Interrupts and System Calls - Computer Science · • 0—31 are for processor interrupts; generally ﬁxed by Intel – E.g., 14 is always for page faults • 32—255 are soeware

CSE306:Opera.ngSystems

InterruptsandSystemCalls

DonPorterCSE306

1


App

LastTime…

2-2

Hardware

Libraries

Kernel

User

Super-visor

App

Libraries

App

Libraries

SystemCallTable(350—1200)

Openfile“hw1.txt”

Ok,here’shandle4


Lecturegoal•  Understandhowsystemcallswork–  AswellashowexcepUons(e.g.,dividebyzero)work

•  Understandthehardwaretoolsavailableforirregularcontrolflow.–  I.e.,thingsotherthanabranchinarunningprogram

•  Buildingblocksforcontextswitching,devicemanagement,etc.

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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:excepUons,systemcalls,etc.

pc Dividebyzero!Programcan’tmake

progress!

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

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

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AsynchronousInterruptExample

User Kernel

Stack Stack

if (x) { printf(“Boo”); ...

printf(va_args…){

...

Disk_handler (){ ...

}

RSP

RIP

RSP

RIP

DiskInterrupt!

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Intelnomenclature•  Interrupt–onlyreferstoasynchronousinterrupts•  ExcepUon–synchronouscontroltransfer

•  Note:fromtheprogrammer’sperspecUve,thesearehandledwiththesameabstracUons

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Lectureoutline•  Overview•  Howinterruptsworkinhardware•  Howinterrupthandlersworkinsoeware•  Howsystemcallswork•  Newsystemcallhardwareonx86

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Interruptoverview•  EachinterruptorexcepUonincludesanumberindicaUngitstype

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

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x86interrupttable

0 255

…

31

… …

47

ReservedfortheCPU

SoewareConfigurable

DeviceIRQs 48=JOSSystemCall

128=LinuxSystemCall

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x86interruptoverview•  Eachtypeofinterruptisassignedanindexfrom0—255.

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

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

•  Mostdevice’sIRQlinecanbeconfigured•  LookupAPICsformoreinfo(Ch4ofBovetandCesaU)

–  0x80issuessystemcallinLinux(moreonthislater)

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Soewareinterrupts•  Theint <num> instrucUonallowssoewaretoraiseaninterrupt–  0x80isjustaLinuxconvenUon.JOSuses0x30.

•  Therearealotofspareindices–  YoucouldhavemulUplesystemcalltablesfordifferentpurposesortypesofprocesses!•  Windowsdoes:oneforthekernelandoneforwin32k

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

–  AnunauthorizedintinstrucUoncausesageneralprotecUonfault•  Interrupt13

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

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

•  Kernelcoderunsandhandlestheinterrupt•  Whenhandlercompletes,resumeprogram(seeiretinstr.)

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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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Summary•  Mostinterrupthandlinghardwarestatesetduringboot

•  EachinterrupthasanIDTentryspecifying:– Whatcodetoexecute,privilegeleveltoraisetheinterrupt

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Lectureoutline•  Overview•  Howinterruptsworkinhardware•  HowinterrupthandlersworkinsoBware•  Howsystemcallswork•  Newsystemcallhardwareonx86

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High-levelgoal•  Respondtosomeevent,returncontroltotheappropriateprocess

•  Whattodoon:–  Networkpacketarrives–  DiskreadcompleUon–  Dividebyzero–  Systemcall

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InterruptHandlers•  Justplainoldkernelcode–  SortoflikeexcepUonhandlersinJava–  Butseparatedfromthecontrolflowoftheprogram

•  TheIDTstoresapointertotherighthandlerrouUne

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Whatisasystemcall?•  AfuncUonprovidedtoapplicaUonsbytheOSkernel–  GenerallytouseahardwareabstracUon(file,socket)–  OrOS-providedsoewareabstracUon(IPC,scheduling)

•  WhynotputthesedirectlyintheapplicaUon?–  ProtecUonoftheOS/hardwarefrombuggy/maliciousprograms

–  ApplicaUonsarenotallowedtodirectlyinteractwithhardware,oraccesskerneldatastructures


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

•  Programselectstheoneitwantsbyplacingindexineaxregister–  ArgumentsgointheotherregistersbycallingconvenUon–  Returnvaluegoesineax

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Howmanysystemcalls?•  Linuxexportsabout350systemcalls•  Windowsexportsabout400systemcallsforcoreAPIs,andanother800forGUImethods


Butwhyuseinterrupts?•  AlsoprotecUon•  ForcesapplicaUonstocallwell-defined“public”funcUons–  RatherthancallingarbitraryinternalkernelfuncUons

•  Example:publicfoo(){

if(!permission_ok())return–EPERM;return_foo();//nopermissioncheck

}

Calling_foo()directlywouldcircumvent

permissioncheck


Summary•  Systemcallsarethe“public”OSAPIs•  KernelleveragesinterruptstorestrictapplicaUonstospecificfuncUons

•  Lab1hint:HowtoissueaLinuxsystemcall?–  int $0x80,withsystemcallnumberineaxregister



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AroundP4era…•  Processorsgotverydeeplypipelined–  Pipelinestalls/flushesbecameveryexpensive–  Cachemissescancausepipelinestalls

•  SystemcallstooktwiceaslongfromP3toP4– Why?–  IDTentrymaynotbeinthecache–  DifferentpermissionsconstraininstrucUonreordering

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Idea•  WhatifwecachetheIDTentryforasystemcallinaspecialCPUregister?–  NomorecachemissesfortheIDT!– MaybewecanalsodomoreopUmizaUons

•  AssumpUon:systemcallsarefrequentenoughtobeworththetransistorbudgettoimplementthis– Whatelsecouldyoudowithextratransistorsthathelpsperformance?

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

•  Adrop-inreplacementforint 0x80•  Everyoneloveditandadopteditwholesale–  EvenIntel!

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

•  Sosystemcallsaredefinitelyfasterasaresult!

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Summary•  InterrupthandlersarespecifiedintheIDT•  Understandhowsystemcallsareexecuted– Whyinterrupts?– WhyspecialsystemcallinstrucUons?

Documents

Interrupts and System Calls - Computer Science · • 0—31 are for processor interrupts; generally ﬁxed by Intel – E.g., 14 is always for page faults • 32—255 are soeware