操作系统内核知识汇总英文版文档格式.docx

操作系统内核知识汇总英文版文档格式.docx

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evenifyoumisssomedetailsyoucangetthegistofwhat’shappening.Themainobstacleisthelackofcontextaroundsomeofthecode,suchaswhenorwhyitrunsortheunderlyingfeaturesofthemachine.Ihopetoprovideabitofthatcontext.Duetobrevity（hah!

）alotoffunstuff–likeinterruptsandmemory–getsonlyanodfornow.ThepostendswiththehighlightsfortheWindowsboot.

AtthispointintheIntelx86bootstorytheprocessorisrunninginreal-mode,isabletoaddress1MBofmemory,andRAMlookslikethisforamodernLinuxsystem:

RAMcontentsafterbootloaderisdone

ThekernelimagehasbeenloadedtomemorybythebootloaderusingtheBIOSdiskI/Oservices.Thisimageisanexactcopyofthefileinyourharddrivethatcontainsthekernel,e.g./boot/vmlinuz-2.6.22-14-server.Theimageissplitintotwopieces:

asmallpartcontainingthereal-modekernelcodeisloadedbelowthe640Kbarrier;

thebulkofthekernel,whichrunsinprotectedmode,isloadedafterthefirstmegabyteofmemory.

Theactionstartsinthereal-modekernelheaderpicturedabove.Thisregionofmemoryisusedtoimplementthe 

Linuxbootprotocol 

betweenthebootloaderandthekernel.Someofthevaluestherearereadbythebootloaderwhiledoingitswork.Theseincludeamenitiessuchasahuman-readablestringcontainingthekernelversion,butalsocrucialinformationlikethesizeofthereal-modekernelpiece.Thebootloaderalso 

writes 

valuestothisregion,suchasthememoryaddressforthecommand-lineparametersgivenbytheuserinthebootmenu.Oncethebootloaderisfinishedithasfilledinalloftheparametersrequiredbythekernelheader.It’sthentimetojumpintothekernelentrypoint.Thediagrambelowshowsthecodesequenceforthekernelinitialization,alongwithsourcedirectories,files,andlinenumbers:

Architecture-specificLinuxKernelInitialization

Theearlykernelstart-upfortheIntelarchitectureisinfile 

arch/x86/boot/header.S.It’sinassemblylanguage,whichisrareforthekernelatlargebutcommonforbootcode.Thestartofthisfileactuallycontainsbootsectorcode,aleftoverfromthedayswhenLinuxcouldworkwithoutabootloader.Nowadaysthisbootsector,ifexecuted,onlyprintsa“bugger_off_msg”totheuserandreboots.Modernbootloadersignorethislegacycode.Afterthebootsectorcodewehavethefirst15bytesofthereal-modekernelheader;

thesetwopiecestogetheraddupto512bytes,thesizeofatypicaldisksectoronIntelhardware.

Afterthese512bytes,atoffset0×

200,wefindtheveryfirstinstructionthatrunsaspartoftheLinuxkernel:

thereal-modeentrypoint.It’sin 

header.S:

110 

anditisa2-bytejumpwrittendirectlyinmachinecodeas0x3aeb.Youcanverifythisbyrunninghexdumponyourkernelimageandseeingthebytesatthatoffset–justasanitychecktomakesureit’snotalladream.Thebootloaderjumpsintothislocationwhenitisfinished,whichinturnjumpsto 

229 

wherewehavearegularassemblyroutinecalledstart_of_setup.Thisshortroutinesetsupastack,zeroesthe 

bss 

segment（theareathatcontainsstaticvariables,sotheystartwithzerovalues）forthereal-modekernelandthenjumpstogoodoldCcodeat 

arch/x86/boot/main.c:

122.

main（）doessomehousekeepinglikedetectingmemorylayout,settingavideomode,etc.Itthencalls 

go_to_protected_mode（）.BeforetheCPUcanbesettoprotectedmode,however,afewtasksmustbedone.Therearetwomainissues:

interruptsandmemory.Inreal-modethe 

interruptvectortable 

fortheprocessorisalwaysatmemoryaddress0,whereasinprotectedmodethelocationoftheinterruptvectortableisstoredinaCPUregistercalledIDTR.Meanwhile,thetranslationoflogicalmemoryaddresses（theonesprogramsmanipulate）tolinearmemoryaddresses（arawnumberfrom0tothetopofthememory）isdifferentbetweenreal-modeandprotectedmode.ProtectedmoderequiresaregistercalledGDTRtobeloadedwiththeaddressofa 

GlobalDescriptorTable 

formemory.Sogo_to_protected_mode（）calls 

setup_idt（） 

and 

setup_gdt（） 

toinstallatemporaryinterruptdescriptortableandglobaldescriptortable.

