13Linux启动流程Word下载.docx
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@andirqsdisabled/*置当前程序状态寄存器,关中断,svc模式*/
mrcp15,0,r9,c0,c0@getprocessorid/*读processorid到r9*/
bl__lookup_processor_type@r5=procinfor9=cupid/*查找运行的cpu的id值,和此linux编译支持的id值是否有相等*/
movsr10,r5@invalidprocessor（r5=0）?
beq__error_p@yes,error'
p'
bl__lookup_machine_type@r5=machinfo/*判断体系类型是否支持*/
movsr8,r5@invalidmachine（r5=0）?
beq__error_a@yes,error'
a'
bl__vet_atags/*剖析__vet_atags内核参数列表，判断第一个参数类型是不是ATAG_CORE*/
bl__create_page_tables/*创建临时页表,供初始化使用*/
ldrr13,__switch_data@addresstojumptoafter
@mmuhasbeenenabled
adrlr,__enable_mmu@return（PIC）address
addpc,r10,#PROCINFO_INITFUNC
下面分被对于上述的四个函数进行分析:
在文件linux/arch/arm/kernel/head-common.S
一、__lookup_processor_type函数
/*
*ReadprocessorIDregister（CP#15,CR0）,andlookupinthelinker-built
*supportedprocessorlist.Notethatwecan'
tusetheabsoluteaddresses
*forthe__proc_infolistssincewearen'
trunningwiththeMMUon
*（andtherefore,wearenotinthecorrectaddressspace）.Wehaveto
*calculatetheoffset.
*
*r9=cpuid
*Returns:
*r3,r4,r6corrupted
*r5=proc_infopointerinphysicaladdressspace
*r9=cpuid（preserved）
*/
.type__lookup_processor_type,%function
__lookup_processor_type:
adrr3,3f/*取标号3的地址*/
ldmdar3,{r5-r7}/*r5:
__proc_info_begin;
r6:
__proc_info_end;
r7:
标号3的地址*/
subr3,r3,r7@getoffsetbetweenvirt&
phys/*由于adrr3,3f取得标号3的物理地址,此时r7是标号3的虚拟地址,通过此指令获得virt&
phys的差值,此时为负值*/
addr5,r5,r3@convertvirtaddressesto
addr6,r6,r3@physicaladdressspace
/*把标号__proc_info_begin,__proc_info_end从虚拟地址转化为物理地址,由于此时mmu没有开启,所以必须以物理地址来进行访问内存*/
1:
ldmiar5,{r3,r4}@value,mask/*从proc_info_list获得cpu_val（r3）和cpu_mask（r4）*/
andr4,r4,r9@maskwantedbits
teqr3,r4/*r9为从cp15读出来的cpuid,判断linux是否支持*/
beq2f/*相等则退出,r5为匹配proc_info的物理地址,
否则在下一个proc_info_list中查找*/
addr5,r5,#PROC_INFO_SZ@sizeof（proc_info_list）
cmpr5,r6
blo1b
movr5,#0@unknownprocessor/*没有找到匹配的,返回r5=0*/
2:
movpc,lr
/****************************************************************************/
*Lookininclude/asm-arm/procinfo.handarch/arm/kernel/arch.[ch]for
*moreinformationaboutthe__proc_infoand__arch_infostructures.
.long__proc_info_begin
.long__proc_info_end
3:
.long.
.long__arch_info_begin
.long__arch_info_end
标号__proc_info_begin__proc_info_end由链接脚本vmlinux.lds.S定义,在进行链接时会把其中内容填充。
__proc_info_begin=.;
*（.proc.info.init）
__proc_info_end=.;
__arch_info_begin=.;
*（.arch.info.init）
__arch_info_end=.;
二、__lookup_machine_type函数
*Lookupmachinearchitectureinthelinker-buildlistofarchitectures.
*Notethatwecan'
tusetheabsoluteaddressesforthe__arch_info
*listssincewearen'
trunningwiththeMMUon（andtherefore,weare
*notinthecorrectaddressspace）.Wehavetocalculatetheoffset.
