的定义并返回。
下面是tqs3c2440开发板,CPU的定义信息,cpuid = 0x41009200,mask = 0xff00fff0。如果是码是运行在tqs3c2440开发板上,那么函数返回下面的定义:
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.section ".proc.info.init", #alloc, #execinstr
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.type __arm920_proc_info,#object
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__arm920_proc_info:
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.long 0x41009200
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.long 0xff00fff0
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.long PMD_TYPE_SECT | \
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PMD_SECT_BUFFERABLE | \
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PMD_SECT_CACHEABLE | \
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PMD_BIT4 | \
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PMD_SECT_AP_WRITE | \
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PMD_SECT_AP_READ
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.long PMD_TYPE_SECT | \
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PMD_BIT4 | \
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PMD_SECT_AP_WRITE | \
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PMD_SECT_AP_READ
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b __arm920_setup
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.long cpu_arch_name
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.long cpu_elf_name
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.long HWCAP_SWP | HWCAP_HALF | HWCAP_THUMB
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.long cpu_arm920_name
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.long arm920_processor_functions
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.long v4wbi_tlb_fns
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.long v4wb_user_fns
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#ifndef CONFIG_CPU_DCACHE_WRITETHROUGH
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.long arm920_cache_fns
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#else
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.long v4wt_cache_fns
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#endif
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.size __arm920_proc_info, . - __arm920_proc_info
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.section ".proc.info.init", #alloc, #execinstr
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.type __arm920_proc_info,#object
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__arm920_proc_info:
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.long 0x41009200
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.long 0xff00fff0
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.long PMD_TYPE_SECT | \
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PMD_SECT_BUFFERABLE | \
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PMD_SECT_CACHEABLE | \
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PMD_BIT4 | \
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PMD_SECT_AP_WRITE | \
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PMD_SECT_AP_READ
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.long PMD_TYPE_SECT | \
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PMD_BIT4 | \
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PMD_SECT_AP_WRITE | \
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PMD_SECT_AP_READ
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b __arm920_setup
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.long cpu_arch_name
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.long cpu_elf_name
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.long HWCAP_SWP | HWCAP_HALF | HWCAP_THUMB
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.long cpu_arm920_name
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.long arm920_processor_functions
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.long v4wbi_tlb_fns
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.long v4wb_user_fns
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#ifndef CONFIG_CPU_DCACHE_WRITETHROUGH
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.long arm920_cache_fns
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#else
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.long v4wt_cache_fns
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#endif
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.size __arm920_proc_info, . - __arm920_proc_info
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__lookup_processor_type:
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adr r3, 3f
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ldmda r3, {r5 - r7}
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sub r3, r3, r7 @ get offset between virt&phys
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add r5, r5, r3 @ convert virt addresses to
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add r6, r6, r3 @ physical address space
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1: ldmia r5, {r3, r4} @ value, mask
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and r4, r4, r9 @ mask wanted bits
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teq r3, r4
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beq 2f
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add r5, r5, #PROC_INFO_SZ @ sizeof(proc_info_list)
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cmp r5, r6
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blo 1b
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mov r5, #0 @ unknown processor
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2: mov pc, lr
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ENDPROC(__lookup_processor_type)
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.long __proc_info_begin
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.long __proc_info_end
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3: .long .
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.long __arch_info_begin
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.long __arch_info_end
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__lookup_processor_type:
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adr r3, 3f
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ldmda r3, {r5 - r7}
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sub r3, r3, r7 @ get offset between virt&phys
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add r5, r5, r3 @ convert virt addresses to
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add r6, r6, r3 @ physical address space
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1: ldmia r5, {r3, r4} @ value, mask
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and r4, r4, r9 @ mask wanted bits
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teq r3, r4
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beq 2f
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add r5, r5, #PROC_INFO_SZ @ sizeof(proc_info_list)
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cmp r5, r6
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blo 1b
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mov r5, #0 @ unknown processor
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2: mov pc, lr
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ENDPROC(__lookup_processor_type)
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.long __proc_info_begin
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.long __proc_info_end
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3: .long .
