NAME

       qemu-cpu-models - QEMU / KVM CPU model configuration

SYNOPSIS

       QEMU / KVM CPU model configuration

DESCRIPTION

       QEMU / KVM virtualization supports two ways to configure CPU models

       Host passthrough
           This  passes  the  host  CPU model features, model, stepping, exactly to the guest. Note that KVM may
           filter out some host CPU model features  if  they  cannot  be  supported  with  virtualization.  Live
           migration is unsafe when this mode is used as libvirt / QEMU cannot guarantee a stable CPU is exposed
           to  the  guest  across  hosts.  This  is  the  recommended CPU to use, provided live migration is not
           required.

       Named model
           QEMU comes with a number of predefined named CPU models, that typically refer to specific generations
           of hardware released by Intel and AMD.  These allow the guest VMs to have a degree of isolation  from
           the host CPU, allowing greater flexibility in live migrating between hosts with differing hardware.

       In  both  cases,  it  is  possible  to optionally add or remove individual CPU features, to alter what is
       presented to the guest by default.

       Libvirt supports a third way to configure CPU models known as "Host model".  This uses  the  QEMU  "Named
       model"  feature,  automatically  picking  a CPU model that is similar the host CPU, and then adding extra
       features to approximate the host model as closely as possible. This does not guarantee  the  CPU  family,
       stepping, etc will precisely match the host CPU, as they would with "Host passthrough", but gives much of
       the benefit of passthrough, while making live migration safe.

       Recommendations for KVM CPU model configuration on x86 hosts

       The  information that follows provides recommendations for configuring CPU models on x86 hosts. The goals
       are to maximise performance, while protecting guest OS against various CPU hardware flaws, and optionally
       enabling live migration between hosts with heterogeneous CPU models.

       Preferred CPU models for Intel x86 hosts

       The following CPU models are preferred  for  use  on  Intel  hosts.  Administrators  /  applications  are
       recommended  to  use  the  CPU model that matches the generation of the host CPUs in use. In a deployment
       with a mixture of host CPU models between machines, if live migration compatibility is required, use  the
       newest CPU model that is compatible across all desired hosts.

       "Skylake-Server"
       "Skylake-Server-IBRS"
           Intel Xeon Processor (Skylake, 2016)

       "Skylake-Client"
       "Skylake-Client-IBRS"
           Intel Core Processor (Skylake, 2015)

       "Broadwell"
       "Broadwell-IBRS"
       "Broadwell-noTSX"
       "Broadwell-noTSX-IBRS"
           Intel Core Processor (Broadwell, 2014)

       "Haswell"
       "Haswell-IBRS"
       "Haswell-noTSX"
       "Haswell-noTSX-IBRS"
           Intel Core Processor (Haswell, 2013)

       "IvyBridge"
       "IvyBridge-IBRS"
           Intel Xeon E3-12xx v2 (Ivy Bridge, 2012)

       "SandyBridge"
       "SandyBridge-IBRS"
           Intel Xeon E312xx (Sandy Bridge, 2011)

       "Westmere"
       "Westmere-IBRS"
           Westmere E56xx/L56xx/X56xx (Nehalem-C, 2010)

       "Nehalem"
       "Nehalem-IBRS"
           Intel Core i7 9xx (Nehalem Class Core i7, 2008)

       "Penryn"
           Intel Core 2 Duo P9xxx (Penryn Class Core 2, 2007)

       "Conroe"
           Intel Celeron_4x0 (Conroe/Merom Class Core 2, 2006)

       Important CPU features for Intel x86 hosts

       The  following  are  important CPU features that should be used on Intel x86 hosts, when available in the
       host CPU. Some of them require explicit configuration to enable, as they are not included by  default  in
       some,  or  all,  of  the  named CPU models listed above. In general all of these features are included if
       using "Host passthrough" or "Host model".

       "pcid"
           Recommended to mitigate the cost of the Meltdown (CVE-2017-5754) fix

           Included by default in Haswell, Broadwell & Skylake Intel CPU models.

           Should be explicitly turned on for Westmere, SandyBridge, and IvyBridge Intel CPU models.  Note  that
           some desktop/mobile Westmere CPUs cannot support this feature.

       "spec-ctrl"
           Required to enable the Spectre v2 (CVE-2017-5715) fix.

           Included by default in Intel CPU models with -IBRS suffix.

           Must be explicitly turned on for Intel CPU models without -IBRS suffix.

           Requires the host CPU microcode to support this feature before it can be used for guest CPUs.

       "stibp"
           Required to enable stronger Spectre v2 (CVE-2017-5715) fixes in some operating systems.

           Must be explicitly turned on for all Intel CPU models.

           Requires the host CPU microcode to support this feature before it can be used for guest CPUs.

       "ssbd"
           Required to enable the CVE-2018-3639 fix

           Not included by default in any Intel CPU model.

           Must be explicitly turned on for all Intel CPU models.

