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.

Two ways to configure CPU models with QEMU / KVM¶

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.

ABI compatibility levels for CPU models¶

The x86_64 architecture has a number of ABI compatibility levels defined. Traditionally most operating systems and toolchains would only target the original baseline ABI. It is expected that in future OS and toolchains are likely to target newer ABIs. The table that follows illustrates which ABI compatibility levels can be satisfied by the QEMU CPU models. Note that the table only lists the long term stable CPU model versions (eg Haswell-v4). In addition to what is listed, there are also many CPU model aliases which resolve to a different CPU model version, depending on the machine type is in use.

x86-64 ABI compatibility levels¶

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".

Preferred CPU models for AMD x86 hosts¶

The following CPU models are preferred for use on AMD 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".

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.

Syntax for configuring CPU models¶

The examples 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

Host passthrough with feature customization:

qemu-system-x86_64 -cpu host,vmx=off,...

Named CPU models:

qemu-system-x86_64 -cpu Westmere

Named CPU models with feature customization:

qemu-system-x86_64 -cpu Westmere,pcid=on,...

Libvirt guest XML¶

Host passthrough:

<cpu mode='host-passthrough'/>

Host passthrough with feature customization:

<cpu mode='host-passthrough'>


    <feature name="vmx" policy="disable"/>


    ...
</cpu>

Host model:

<cpu mode='host-model'/>

Host model with feature customization:

<cpu mode='host-model'>


    <feature name="vmx" policy="disable"/>


    ...
</cpu>

Named model:

<cpu mode='custom'>


    <model name="Westmere"/>
</cpu>

Named model with feature customization:

<cpu mode='custom'>


    <model name="Westmere"/>


    <feature name="pcid" policy="require"/>


    ...
</cpu>

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: