interrupts and system calls - computer science · 2016. 7. 29. · • lab 1 due friday 2/5 ... 11...

CSE506:Opera.ngSystems

InterruptsandSystemCalls

DonPorterCSE506

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Housekeeping•  NextTuesday’sclasshasareadingassignment•  Lab1dueFriday2/5•  AllstudentsshouldhaveVMsandprivaterepossoon–  Alotofthelabisreading,soyoucangoaheadandstartiftheyarenotavailable

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LogicalDiagram

MemoryManagement

CPUScheduler

User

Kernel

Hardware

BinaryFormats

Consistency

SystemCalls

Interrupts Disk Net

RCU FileSystem

DeviceDrivers

Networking Sync

MemoryAllocators Threads

Today’sLecture

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Background:ControlFlow

// x = 2, y = true if (y) {

2 /= x; printf(x);

} //...

void printf(va_args) {

//...

}

Regularcontrolflow:branchesandcalls(logicallyfollowssourcecode)

pc

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Background:ControlFlow

// x = 0, y = true if (y) {

2 /= x; printf(x);

} //...

void handle_divzero(){

x = 2;

}

Irregularcontrolflow:excepYons,systemcalls,etc.

pc Dividebyzero!Programcan’tmake

progress!

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Lecturegoal•  Understandthehardwaretoolsavailableforirregularcontrolflow.–  I.e.,thingsotherthanabranchinarunningprogram

•  Buildingblocksforcontextswitching,devicemanagement,etc.

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Twotypesofinterrupts•  Synchronous:willhappeneveryYmeaninstrucYonexecutes(withagivenprogramstate)–  Dividebyzero–  Systemcall–  Badpointerdereference

•  Asynchronous:causedbyanexternalevent–  UsuallydeviceI/O–  TimerYcks(well,clockscanbeconsideredadevice)

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Intelnomenclature•  Interrupt–onlyreferstoasynchronousinterrupts•  ExcepYon–synchronouscontroltransfer

•  Note:fromtheprogrammer’sperspecYve,thesearehandledwiththesameabstracYons

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Lectureoutline•  Overview•  Howinterruptsworkinhardware•  Howinterrupthandlersworkinsoaware•  Howsystemcallswork•  Newsystemcallhardwareonx86

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Interruptoverview•  EachinterruptorexcepYonincludesanumberindicaYngitstype

•  E.g.,14isapagefault,3isadebugbreakpoint•  Thisnumberistheindexintoaninterrupttable

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x86interrupttable

0 255

…

31

… …

47

ReservedfortheCPU

SoawareConfigurable

DeviceIRQs 48=JOSSystemCall

128=LinuxSystemCall

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x86interruptoverview•  Eachtypeofinterruptisassignedanindexfrom0—255.

•  0—31areforprocessorinterrupts;generallyfixedbyIntel–  E.g.,14isalwaysforpagefaults

•  32—255aresoawareconfigured–  32—47arefordeviceinterrupts(IRQs)inJOS

•  Mostdevice’sIRQlinecanbeconfigured•  LookupAPICsformoreinfo(Ch4ofBovetandCesaY)

–  0x80issuessystemcallinLinux(moreonthislater)

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Soawareinterrupts•  Theint instrucYonallowssoawaretoraiseaninterrupt–  0x80isjustaLinuxconvenYon.JOSuses0x30.

•  Therearealotofspareindices–  YoucouldhavemulYplesystemcalltablesfordifferentpurposesortypesofprocesses!•  Windowsdoes:oneforthekernelandoneforwin32k

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Soawareinterrupts,cont•  OSsetsringlevelrequiredtoraiseaninterrupt–  Generally,userprogramscan’tissueanint 14 (pagefault)manually

–  AnunauthorizedintinstrucYoncausesageneralprotecYonfault•  Interrupt13

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Whathappens(generally):•  Controljumpstothekernel–  Ataprescribedaddress(theinterrupthandler)

•  Theregisterstateoftheprogramisdumpedonthekernel’sstack–  SomeYmes,extrainfoisloadedintoCPUregisters–  E.g.,pagefaultsstoretheaddressthatcausedthefaultinthecr2register

•  Kernelcoderunsandhandlestheinterrupt•  Whenhandlercompletes,resumeprogram(seeiretinstr.)

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Howitworks(HW)•  HowdoesHWknowwhattoexecute?•  WheredoestheHWdumptheregisters;whatdoesituseastheinterrupthandler’sstack?

