CPU model configuration for QEMU/KVM on x86 hosts

With the various CPU hardware vulnerabilities reported this year, guest CPU configuration is now a security critical task. This blog post contains content I’ve written that is on its way to become part of the QEMU documentation.

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 hetergeneous 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 (CVE-2017-5753 and CVE-2017-5715) fix, in cases where retpolines are not sufficient. 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.

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.

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 (CVE-2017-5753 and CVE-2017-5715) fix, in cases where retpolines are not sufficient. 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.

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 compatability 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 genarations 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.

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>Westmere</model>
   </cpu>

With feature customization:

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