*r1=machinearchitecturenumber
*r5=mach_infopointerinphysicaladdressspace
.type__lookup_machine_type,%function
__lookup_machine_type:
/*此函数的实现同__lookup_processor_type,分析略*/
adrr3,3b
ldmiar3,{r4,r5,r6}
subr3,r3,r4@getoffsetbetweenvirt&
phys
addr6,r6,r3@physicaladdressspace
ldrr3,[r5,#MACHINFO_TYPE]@getmachinetype
teqr3,r1@matchesloadernumber?
beq2f@found
addr5,r5,#SIZEOF_MACHINE_DESC@nextmachine_desc
movr5,#0@unknownmachine
3、__vet_atags函数
内核参数列表一般放在内核前面16K地址空间处。
列表的表项由structtag构成，每个structtag有常见的以下类型：
：
ATAG_CORE、ATAG_MEM、ATAG_CMDLINE、ATAG_RAMDISK、ATAG_INITRD等。
这些类型是宏定义，比如#defineATAG_CORE0x54410001
arch/arm/include/asm/setup.h
structtag_header{
__u32size;
__u32tag;
};
structtag{
structtag_headerhdr;
union{
structtag_corecore;
//有效的内核
structtag_mem32mem;
structtag_videotextvideotext;
structtag_ramdiskramdisk;
//文件系统
structtag_initrdinitrd;
//临时根文件系统
structtag_serialnrserialnr;
structtag_revisionrevision;
structtag_videolfbvideolfb;
structtag_cmdlinecmdline;
//命令行
}u;
/*Determinevalidityofther2atagspointer.Theheuristicrequires
*thatthepointerbealigned,inthefirst16kofphysicalRAMand
*thattheATAG_COREmarkerisfirstandpresent.Futurerevisions
*ofthisfunctionmaybemorelenientwiththephysicaladdressand
*mayalsobeabletomovetheATAGSblockifnecessary.
*
*r8=machinfo
*Returns:
*r2eithervalidatagspointer,orzero
*r5,r6corrupted
*/
__vet_atags:
tstr2,#0x3@aligned?
bne1f
ldrr5,[r2,#0]@isfirsttagATAG_CORE?
subsr5,r5,#ATAG_CORE_SIZE
ldrr5,[r2,#4]
ldrr6,=ATAG_CORE
cmpr5,r6
movpc,lr@atagpointerisok
四、__create_page_tables函数
本例中__create_page_tables函数在linux/arch/arm/kernel/head.S中。
/*
*Setuptheinitialpagetables.Weonlysetupthebarest
*amountwhicharerequiredtogetthekernelrunning,which
*generallymeansmappinginthekernelcode.
*r8=machinfo
*r10=procinfo
*r0,r3,r6,r7corrupted
*r4=physicalpagetableaddress
.type__create_page_tables,%function
__create_page_tables:
/*假设ram的物理起始地址为0x40000000*/
ldrr5,=PHYS_OFFSET
pgtblr4,r5@pagetableaddress/*r4=0x40004000页表在物理地址中基址*/
*Clearthe16Klevel1swapperpagetable
movr0,r4
movr3,#0
addr6,r0,#0x4000
strr3,[r0],#4/*多条同样的指令在多级流水线体系结构中效率会很高*/
strr3,[r0],#4
teqr0,r6
bne1b/*把物理地址0x40004000至0x40008000清零*/
ldrr7,[r10,#PROCINFO_MMUFLAGS]@mmuflags
*CreateidentitymappingforfirstMBofkernelto
*caterfortheMMUenable.Thisidentitymapping
*willberemovedbypaging_init（）.Weuseourcurrentprogram
*countertodeterminecorrespondingsectionbaseaddress.