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Howisthisconfigured?•  KernelcreatesanarrayofInterruptdescriptorsinmemory,calledInterruptDescriptorTable,orIDT–  Canbeanywhereinmemory–  Pointedtobyspecialregister(idtr)

•  c.f.,segmentregistersandgdtr andldtr

•  Entry0configuresinterrupt0,andsoon

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x86interrupttable

0 255

…

31

… …

47

idtr

LinearAddressofInterruptTable

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x86interrupttable

0 255

…

31

… …

47

idtr

Code Segment: Kernel Code Segment Offset: &page_fault_handler //linear addr Ring: 0 // kernel Present: 1 Gate Type: Exception

14

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InterruptDescriptor•  Codesegmentselector–  Almostalwaysthesame(kernelcodesegment)–  Recall,thiswasdesignedbeforepagingonx86!

•  Segmentoffsetofthecodetorun–  Kernelsegmentis“flat”,sothisisjustthelinearaddress

•  PrivilegeLevel(ring)–  RingthatcanraisethisinterruptwithanintinstrucYon

•  Presentbit–disableunusedinterrupts•  Gatetype(interruptortrap/excepYon)–moreinabit

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x86interrupttable

0 255

…

31

… …

47

idtr

Code Segment: Kernel Code Segment Offset: &breakpoint_handler //linear addr Ring: 3 // user Present: 1 Gate Type: Exception

3

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InterruptDescriptors,ctd.•  In-memorylayoutisabitconfusing–  Likealotofthex86architecture,manyinterfaceswerelaterdeprecated

•  WorthcomparingCh9.5ofthei386manualwithinc/mmu.hintheJOSsourcecode

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Howitworks(HW)•  HowdoesHWknowwhattoexecute?–  Interruptdescriptortablespecifieswhatcodetorun

•  Andatwhatprivilege(viacodesegment)–  Thiscanbesetuponceduringbootforthewholesystem

•  WheredoestheHWdumptheregisters;whatdoesituseastheinterrupthandler’sstack?–  SpecifiedintheTaskStateSegment

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TaskStateSegment(TSS)•  Anothersegment,justlikethecodeanddatasegment–  AdescriptorcreatedintheGDT(cannotbeinLDT)–  Selectedbyspecialtaskregister(tr)–  Unlikeothers,hasahardware-specifiedlayout

•  Lotsoffieldsforrarely-usedfeatures•  TwofeatureswecareaboutinamodernOS:–  1)LocaYonofkernelstack(fieldsss0/esp0)–  2)I/OPortprivileges(moreinalaterlecture)

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TSS,cont.•  Simplemodel:specifyaTSSforeachprocess–  Note:Only2^13entriesintheGDT

•  OpYmizaYon(JOS):–  Ourkernelispreuysimple(uniprocessoronly)– WhynotjustshareoneTSSandkernelstackper-process?

•  LinuxgeneralizaYon:–  OneTSSperCPU– ModifyTSSfieldsaspartofcontextswitching

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Summary•  Mostinterrupthandlinghardwarestatesetduringboot

•  EachinterrupthasanIDTentryspecifying:– Whatcodetoexecute,privilegeleveltoraisetheinterrupt

•  StacktousespecifiedintheTSS

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Comment•  Again,segmentaYonrearsitshead•  Youcan’tprogramOS-levelcodeonx86withoutgevngyourhandsdirtywithit

•  Helpstoknowwhichfeaturesareimportantwhenreadingthemanuals

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Lectureoutline•  Overview•  Howinterruptsworkinhardware•  Howinterrupthandlersworkinso>ware•  Howsystemcallswork•  Newsystemcallhardwareonx86

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High-levelgoal•  Respondtosomeevent,returncontroltotheappropriateprocess

•  Whattodoon:–  Networkpacketarrives–  DiskreadcompleYon–  Dividebyzero–  Systemcall

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InterruptHandlers•  Justplainoldkernelcode

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Example

User Kernel

Stack Stack

if (x) { printf(“Boo”); ...

printf(va_args…){

...

Disk_handler (){ ...

}

RSP

RIP

RSP

RIP

DiskInterrupt!

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ComplicaYon:•  WhathappensifI’minaninterrupthandler,andanotherinterruptcomesin?–  Note:kernelstackonlychangesonprivilegelevelchange–  Nestedinterruptsjustpushthenextframeonthestack

•  Whatcouldgowrong?–  Violatecodeinvariants–  Deadlock–  Exhaustthestack(iftoomanyfireatonce)

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Example

User Kernel

Stack Stack

if (x) { printf(“Boo”); ...

printf(va_args…){

...

disk_handler (){ lock_kernel(); ... unlock_kernel();

...