           Requires the host CPU microcode to support this feature before it can be used for guest CPUs.

       "pdpe1gb"
           Recommended to allow guest OS to use 1GB size pages

           Not included by default in any Intel CPU model.

           Should be explicitly turned on for all Intel CPU models.

           Note that not all CPU hardware will support this feature.

       "md-clear"
           Required to confirm the MDS (CVE-2018-12126, CVE-2018-12127, CVE-2018-12130, CVE-2019-11091) fixes.

           Not included by default in any Intel CPU model.

           Must be explicitly turned on for all Intel CPU models.

           Requires the host CPU microcode to support this feature before it can be used for guest CPUs.

       Preferred CPU models for AMD x86 hosts

       The  following  CPU  models  are  preferred  for  use  on  Intel hosts. Administrators / applications are
       recommended to use the CPU model that matches the generation of the host CPUs in  use.  In  a  deployment
       with  a mixture of host CPU models between machines, if live migration compatibility is required, use the
       newest CPU model that is compatible across all desired hosts.

       "EPYC"
       "EPYC-IBPB"
           AMD EPYC Processor (2017)

       "Opteron_G5"
           AMD Opteron 63xx class CPU (2012)

       "Opteron_G4"
           AMD Opteron 62xx class CPU (2011)

       "Opteron_G3"
           AMD Opteron 23xx (Gen 3 Class Opteron, 2009)

       "Opteron_G2"
           AMD Opteron 22xx (Gen 2 Class Opteron, 2006)

       "Opteron_G1"
           AMD Opteron 240 (Gen 1 Class Opteron, 2004)

       Important CPU features for AMD x86 hosts

       The following are important CPU features that should be used on AMD x86 hosts, when available in the host
       CPU. Some of them require explicit configuration to enable, as they are not included by default in  some,
       or  all,  of  the  named  CPU models listed above. In general all of these features are included if using
       "Host passthrough" or "Host model".

       "ibpb"
           Required to enable the Spectre v2 (CVE-2017-5715) fix.

           Included by default in AMD CPU models with -IBPB suffix.

           Must be explicitly turned on for AMD CPU models without -IBPB suffix.

           Requires the host CPU microcode to support this feature before it can be used for guest CPUs.

       "stibp"
           Required to enable stronger Spectre v2 (CVE-2017-5715) fixes in some operating systems.

           Must be explicitly turned on for all AMD CPU models.

           Requires the host CPU microcode to support this feature before it can be used for guest CPUs.

       "virt-ssbd"
           Required to enable the CVE-2018-3639 fix

           Not included by default in any AMD CPU model.

           Must be explicitly turned on for all AMD CPU models.

           This  should  be  provided  to  guests,  even  if  amd-ssbd  is  also  provided,  for  maximum  guest
           compatibility.

           Note  for  some  QEMU  /  libvirt  versions, this must be force enabled when when using "Host model",
           because this is a virtual feature that doesn't exist in the physical host CPUs.

       "amd-ssbd"
           Required to enable the CVE-2018-3639 fix

           Not included by default in any AMD CPU model.

           Must be explicitly turned on for all AMD CPU models.

           This provides higher performance than virt-ssbd so should be exposed to guests whenever available  in
           the  host.  virt-ssbd  should  none  the less also be exposed for maximum guest compatibility as some
           kernels only know about virt-ssbd.

       "amd-no-ssb"
           Recommended to indicate the host is not vulnerable CVE-2018-3639

           Not included by default in any AMD CPU model.

           Future hardware generations of CPU will not be vulnerable to CVE-2018-3639, and thus the guest should
           be told not to enable its mitigations, by exposing amd-no-ssb. This is mutually exclusive with  virt-
           ssbd and amd-ssbd.

       "pdpe1gb"
           Recommended to allow guest OS to use 1GB size pages

           Not included by default in any AMD CPU model.

           Should be explicitly turned on for all AMD CPU models.

           Note that not all CPU hardware will support this feature.

       Default x86 CPU models

       The default QEMU CPU models are designed such that they can run on all hosts.  If an application does not
       wish  to  do  perform any host compatibility checks before launching guests, the default is guaranteed to
       work.

       The default CPU models will, however, leave the guest OS vulnerable to various  CPU  hardware  flaws,  so
       their  use is strongly discouraged. Applications should follow the earlier guidance to setup a better CPU
       configuration, with host passthrough recommended if live migration is not needed.

       "qemu32"
       "qemu64"
           QEMU Virtual CPU version 2.5+ (32 & 64 bit variants)

           qemu64 is used for x86_64 guests and qemu32 is used for i686 guests, when no -cpu argument  is  given
           to QEMU, or no <cpu> is provided in libvirt XML.

       Other non-recommended x86 CPUs

       The  following  CPUs  models  are  compatible  with  most  AMD  and  Intel  x86 hosts, but their usage is
       discouraged, as they expose a very limited featureset, which prevents guests having optimal performance.