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.long __arch_info_begin
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.long __arch_info_end
4 __lookup_machine_type 函数
__lookup_machine_type
和__lookup_processor_type像对孪生兄弟,它们的行为都是很类似的:__lookup_machine_type根据r1寄存器的
机器编号到.arch.info.init段的数组中依次查找机器编号与r1相同的记录。它使了与它孪生兄弟同样的手法进行虚拟地址到物理地址的转换计
算。
在介绍函数,我们先分析tqs3c2440开发板的机器信息的定义:
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Arch/arm/include/asm/mach/arch.h
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#define MACHINE_START(_type,_name) \
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static const struct machine_desc __mach_desc_##_type \
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__used \
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__attribute__((__section__(".arch.info.init"))) = { \
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.nr = MACH_TYPE_##_type, \
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.name = _name,
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#define MACHINE_END \
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};
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Arch/arm/include/asm/mach/arch.h
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#define MACHINE_START(_type,_name) \
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static const struct machine_desc __mach_desc_##_type \
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__used \
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__attribute__((__section__(".arch.info.init"))) = { \
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.nr = MACH_TYPE_##_type, \
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.name = _name,
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#define MACHINE_END \
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};
MACHINE_START宏用于定义一个.arch.info.init段的数组元素。.nr元素就是函数要比较的变量。Tqs3c2440开发板相应的定义如下:
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MACHINE_START(S3C2440, "TQ2440")
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.phys_io = S3C2410_PA_UART,
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.io_pg_offst = (((u32)S3C24XX_VA_UART) >> 18) & 0xfffc,
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.boot_params = S3C2410_SDRAM_PA + 0x100,
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.init_irq = s3c24xx_init_irq,
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.map_io = tq2440_map_io,
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.init_machine = tq2440_machine_init,
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.timer = &s3c24xx_timer,
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MACHINE_END
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MACHINE_START(S3C2440, "TQ2440")
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.phys_io = S3C2410_PA_UART,
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.io_pg_offst = (((u32)S3C24XX_VA_UART) >> 18) & 0xfffc,
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.boot_params = S3C2410_SDRAM_PA + 0x100,
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.init_irq = s3c24xx_init_irq,
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.map_io = tq2440_map_io,
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.init_machine = tq2440_machine_init,
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.timer = &s3c24xx_timer,
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MACHINE_END
这是一个struct machine_desc结构,在后面的C代码(start_kernel开始执行的代码)会使用该变量对象。在tqs3c2440开发中的__lookup_machine_type函数就是返回该对象指针。
这里涉及很多函数指针,它们都是在start_kernel函数里在各种阶段进行初始化的回函数。如map_io指向的tq2440_map_io就是在建立好内核页表后,再调用它来针对开发板的各种IO端口来建立相关的映射和页表。
至于__loopup_machine_type的代码就不作详细分析,请对比__lookup_processor_type来自行分析。代码如下:
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__lookup_machine_type:
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adr r3, 3b
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ldmia r3, {r4, r5, r6}
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sub r3, r3, r4 @ get offset between virt&phys
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add r5, r5, r3 @ convert virt addresses to
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add r6, r6, r3 @ physical address space
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1: ldr r3, [r5, #MACHINFO_TYPE] @ get machine type
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teq r3, r1 @ matches loader number?
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beq 2f @ found
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add r5, r5, #SIZEOF_MACHINE_DESC @ next machine_desc
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cmp r5, r6
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blo 1b
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mov r5, #0 @ unknown machine
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2: mov pc, lr
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ENDPROC(__lookup_machine_type)
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__lookup_machine_type:
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adr r3, 3b
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ldmia r3, {r4, r5, r6}
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sub r3, r3, r4 @ get offset between virt&phys
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add r5, r5, r3 @ convert virt addresses to
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add r6, r6, r3 @ physical address space
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1: ldr r3, [r5, #MACHINFO_TYPE] @ get machine type
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teq r3, r1 @ matches loader number?