We’renowreadyfortheplungeintoprotectedmode,whichisdoneby 

protected_mode_jump,anotherassemblyroutine.ThisroutineenablesprotectedmodebysettingthePEbitintheCR0CPUregister.Atthispointwe’rerunningwith 

paging 

disabled;

pagingisanoptionalfeatureoftheprocessor,eveninprotectedmode,andthere’snoneedforityet.What’simportantisthatwe’renolongerconfinedtothe640Kbarrierandcannowaddressupto4GBofRAM.Theroutinethencallsthe32-bitkernelentrypoint,whichis 

startup_32 

forcompressedkernels.Thisroutinedoessomebasicregisterinitializationsandcalls 

decompress_kernel（）,aCfunctiontodotheactualdecompression.

decompress_kernel（）printsthefamiliar“DecompressingLinux…”message.Decompressionhappensin-placeandonceit’sfinishedtheuncompressedkernelimagehasoverwrittenthecompressedonepicturedinthefirstdiagram.Hencetheuncompressedcontentsalsostartat1MB.decompress_kernel（）thenprints“done.”andthecomforting“Bootingthekernel.”By“Booting”itmeansajumptothefinalentrypointinthiswholestory,giventoLinusbyGodhimselfatop 

MountainHalti,whichistheprotected-modekernelentrypointatthestartofthesecondmegabyteofRAM（0×

100000）.Thatsacredlocationcontainsaroutinecalled,uh, 

startup_32.But 

this 

oneisinadifferentdirectory,yousee.

Thesecondincarnationofstartup_32isalsoanassemblyroutine,butitcontains32-bitmodeinitializations.Itclearsthebsssegmentfortheprotected-modekernel（whichisthe 

true 

kernelthatwillnowrununtilthemachinerebootsorshutsdown）,setsupthefinalglobaldescriptortableformemory,buildspagetablessothatpagingcanbeturnedon,enablespaging,initializesastack,createsthefinalinterruptdescriptortable,andfinallyjumpstotothearchitecture-independentkernelstart-up, 

start_kernel（）.Thediagrambelowshowsthecodeflowforthelastlegoftheboot:

Architecture-independentLinuxKernelInitialization

start_kernel（）looksmoreliketypicalkernelcode,whichisnearlyallCandmachineindependent.Thefunctionisalonglistofcallstoinitializationsofthevariouskernelsubsystemsanddatastructures.Theseincludethescheduler,memoryzones,timekeeping,andsoon.start_kernel（）thencallsrest_init（）,atwhichpointthingsarealmostallworking.rest_init（）createsakernelthreadpassinganotherfunction, 

kernel_init（）,astheentrypoint.rest_init（）thencalls 

schedule（） 

tokickstarttaskschedulingandgoestosleepbycalling 

cpu_idle（）,whichistheidlethreadfortheLinuxkernel.cpu_idle（）runsforeverandsodoesprocesszero,whichhostsit.Wheneverthereisworktodo–arunnableprocess–processzerogetsbootedoutoftheCPU,onlytoreturnwhennorunnableprocessesareavailable.

Buthere’sthekickerforus.Thisidleloopistheendofthelongthreadwefollowedsinceboot,it’sthefinaldescendentoftheveryfirst 

jump 

executedbytheprocessorafterpowerup.Allofthismess,fromresetvectortoBIOStoMBRtobootloadertoreal-modekerneltoprotected-modekernel,allofitleadsrighthere,jumpbyjumpbyjumpitendsintheidleloopforthebootprocessor,cpu_idle（）.Whichisreallykindofcool.However,thiscan’tbethewholestoryotherwisethecomputerwoulddonowork.

Atthispoint,thekernelthreadstartedpreviouslyisreadytokickin,displacingprocess0anditsidlethread.Andsoitdoes,atwhichpointkernel_init（）startsrunningsinceitwasgivenasthethreadentrypoint. 

kernel_init（） 

isresponsibleforinitializingtheremainingCPUsinthesystem,whichhavebeenhaltedsinceboot.Allofthecodewe’veseensofarhasbeenexecutedinasingleCPU,calledthebootprocessor.AstheotherCPUs,calledapplicationprocessors,arestartedtheycomeupinreal-modeandmustrunthroughseveralinitializationsaswell.Manyofthecodepathsarecommon,asyoucanseeinthecodefor 

startup_32,butthereareslightforkstakenbythelate-comingapplicationprocessors.Finally,kernel_init（）calls 

init_post（）,whichtriestoexecuteauser-modeprocessinthefollowingorder:

/sbin/init,/etc/init,/bin/init,and/bin/sh.Ifallfail,thekernelwillpanic.Luckilyinitisusuallythere,andstartsrunningasPID1.Itchecksitsconfigurationfiletofigureoutwhichprocessestolaunch,whichmightincludeX11Windows,programsforlogginginontheconsole,networkdaemons,andsoon.ThusendsthebootprocessasyetanotherLinuxboxstartsrunningsomewhere.Mayyouruptimebelonganduntroubled.

TheprocessforWindowsissimilarinmanyways,giventhecommonarchitecture.Manyofthesameproblemsarefacedandsimilarinitializationsmustbedone.WhenitcomestobootoneofthebiggestdifferencesisthatWindowspacksallofthereal-modekernelcode,andsomeoftheinitialprotectedmodecode,intothebootloaderitself（C:

\NTLDR）.Soinsteadofhavingtworegionsinthesamekernelimage,Windowsusesdifferentbinaryimages.PlusLinuxcompletelyseparatesbootloaderandkernel;

inawaythisautomatic