Splice, Tee & VMsplice: zero copy in Linux

Splice,Tee&Vmsplice:ZeroCopyinLinuxUnabletohandlekernelpagingrequestatvirtualaddress4d1b65e8 Unabletohandlekernelpagingrequestatvirtualaddress4d1b65e8 pgd=c0280000 pgd=c0280000 [4d1b65e8]*pgd=00000000[4d1b65e8]*pgd=00000000 Internalerror:Oops:f5[#1] Internalerror:Oops:f5[#1] Moduleslinkedin:Moduleslinkedin:hx4700_udchx4700_udcasic3_baseasic3_base CPU:0 CPU:0 PCisatset_pxa_fb_info+0x2c/0x44 PCisatset_pxa_fb_info+0x2c/0x44 LRisathx4700_udc_init+0x1c/0x38[hx4700_udc] LRisathx4700_udc_init+0x1c/0x38[hx4700_udc] pc:[]lr:[]Nottainted Herzelinux sp:c076df78ip:60000093fp:c076df84 http://tuxology.net pc:[]lr:[]Nottainted

Copyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd.

Forfullcopyrightinformationseelastpage. CreativeCommonsAttributionShareAlike2.0license

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RightstocopyThiskitcontainsworkbytheAttributionShareAlike2.0 Youarefree tocopy,distribute,display,andperformthework tomakederivativeworks tomakecommercialuseofthework Underthefollowingconditions Attribution.Youmustgivetheoriginalauthorcredit. ShareAlike.Ifyoualter,transform,orbuilduponthiswork, youmaydistributetheresultingworkonlyunderalicense identicaltothisone. Foranyreuseordistribution,youmustmakecleartoothersthe licensetermsofthiswork. Anyoftheseconditionscanbewaivedifyougetpermissionfrom thecopyrightholder. Yourfairuseandotherrightsareinnowayaffectedbytheabove. Licensetext:http://creativecommons.org/licenses/bysa/2.0/legalcodeCopyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd.

followingauthors: Copyright20042006 MichaelOpdenacker [email protected] http://www.freeelectrons.com Copyright20032006 OronPeled [email protected] http://www.actcom.co.il/~oron Copyright20042008 Codefidenceltd. [email protected] http://www.codefidence.com


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KernelarchitectureApp1 App2 Clibrary Systemcallinterface Process management Memory management Filesystem support Filesystem types CPUsupport code CPU/MMU supportcode Storage drivers Character devicedrivers Network devicedrivers Hardware CPU RAM Storage Device control Networking ... User space

Kernel space



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KernelModevs.UserModeAllmodernCPUssupportadualmodeofoperation: Usermode,forregulartasks. Supervisor(orprivileged)mode,forthekernel. ThemodetheCPUisindetermineswhichinstructionstheCPUis willingtoexecute: SensitiveinstructionswillnotbeexecutedwhentheCPUisin usermode. TheCPUmodeisdeterminedbyoneoftheCPUregisters,whichstores thecurrentRingLevel 0forsupervisormode,3forusermode,12unusedbyLinux.Copyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd. Forfullcopyrightinformationseelastpage. CreativeCommonsAttributionShareAlike2.0license

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TheSystemCallInterfaceWhenauserspacetasksneedstouseakernelservice,itwillmakea SystemCall. TheClibraryplacesparametersandnumberofsystemcallinregisters andthenissuesaspecialtrapinstruction. Thetrapatomicallychangestheringleveltosupervisormodeandthe setstheinstructionpointertothekernel. Thekernelwillfindtherequiredsystemcalledviathesystemcalltable andexecuteit. Returningfromthesystemcalldoesnotrequireaspecialinstruction, sinceinsupervisormodetheringlevelcanbechangeddirectly.



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LinuxSystemCallPathKernel do_name() sys_name() entry.S Function call Trap

Task

Glibc Task



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ExchangingDataWithUserSpace(1)Inkernelcode,youcan'tjustmemcpybetween anaddresssuppliedbyuserspaceand theaddressofabufferinkernelspace! Correspondtocompletelydifferent addressspaces(thankstovirtualmemory). Theuserspaceaddressmaybeswappedouttodisk. Theuserspaceaddressmaybeinvalid (userspaceprocesstryingtoaccessunauthorizeddata).



