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.
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''.
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.
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.
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.
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.
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.
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.
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''.
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.
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.
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.
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.
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.
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.
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.
Legacy models just for historical compatibility with ancient QEMU versions.
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.
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.
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.
Preferred CPU models for MIPS hosts
The following CPU models are preferred for use on different MIPS hosts:
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
$ qemu-system-x86_64 -cpu host
With feature customization:
$ qemu-system-x86_64 -cpu host,-vmx,...
$ qemu-system-x86_64 -cpu Westmere
With feature customization:
$ qemu-system-x86_64 -cpu Westmere,+pcid,...
Libvirt guest XML
<cpu mode='host-passthrough'/>
With feature customization:
<cpu mode='host-passthrough'>
<feature name="vmx" policy="disable"/>
...
</cpu>
<cpu mode='host-model'/>
With feature customization:
<cpu mode='host-model'>
<feature name="vmx" policy="disable"/>
...
</cpu>
<cpu mode='custom'>
<model name="Westmere"/>
</cpu>
With feature customization:
<cpu mode='custom'>
<model name="Westmere"/>
<feature name="pcid" policy="require"/>
...
</cpu>