       "kvm32"
       "kvm64"
           Common KVM processor (32 & 64 bit variants)

           Legacy models just for historical compatibility with ancient QEMU versions.

       486
       "athlon"
       "phenom"
       "coreduo"
       "core2duo"
       "n270"
       "pentium"
       "pentium2"
       "pentium3"
           Various  very  old  x86  CPU  models,  mostly  predating  the  introduction  of   hardware   assisted
           virtualization, that should thus not be required for running virtual machines.

       Supported CPU model configurations on MIPS hosts

       QEMU supports variety of MIPS CPU models:

       Supported CPU models for MIPS32 hosts

       The  following  CPU  models  are  supported  for  use  on MIPS32 hosts. Administrators / applications are
       recommended to use the CPU model that matches the generation of the host CPUs in  use.  In  a  deployment
       with  a mixture of host CPU models between machines, if live migration compatibility is required, use the
       newest CPU model that is compatible across all desired hosts.

       "mips32r6-generic"
           MIPS32 Processor (Release 6, 2015)

       "P5600"
           MIPS32 Processor (P5600, 2014)

       "M14K"
       "M14Kc"
           MIPS32 Processor (M14K, 2009)

       "74Kf"
           MIPS32 Processor (74K, 2007)

       "34Kf"
           MIPS32 Processor (34K, 2006)

       "24Kc"
       "24KEc"
       "24Kf"
           MIPS32 Processor (24K, 2003)

       "4Kc"
       "4Km"
       "4KEcR1"
       "4KEmR1"
       "4KEc"
       "4KEm"
           MIPS32 Processor (4K, 1999)

       Supported CPU models for MIPS64 hosts

       The following CPU models are supported for  use  on  MIPS64  hosts.  Administrators  /  applications  are
       recommended  to  use  the  CPU model that matches the generation of the host CPUs in use. In a deployment
       with a mixture of host CPU models between machines, if live migration compatibility is required, use  the
       newest CPU model that is compatible across all desired hosts.

       "I6400"
           MIPS64 Processor (Release 6, 2014)

       "Loongson-2F"
           MIPS64 Processor (Loongson 2, 2008)

       "Loongson-2E"
           MIPS64 Processor (Loongson 2, 2006)

       "mips64dspr2"
           MIPS64 Processor (Release 2, 2006)

       "MIPS64R2-generic"
       "5KEc"
       "5KEf"
           MIPS64 Processor (Release 2, 2002)

       "20Kc"
           MIPS64 Processor (20K, 2000)

       "5Kc"
       "5Kf"
           MIPS64 Processor (5K, 1999)

       "VR5432"
           MIPS64 Processor (VR, 1998)

       "R4000"
           MIPS64 Processor (MIPS III, 1991)

       Supported CPU models for nanoMIPS hosts

       The  following  CPU  models  are  supported  for use on nanoMIPS hosts. Administrators / applications are
       recommended to use the CPU model that matches the generation of the host CPUs in  use.  In  a  deployment
       with  a mixture of host CPU models between machines, if live migration compatibility is required, use the
       newest CPU model that is compatible across all desired hosts.

       "I7200"
           MIPS I7200 (nanoMIPS, 2018)

       Preferred CPU models for MIPS hosts

       The following CPU models are preferred for use on different MIPS hosts:

       "MIPS III"
           R4000

       "MIPS32R2"
           34Kf

       "MIPS64R6"
           I6400

       "nanoMIPS"
           I7200

       Syntax for configuring CPU models

       The example below illustrate the approach to configuring the various CPU models / features  in  QEMU  and
       libvirt

       QEMU command line

       Host passthrough
                      $ qemu-system-x86_64 -cpu host

           With feature customization:

                      $ qemu-system-x86_64 -cpu host,-vmx,...

       Named CPU models
                      $ qemu-system-x86_64 -cpu Westmere

           With feature customization:

                      $ qemu-system-x86_64 -cpu Westmere,+pcid,...

       Libvirt guest XML

       Host passthrough
                      <cpu mode='host-passthrough'/>

           With feature customization:

                      <cpu mode='host-passthrough'>
                          <feature name="vmx" policy="disable"/>
                          ...
                      </cpu>

       Host model
                      <cpu mode='host-model'/>

           With feature customization:

                      <cpu mode='host-model'>
                          <feature name="vmx" policy="disable"/>
                          ...
                      </cpu>

       Named model
                      <cpu mode='custom'>
                          <model name="Westmere"/>
                      </cpu>

           With feature customization:

                      <cpu mode='custom'>
                          <model name="Westmere"/>
                          <feature name="pcid" policy="require"/>
                          ...
                      </cpu>

SEE ALSO

       The HTML documentation of QEMU for more precise information and Linux user mode emulator invocation.

AUTHOR

       Daniel P. Berrange

                                                   2024-10-20                               QEMU-CPU-MODELS.7(7)