layerssandybridge

KVMandCPUfeatureenablementEduardoHabkostDeveloperConference2014AgendaBasicconceptsExistingmechanismsandcurrentchallengesCurrentworkandfutureplans!
2BasicsIntroduction:Basics!
4HostCPU(management)KernelQEMUKVMVMGuestOSlibvirtIntroduction:Basics!
4command-line,monitor(QMP)HostCPU(management)KernelQEMUKVMVMGuestOSlibvirtIntroduction:StableguestABIGuestOSshouldseethe"same"machine,evenifthehostsystemhaschangedHardrequirementforlivemigrationSoftrequirementfornon-livemigrationHostsystemmaychangealot,butVMshouldlookthesame!
5x86CPUIDinstructionReturnsinformationabouttherunningCPUMostinformationshownon/proc/cpuinfoFeatureagsindicatingafeatureispresentOthermorecomplexdatae.
g.
:cacheandtopologyinformationCPUIDdataispartofguestABI!
6ExistingMechanismsCPUIDhandling!
8HOSTCPUQEMUGuestOSKernelCPUIDhandling!
8HOSTCPUQEMUGuestOSKernelGET_SUPPORTED_CPUIDHostCPUIDinstructionCPUIDhandling!
8HOSTCPUQEMUGuestOSCPUIDtableSET_CPUIDKernelGET_SUPPORTED_CPUIDHostCPUIDinstructionCPUIDhandling!
8HOSTCPUQEMUGuestOSCPUIDtableSET_CPUIDKernelGET_SUPPORTED_CPUIDHostCPUIDinstructionemulateCPUIDGuestCPUIDinstructionHostCPU(management)KernelQEMUKVMVMGuestOSlibvirtTheStack!
9Decisiontoenableafeature(shouldbe)takenintheupperlayersLowerlayersaffecttheabilitytoenableafeatureEnablingnewfeaturesWecan'tsilentlyenableordisableafeature:ItbreaksguestABIMayunexpectedlypreventmigrationtoother(lesspowerful)hosts!
10CPUmodelsCPUmodeltable,differentCPUIDdataoneachentryqemu-system-x86_64-cpuSandyBridgeqemu-system-x86_64-cpuHaswellControllingindividualfeatures.
e.
g.
:-cpuNehalem,+aesCPUmodelentriesmaychange,machine-typeskeepcompatibilityqemu-system-x86_64-machinepc-1.
6-cpuSandyBridgeqemu-system-x86_64-machinepc-1.
7-cpuSandyBridgeenforceag.
e.
g.
:-cpuSandyBridge,enforceRequiredtoensurepredictableresults!
11CPUmodelsSpecialCPUmodel:-cpuhostWillenableeverythingthat'ssupportedbythehostNostableguestABI!
12ManagementrequirementsEnsuringthattheresultingCPUIDdataiswhatwasaskedforKnowingwhichCPUmodelscanbeenabledinahostKnowingwhichfeaturesareavailableinahostKnowingtowhichhostsaVMcanbemigrated!
13IssuesHostCPU(management)KernelQEMUKVMVMGuestOSlibvirtProblem:queryingCPUmodelinformationlibvirthasitsownlistofCPUmodelslibvirtdoesn'tknowQEMUCPUmodelscanchangeovertimeQEMU'sfault,there'snogoodAPIforthat(yet!
)!
15CPUmodellistQEMUCPUmodellistHostCPU(management)KernelQEMUKVMVMGuestOSlibvirtProblem:no"enforce"modelibvirtdoesn'tusetheenforceagErrorreportingnotmachine-friendlyMostseriousissuesofarFixinvolvesimplementingCPUmodelandhostcapabilityAPIs!
16X,Z-cpu…,+X,+Y,+ZX!
(noY)!
(noZ)X,YHostCPU(management)KernelQEMUKVMVMGuestOSlibvirtProblem:queryinghostcapabilitieslibvirtquerieshostCPUfeaturesdirectlyusingCPUIDinstructionIgnoresKVMcapabilitiesIgnoresQEMUcapabilitiesIgnoresfeaturesthatrequireextraCPUcapabilitiesQEMU'sfault,there'snogoodAPIforthat(yet!
)!
17SolutionsSolutionsExistinginterfaces:CPU-specicoptionsandcommands-cpu,cpu-add,query-cpu-definitionsNewinterfaces:basedoncommoninfrastructure(QDev,QOM)!
19QDevQDev=QEMUDeviceModelQOM=QEMUObjectModelQDevdevicesareQOMobjects-devicecommand-lineoptionQMPcommands:Addingdevices/objects(device_add,object-add)Removingdevices/objects(device_del,object-del)Getting/settingdevicesproperties(qom-get,qom-set)Listingobjectsandobjectclasses(qom-list,qom-list-types)!
20QDev-basedsolutionCPUsareQDevdevices(done)CPUdevicesanditspropertiesvisiblethroughQMPfeature-wordsproperty(done)QueryCPUmodelinfoQueryhostcapabilities("host"CPUmodel)Incomplete:nomachine-type-specicdatafiltered-featuresproperty(done)Usedtoemulate"enforce"modewithbettererrorreportingNotusedbylibvirtyet!
21What'smissing(1/2)QueryingCPUmodelinformationwithoutre-runningQEMUSolution:separateQOMtypesforeachCPUmodelExposingmachine-type-specicdataNodenedsolutionyetUseQOMpropertiestocontrolallfeatureagsChanginglibvirttousethenewstuff!
22What'smissing(2/2)Longtermplans:Deprecate-cpu,cpu-addanduseonlyQDevcommands(-device,device_add)BetterinterfacestospecifyCPUtopology(NUMAnodes,sockets,cores,threads)!
23FutureReportingcapabilitiesreliablysmartermanagementsystemsUsability(automaticallychoosinggooddefaults)SmarterVMschedulingMayrequireextendinglibvirtAPI!
24ThanksFeedback:http://devconf.
cz/f/34!
Additionalinfo/pointers:http://wiki.
qemu.
org/Features/CPUModelsehabkost@redhat.
com!
Questions!
25