RSP

RIP

net_handler (){ lock_kernel(); …

NetworkInterrupt!

WillHangForever!AlreadyLocked!!!

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BouomLine:•  InterruptservicerouYnesmustbereentrantorsynchronize

•  Period.

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Hardwareinterruptsync.•  WhileaCPUisservicinganinterruptonagivenIRQline,thesameIRQwon’traiseanotherinterruptunYltherouYnecompletes–  Bouom-line:deviceinterrupthandlerdoesn’thavetoworryaboutbeinginterruptedbyitself

•  Adifferentdevicecaninterruptthehandler–  ProblemaYciftheysharedatastructures–  Likealistoffreephysicalpages…– Whatifbothtrytograbalockforthefreelist?

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Disablinginterrupts•  Anx86CPUcandisableI/Ointerrupts–  Clearbit9oftheEFLAGSregister(IFFlag)–  cliandstiinstrucYonsclearandsetthisflag

•  Beforetouchingashareddatastructure(orgrabbingalock),aninterrupthandlershoulddisableI/Ointerrupts

36


Gatetypes•  Recall:anIDTentrycanbeaninterruptoranexcepYongate

•  Difference?–  AninterruptgateautomaYcallydisablesallotherinterrupts(i.e.,clearsandsetsIFonenter/exit)

–  AnexcepYongatedoesn’t•  Thisisjustaprogrammerconvenience:youcoulddothesamethinginsoaware

37


ExcepYons•  Youcan’tmaskexcepYons– Whynot?

•  Can’tmakeprogressaaeradivide-by-zero–  DoubleandTriplefaultsdetectfaultsinthekernel

•  DoexcepYonhandlersneedtobereentrant?–  Notifyourkernelhasnobugs(orsystemcallsinitself)–  Incertaincases,Linuxallowsnestedpagefaults

•  E.g.,todetecterrorscopyinguser-providedbuffers

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Summary•  Interrupthandlersneedtosynchronize,bothwithlocks(mulY-processor)andbydisablinginterrupts(sameCPU)

•  ExcepYonhandlerscan’tbemasked–  NestedexcepYonsgenerallyavoided

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Systemcall“interrupt”•  Originally,systemcallsissuedusingintinstrucYon•  DispatchrouYnewasjustaninterrupthandler•  Likeinterrupts,systemcallsarearrangedinatable–  Seearch/x86/kernel/syscall_table*.SinLinuxsource

•  Programselectstheoneitwantsbyplacingindexineaxregister–  ArgumentsgointheotherregistersbycallingconvenYon–  Returnvaluegoesineax

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AroundP4era…•  Processorsgotverydeeplypipelined–  Pipelinestalls/flushesbecameveryexpensive–  Cachemissescancausepipelinestalls

•  SystemcallstooktwiceaslongfromP3toP4– Why?–  IDTentrymaynotbeinthecache–  DifferentpermissionsconstraininstrucYonreordering

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Idea•  WhatifwecachetheIDTentryforasystemcallinaspecialCPUregister?–  NomorecachemissesfortheIDT!– MaybewecanalsodomoreopYmizaYons

•  AssumpYon:systemcallsarefrequentenoughtobeworththetransistorbudgettoimplementthis– Whatelsecouldyoudowithextratransistorsthathelpsperformance?

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AMD:syscall/sysret•  TheseinstrucYonsuseMSRs(machinespecificregisters)tostore:–  Syscallentrypointandcodesegment–  Kernelstack

•  Adrop-inreplacementforint 0x80•  Everyoneloveditandadopteditwholesale–  EvenIntel!

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Aaermath•  Getpid()onmydesktopmachine(recentAMD6-core):–  Int80:371cycles–  Syscall:231cycles

•  Sosystemcallsaredefinitelyfasterasaresult!

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InJOS•  YouwillusetheintinstrucYontoimplementsystemcalls

•  Thereisachallengeprobleminlab3(i.e.,extracredit)tousesystenter/sysexit–  Notethattherearesomemoredetailsaboutregistersavingtodealwith

–  Syscall/sysretisabittootrivialforextracredit•  ButsYllcoolifyougetitworking!

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Summary•  InterrupthandlersarespecifiedintheIDT•  Understandwhennestedinterruptscanhappen–  Andhowtopreventthemwhenunsafe

•  UnderstandopYmizedsystemcallinstrucYons–  Beabletoexplainsyscallvs.int80

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interrupts and system calls - computer science · 2016. 7. 29. · • lab 1 due friday 2/5 ... 11...

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