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beq 2f @ found
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add r5, r5, #SIZEOF_MACHINE_DESC @ next machine_desc
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cmp r5, r6
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blo 1b
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mov r5, #0 @ unknown machine
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2: mov pc, lr
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ENDPROC(__lookup_machine_type)
5. 为kernel建立临时页表
前面提及
到,kernel里面的所有符号在链接时,都使用了虚拟地址值。在完成基本的初始化后,kernel代码将跳到第一个C语言函数start_kernl来
执行,在哪个时候,这些虚拟地址必须能够对它所存放在真正内存位置,否则运行将为出错。为此,CPU必须开启MMU,但在开启MMU前,必须为虚拟地址到
物理地址的映射建立相应的面表。在开启MMU后,kernel指并不马上将PC值指向start_kernl,而是要做一些C语言运行期的设置,如堆栈,
重定义等工作后才跳到start_kernel去执行。在此过程中,PC值还是物理地址,因此还需要为这段内存空间建立va
= pa的内存映射关系。当然,本函数建立的所有页表都会在将来paging_init销毁再重建,这是临时过度性的映射关系和页表。
在介绍
__create_table_pages前,先认识一个macro pgtbl,它将KERNL_RAM_PADDR –
0x4000的值赋给rd寄存器,从下面的使用中可以看它,该值是页表在物理内存的基础,也即页表放在kernel开始地址下的16K的地方。
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.macro pgtbl, rd
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ldr \rd, =(KERNEL_RAM_PADDR - 0x4000)
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.endm
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.macro pgtbl, rd
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ldr \rd, =(KERNEL_RAM_PADDR - 0x4000)
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.endm
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__create_page_tables:
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pgtbl r4 @ page table address
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mov r0, r4
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mov r3, #0
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add r6, r0, #0x4000
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1: str r3, [r0], #4
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str r3, [r0], #4
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str r3, [r0], #4
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str r3, [r0], #4
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teq r0, r6
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bne 1b
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ldr r7, [r10, #PROCINFO_MM_MMUFLAGS] @ mm_mmuflags
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str r3, [r4, r6, lsl #2] @ identity mapping
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str r3, [r0, #(KERNEL_START & 0x00f00000) >> 18]!
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ldr r6, =(KERNEL_END - 1)
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add r0, r0, #4
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add r6, r4, r6, lsr #18
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1: cmp r0, r6
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add r3, r3, #1 << 20
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strls r3, [r0], #4
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bls 1b
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#ifdef CONFIG_XIP_KERNEL
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#endif
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add r0, r4, #PAGE_OFFSET >> 18
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orr r6, r7, #(PHYS_OFFSET & 0xff000000)
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.if (PHYS_OFFSET & 0x00f00000)
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orr r6, r6, #(PHYS_OFFSET & 0x00f00000)
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.endif
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str r6, [r0]
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#ifdef CONFIG_DEBUG_LL
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#if defined(CONFIG_ARCH_NETWINDER) || defined(CONFIG_ARCH_CATS)
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#endif
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#ifdef CONFIG_ARCH_RPC
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#endif
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#endif
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mov pc, lr
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ENDPROC(__create_page_tables)
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__create_page_tables:
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pgtbl r4 @ page table address
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mov r0, r4
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mov r3, #0
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add r6, r0, #0x4000
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1: str r3, [r0], #4
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str r3, [r0], #4
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str r3, [r0], #4
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str r3, [r0], #4
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teq r0, r6
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bne 1b
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ldr r7, [r10, #PROCINFO_MM_MMUFLAGS] @ mm_mmuflags
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str r3, [r4, r6, lsl #2] @ identity mapping
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str r3, [r0, #(KERNEL_START & 0x00f00000) >> 18]!