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ExchangingDataWithUserSpace(2)Youmustusededicatedfunctionssuchasthefollowingonesinyour readandwritefileoperationscode: include unsignedlongcopy_to_user(void__user*to, constvoid*from, unsignedlongn); unsignedlongcopy_from_user(void*to, constvoid__user*from, unsignedlongn); Makesurethatthesefunctionsreturn0! Anotherreturnvaluewouldmeanthattheyfailed.Copyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd. Forfullcopyrightinformationseelastpage. CreativeCommonsAttributionShareAlike2.0license

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DMAOffLoadEngineDMA(DirectMemoryAccess)offloadengineisapieceof hardwarethatdoesmemcpybyhardwareotherthentheCPU.Example:IntelI/OAT(I/OAccelerationTechnology).

MakesthecopythejobofanentityotherthentheCPU. It'szerocopy,ifbycopyyoumeancopybytheCPU.



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SimpleClient/ServerCopiesClientKernel

Server

Rx

Tx

Kernel

Copy to user ... ret = recv(s, buf) ... User space Application

Copy from user ... ret = send(s, buf) ... User space Application



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SimpleClient/ServerCopiesClient Server

RxKernel

Tx

Kernel

Copy to user ... ret = recv(s, buf) ... User space Application

DMA

Copy from user DMA ... ret = read(s, buf) ... ret = send(s, buf) ... User space Application DMA

Disk



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ZeroCopyInkernelbufferthattheuserhascontrolover. Thebufferisimplementedasasetofreferencecountedpointerswhich thekernelcopiesaroundwithoutactuallycopyingthedata. splice()movesdatato/fromthebufferfrom/toanarbitraryfiledescriptor tee()Movesdatato/fromonebuffertoanother vmsplice()doesthesamethansplice(),butinsteadofsplicingfromfdto fdassplice()does,itsplicesfromauseraddressrangeintoafile. Canbeusedanywherewhereaprocessneedstosendsomethingfrom oneendtoanother,butitdoesn'tneedtotouchorevenlookatthedata, justforwardit.Copyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd. Forfullcopyrightinformationseelastpage. CreativeCommonsAttributionShareAlike2.0license

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ZeroCopyInkernelbufferthattheuserhascontrolover. Implementedasapipe. Thepipebufferisimplementedasasetofreferencecounted pointerswhichthekernelcopiesaroundwithoutactually copyingthedata. tee(),splice()andvmsplice()movedatafromuserprogramto thepipeandfromonepipetothenext,withoutcopying Usewhenaprocessneedstosendsomethingfromoneendto another,butdoesn'tneedtotouchorevenlookatthedata.Copyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd. Forfullcopyrightinformationseelastpage. CreativeCommonsAttributionShareAlike2.0license

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Splicesplice(intfd_in,off_t*off_in,intfd_out,off_t*off_out, size_tlen,unsignedintflags); splice()movesdatato(from)thepipefrom(to)anarbitrary filedescriptor. sendfile()isnowinternallyimplementedassplice(). MustuseSPLICE_F_MOVEflagtoachievezerocopy,if possible:bufferref.countofzeroofwholepages. Otherflags:SPLICE_F_NONBLOCK,SPLICE_F_MORE whichworkslikeTCP_CORK.Copyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd. Forfullcopyrightinformationseelastpage. CreativeCommonsAttributionShareAlike2.0license

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Teelongtee(intfd_in,intfd_out,size_tlen,unsignedint flags); tee()moves(read:copiesreferenceto)datato(from)one pipebuffertotheother. Sourcepipestillholdsthedata. OnlyusefulflagisSPLICE_F_NONBLOCK.



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ZeroCopyofExample1Splice() *Only pointer is copied

User space

FilePointer to page cache page

Socket BufPointer to page as part of frag list

Kernel Memory

Data

Copy (using DMA)

Hardware

HD Controller

Network ChipForfullcopyrightinformationseelastpage. CreativeCommonsAttributionShareAlike2.0license

* In reality you have to do two splice calls: one from the file to an intermediate pipe and one from the pipe to the socket buffers.Copyright20062004,MichaelOpdenacker Copyright20032006,OronPeled Copyright20042006CodefidenceLtd.

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TeeImplementedusingTee&Splice#define_GNU_SOURCE #include #include #include #include #include #include #include intmain(intargc,char*argv[]) { intfd; intlen,slen; assert(argc==2); fd=open(argv[1],O_WRONLY|O_CREAT|O_TRUNC,\ 0644); if(fd==1){ perror("open"); exit(EXIT_FAILURE); } do{ /* *teestdintostdout. */ len=tee(STDIN_FILENO,STDOUT_FILENO, INT_MAX,SPLICE_F_NONBLOCK); if(len0){ slen=splice(STDIN_FILENO,NULL,fd,NULL, len,SPLICE_F_MOVE); if(slen

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Splice, Tee & VMsplice: zero copy in Linux

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