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---
features:
- |
The libvirt driver now allows specifying individual CPU feature
flags for guests, via a new configuration attribute
``[libvirt]/cpu_model_extra_flags`` -- this is valid in combination
with all the three possible values for ``[libvirt]/cpu_mode``:
``custom``, ``host-model``, or ``host-passthrough``. The
``cpu_model_extra_flags`` also allows specifying multiple CPU flags.
Refer to its documentation in ``nova.conf`` for usage details.
One of the motivations for this is to alleviate the performance
degradation (caused as a result of applying the "Meltdown" CVE
fixes) for guests running with certain Intel-based virtual CPU
models. This guest performance impact is reduced by exposing the
CPU feature flag 'PCID' ("Process-Context ID") to the *guest* CPU,
assuming that it is available in the physical hardware itself.
Note that besides ``custom``, Nova's libvirt driver has two other
CPU modes: ``host-model`` (which is the default), and
``host-passthrough``. Refer to the
``[libvirt]/cpu_model_extra_flags`` documentation for what to do
when you are using either of those CPU modes in context of 'PCID'.
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