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
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

(1) **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.

(2) **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.  @end table

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.


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.

``Cascadelake-Server``, ``Cascadelake-Server-noTSX``
    Intel Xeon Processor (Cascade Lake, 2019), with "stepping" levels 6
    or 7 only.  (The Cascade Lake Xeon processor with *stepping 5 is
    vulnerable to MDS variants*.)

``Skylake-Server``, ``Skylake-Server-IBRS``, ``Skylake-Server-IBRS-noTSX``
    Intel Xeon Processor (Skylake, 2016)

``Skylake-Client``, ``Skylake-Client-IBRS``, ``Skylake-Client-noTSX-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-IBR``
    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.

``mds-no``
  Recommended to inform the guest OS that the host is *not* vulnerable
  to any of the MDS variants ([MFBDS] CVE-2018-12130, [MLPDS]
  CVE-2018-12127, [MSBDS] CVE-2018-12126).

  This is an MSR (Model-Specific Register) feature rather than a CPUID feature,
  so it will not appear in the Linux ``/proc/cpuinfo`` in the host or
  guest.  Instead, the host kernel uses it to populate the MDS
  vulnerability file in ``sysfs``.

  So it should only be enabled for VMs if the host reports @code{Not
  affected} in the ``/sys/devices/system/cpu/vulnerabilities/mds`` file.

``taa-no``
  Recommended to inform that the guest that the host is ``not``
  vulnerable to CVE-2019-11135, TSX Asynchronous Abort (TAA).

  This too is an MSR feature, so it does not show up in the Linux
  ``/proc/cpuinfo`` in the host or guest.

  It should only be enabled for VMs if the host reports ``Not affected``
  in the ``/sys/devices/system/cpu/vulnerabilities/tsx_async_abort``
  file.

``tsx-ctrl``
  Recommended to inform the guest that it can disable the Intel TSX
  (Transactional Synchronization Extensions) feature; or, if the
  processor is vulnerable, use the Intel VERW instruction (a
  processor-level instruction that performs checks on memory access) as
  a mitigation for the TAA vulnerability.  (For details, refer to
  Intel's `deep dive into MDS
  <https://software.intel.com/security-software-guidance/insights/deep-dive-intel-analysis-microarchitectural-data-sampling>`_.)

  Expose this to the guest OS if and only if: (a) the host has TSX
  enabled; *and* (b) the guest has ``rtm`` CPU flag enabled.

  By disabling TSX, KVM-based guests can avoid paying the price of
  mitigating TSX-based attacks.

  Note that ``tsx-ctrl`` too is an MSR feature, so it does not show
  up in the Linux ``/proc/cpuinfo`` in the host or guest.

  To validate that Intel TSX is indeed disabled for the guest, there are
  two ways: (a) check for the *absence* of ``rtm`` in the guest's
  ``/proc/cpuinfo``; or (b) the
  ``/sys/devices/system/cpu/vulnerabilities/tsx_async_abort`` file in
  the guest should report ``Mitigation: TSX disabled``.


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.


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:

.. parsed-literal::

  |qemu_system| -cpu host

Host passthrough with feature customization:

.. parsed-literal::

  |qemu_system| -cpu host,-vmx,...

Named CPU models:

.. parsed-literal::

  |qemu_system| -cpu Westmere

Named CPU models with feature customization:

.. parsed-literal::

  |qemu_system| -cpu Westmere,+pcid,...

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>