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ldr r6, =(KERNEL_END - 1)
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add r0, r0, #4
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add r6, r4, r6, lsr #18
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1: cmp r0, r6
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add r3, r3, #1 << 20
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strls r3, [r0], #4
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bls 1b
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#ifdef CONFIG_XIP_KERNEL
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#endif
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add r0, r4, #PAGE_OFFSET >> 18
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orr r6, r7, #(PHYS_OFFSET & 0xff000000)
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.if (PHYS_OFFSET & 0x00f00000)
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orr r6, r6, #(PHYS_OFFSET & 0x00f00000)
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.endif
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str r6, [r0]
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-
#ifdef CONFIG_DEBUG_LL
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-
#if defined(CONFIG_ARCH_NETWINDER) || defined(CONFIG_ARCH_CATS)
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-
#endif
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-
#ifdef CONFIG_ARCH_RPC
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-
#endif
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-
#endif
-
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mov pc, lr
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ENDPROC(__create_page_tables)
一口气将__create_pages_table分析完,但里涉及的代码还是需要细细品读。尤其是右移20位和18位两个地方与页表目录项的地址关系比较复杂。执行完该函数后,虚拟内存和物理内存的映射关系如下图所示:
6. 开启MMU
看完页表的建立,想必开启MMU的代码也是小菜一碟吧。此函数的主要功能是将页表的基址加到cp15中的面表指针寄存器,同时设置域访问(domain access)寄存器。
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-
-
-
-
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__enable_mmu:
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#ifdef CONFIG_ALIGNMENT_TRAP
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orr r0, r0, #CR_A
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#else
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bic r0, r0, #CR_A
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#endif
-
-
#ifdef CONFIG_CPU_DCACHE_DISABLE
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bic r0, r0, #CR_C
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#endif
-
-
#ifdef CONFIG_CPU_BPREDICT_DISABLE
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bic r0, r0, #CR_Z
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#endif
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-
#ifdef CONFIG_CPU_ICACHE_DISABLE
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bic r0, r0, #CR_I
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#endif
-
-
-
-
-
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mov r5, #(domain_val(DOMAIN_USER, DOMAIN_MANAGER) | \
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domain_val(DOMAIN_KERNEL, DOMAIN_MANAGER) | \
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domain_val(DOMAIN_TABLE, DOMAIN_MANAGER) | \
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domain_val(DOMAIN_IO, DOMAIN_CLIENT))
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mcr p15, 0, r5, c3, c0, 0 @ load domain access register
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mcr p15, 0, r4, c2, c0, 0 @ load page table pointer
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b __turn_mmu_on
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ENDPROC(__enable_mmu)
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-
-
-
-
-
-
-
-
-
-
-
-
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.align 5
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__turn_mmu_on:
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mov r0, r0
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-
-
-
-
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mcr p15, 0, r0, c1, c0, 0 @ write control reg
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mrc p15, 0, r3, c0, c0, 0 @ read id reg
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-
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mov r3, r3
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mov r3, r3
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-
-
-
-
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mov pc, r13
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ENDPROC(__turn_mmu_on)
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-
-
-
-
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__enable_mmu:
-
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#ifdef CONFIG_ALIGNMENT_TRAP
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orr r0, r0, #CR_A
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#else
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bic r0, r0, #CR_A
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#endif
-
-
#ifdef CONFIG_CPU_DCACHE_DISABLE
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bic r0, r0, #CR_C
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#endif