movr6,pc,lsr#20@startofkernelsection/*r6=0x300*/
orrr3,r7,r6,lsl#20@flags+kernelbase/*r3=0x40000000|muflag=0x40000c1e*/
strr3,[r4,r6,lsl#2]@identitymapping/*0x40004c00中的内容为0x40000c1e*/
/*这一部分把物理地址0x40000000,映射到0x40000000,也就是完全映射,保证了mmu开启,都可以正常取指运行*/
*Nowsetupthepagetablesforourkerneldirect
*mappedregion.
addr0,r4,#（KERNEL_START&
0xff000000）>
>
18
@startofkernel/*r0=0x40007000*/
strr3,[r0,#（KERNEL_START&
0x00f00000）>
18]!
/*0x40007000中的内容为0x40000c1e*/
ldrr6,=（KERNEL_END-1）
addr0,r0,#4
addr6,r4,r6,lsr#18
cmpr0,r6
addr3,r3,#1<
<
20
strlsr3,[r0],#4
bls1b
*Thenmapfirst1MBoframincaseitcontainsourbootparams.
addr0,r4,#PAGE_OFFSET>
18
orrr6,r7,#PHYS_OFFSET
strr6,[r0]/*r0=0x40007000,r6=0x40000c1e*/
movpc,lr
下面以从虚拟地址0xc0005123到物理地址0x40005123的转换为例来说明mmu和页表共同的工作过程。
cp15的寄存器c2存放页表的基地址0x40004000,0x40004000&
0xffffc000|（（0xc0005123>
18）&
0x3ffc）组成地址0x40007000,以此地址作为一级描述符地址,根据上述映射过程可以得到0x40007000中的内容是0x40000c1e,其中c1e说明是段地址,有读写权限（详见一级描述符格式说明）,（0x40000c1e&
0xfff00000）|（0xc0005123&
0xfffff）的到地址0x4005123,此地址就是虚拟地址0xc0005123对应的物理地址。
/*****************************************************************************/
回到linux/arch/arm/kernel/head.S中
*ThefollowingcallsCPUspecificcodeinapositionindependent
*manner.Seearch/arm/mm/proc-*.Sfordetails.r10=baseof
*xxx_proc_infostructureselectedby__lookup_machine_type
*above.Onreturn,theCPUwillbereadyfortheMMUtobe
*turnedon,andr0willholdtheCPUcontrolregistervalue.
addpc,r10,#PROCINFO_INITFUNC/*此命令进行mmu寄存器的初始化可以参照cp15寄存器手册来分析实现的功能,执行完成后,执行函数__enable_mmu*/
*SetupcommonbitsbeforefinallyenablingtheMMU.Essentially
*thisisjustloadingthepagetablepointeranddomainaccess
*registers.
.type__enable_mmu,%function
__enable_mmu:
#ifdefCONFIG_ALIGNMENT_TRAP
orrr0,r0,#CR_A
#else
bicr0,r0,#CR_A
#endif
#ifdefCONFIG_CPU_DCACHE_DISABLE
bicr0,r0,#CR_C
#ifdefCONFIG_CPU_BPREDICT_DISABLE
bicr0,r0,#CR_Z
#ifdefCONFIG_CPU_ICACHE_DISABLE
bicr0,r0,#CR_I
movr5,#（domain_val（DOMAIN_USER,DOMAIN_MANAGER）|\
domain_val（DOMAIN_KERNEL,DOMAIN_MANAGER）|\
domain_val（DOMAIN_TABLE,DOMAIN_MANAGER）|\
domain_val（DOMAIN_IO,DOMAIN_CLIENT））
mcrp15,0,r5,c3,c0,0@loaddomainaccessregister
mcrp15,0,r4,c2,c0,0@loadpagetablepointer
b__turn_mmu_on
*EnabletheMMU.Thiscompletelychangesthestructureofthevisible
*memoryspace.Youwillnotbeabletotraceexecutionthroughthis.
*Ifyouhaveanenquiryaboutthis,*please*checkthelinux-arm-kernel
*mailinglistar
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