-
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#ifdef CONFIG_CPU_BPREDICT_DISABLE
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bic r0, r0, #CR_Z
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#endif
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#ifdef CONFIG_CPU_ICACHE_DISABLE
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bic r0, r0, #CR_I
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#endif
-
-
-
-
-
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mov r5, #(domain_val(DOMAIN_USER, DOMAIN_MANAGER) | \
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domain_val(DOMAIN_KERNEL, DOMAIN_MANAGER) | \
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domain_val(DOMAIN_TABLE, DOMAIN_MANAGER) | \
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domain_val(DOMAIN_IO, DOMAIN_CLIENT))
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mcr p15, 0, r5, c3, c0, 0 @ load domain access register
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mcr p15, 0, r4, c2, c0, 0 @ load page table pointer
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b __turn_mmu_on
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ENDPROC(__enable_mmu)
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-
-
-
-
-
-
-
-
-
-
-
-
-
.align 5
-
__turn_mmu_on:
-
mov r0, r0
-
-
-
-
-
-
mcr p15, 0, r0, c1, c0, 0 @ write control reg
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mrc p15, 0, r3, c0, c0, 0 @ read id reg
-
-
-
mov r3, r3
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mov r3, r3
-
-
-
-
-
-
mov pc, r13
-
ENDPROC(__turn_mmu_on)
7.__mmap_switched函数
__mmap_switched函数专用来设置C语言的执行环境,比如重定位工作,堆栈,以及BSS段的清零。
__switch_data变量先定义了一系里面处量的数据,如重定位和数据段的地址,BSS段的地址,pocessor_id和__mach_arch_type变量的地址等。
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.type __switch_data, %object
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__switch_data:
-
.long __mmap_switched
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.long __data_loc @ r4
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.long _data @ r5
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.long __bss_start @ r6
-
.long _end @ r7
-
.long processor_id @ r4
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.long __machine_arch_type @ r5
-
.long __atags_pointer @ r6
-
.long cr_alignment @ r7
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.long init_thread_union + THREAD_START_SP @ sp
-
-
-
-
-
-
-
-
-
-
-
__mmap_switched:
-
adr r3, __switch_data + 4
-
-
ldmia r3!, {r4, r5, r6, r7}
-
-
-
-
-
-
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cmp r4, r5 @ Copy data segment if needed
-
1: cmpne r5, r6
-
ldrne fp, [r4], #4
-
strne fp, [r5], #4
-
bne 1b
-
-
-
-
mov fp, #0 @ Clear BSS (and zero fp)
-
1: cmp r6, r7
-
strcc fp, [r6],#4
-
bcc 1b
-
-
-
-
-
-
-
-
-
-
-
-
-
ldmia r3, {r4, r5, r6, r7, sp}
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str r9, [r4] @ Save processor ID
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str r1, [r5] @ Save machine type
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str r2, [r6] @ Save atags pointer
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bic r4, r0, #CR_A @ Clear 'A' bit
-
-
-
-
-
-
-
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stmia r7, {r0, r4} @ Save control register values
-
-
-
-
-
-
b start_kernel
-
ENDPROC(__mmap_switched)
-
.type __switch_data, %object
-
__switch_data:
-
.long __mmap_switched
-
.long __data_loc @ r4
-
.long _data @ r5
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.long __bss_start @ r6
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.long _end @ r7
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.long processor_id @ r4
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.long __machine_arch_type @ r5
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.long __atags_pointer @ r6
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.long cr_alignment @ r7
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.long init_thread_union + THREAD_START_SP @ sp
-
-
-
-
-
-
-
-
-
-
-
__mmap_switched:
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adr r3, __switch_data + 4
-
-
ldmia r3!, {r4, r5, r6, r7}
-
-
-
-
-
-
-
cmp r4, r5 @ Copy data segment if needed
-
1: cmpne r5, r6
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ldrne fp, [r4], #4
-
strne fp, [r5], #4
-
bne 1b
-
-
-
-
mov fp, #0 @ Clear BSS (and zero fp)
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1: cmp r6, r7
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strcc fp, [r6],#4
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bcc 1b
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-
-
-
-
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-
-
-
-
-
-
-
ldmia r3, {r4, r5, r6, r7, sp}
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str r9, [r4] @ Save processor ID
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str r1, [r5] @ Save machine type
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str r2, [r6] @ Save atags pointer
-
bic r4, r0, #CR_A @ Clear 'A' bit
-
-
-
-
-
-
-
-
stmia r7, {r0, r4} @ Save control register values
-
-
-
-
-
-
b start_kernel
-
ENDPROC(__mmap_switched)
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