Browse Source

Merge "Explain nested guest support" into stable/queens

Zuul Gerrit Code Review 4 months ago
1 changed files with 121 additions and 4 deletions
  1. +121

+ 121
- 4
doc/source/admin/configuration/hypervisor-kvm.rst View File

@@ -291,8 +291,14 @@ If your ``nova.conf`` file contains ``cpu_mode=host-model``, libvirt identifies
the CPU model in ``/usr/share/libvirt/cpu_map.xml`` file that most closely
matches the host, and requests additional CPU flags to complete the match. This
configuration provides the maximum functionality and performance and maintains
good reliability and compatibility if the guest is migrated to another host
with slightly different host CPUs.
good reliability.

With regard to enabling and facilitating live migration between
compute nodes, you should assess whether ``host-model`` is suitable
for your compute architecture. In general, using ``host-model`` is a
safe choice if your compute node CPUs are largely identical. However,
if your compute nodes span multiple processor generations, you may be
better advised to select a ``custom`` CPU model.

Host pass through
@@ -302,7 +308,21 @@ tells KVM to pass through the host CPU with no modifications. The difference
to host-model, instead of just matching feature flags, every last detail of the
host CPU is matched. This gives the best performance, and can be important to
some apps which check low level CPU details, but it comes at a cost with
respect to migration. The guest can only be migrated to a matching host CPU.
respect to migration.

In ``host-passthrough`` mode, the guest can only be live-migrated to a
target host that matches the source host extremely closely. This
definitely includes the physical CPU model and running microcode, and
may even include the running kernel. Use this mode only if

* your compute nodes have a very large degree of homogeneity
(i.e. substantially all of your compute nodes use the exact same CPU
generation and model), and you make sure to only live-migrate
between hosts with exactly matching kernel versions, `or`

* you decide, for some reason and against established best practices,
that your compute infrastructure should not support any live
migration at all.

@@ -318,14 +338,111 @@ option. For example, to configure the KVM guests to expose Nehalem CPUs, your
cpu_mode = custom
cpu_model = Nehalem

In selecting the ``custom`` mode, along with a ``cpu_model`` that
matches the oldest of your compute node CPUs, you can ensure that live
migration between compute nodes will always be possible. However, you
should ensure that the ``cpu_model`` you select passes the correct CPU
feature flags to the guest.

None (default for all libvirt-driven hypervisors other than KVM & QEMU)

If your ``nova.conf`` file contains ``cpu_mode=none``, libvirt does not specify
a CPU model. Instead, the hypervisor chooses the default model.

Set CPU feature flags

Regardless of whether your selected ``cpu_mode`` is
``host-passthrough``, ``host-model``, or ``custom``, it is also
possible to selectively enable additional feature flags. Suppose your
selected ``custom`` CPU model is ``IvyBridge``, which normally does
not enable the ``pcid`` feature flag --- but you do want to pass
``pcid`` into your guest instances. In that case, you would set:

.. code-block:: ini

cpu_mode = custom
cpu_model = IvyBridge
cpu_model_extra_flags = pcid

Nested guest support

You may choose to enable support for nested guests --- that is, allow
your Nova instances to themselves run hardware-accelerated virtual
machines with KVM. Doing so requires a module parameter on
your KVM kernel module, and corresponding ``nova.conf`` settings.

Nested guest support in the KVM kernel module

To enable nested KVM guests, your compute node must load the
``kvm_intel`` or ``kvm_amd`` module with ``nested=1``. You can enable
the ``nested`` parameter permanently, by creating a file named
``/etc/modprobe.d/kvm.conf`` and populating it with the following

.. code-block:: none

options kvm_intel nested=1
options kvm_amd nested=1

A reboot may be required for the change to become effective.

Nested guest support in ``nova.conf``

To support nested guests, you must set your ``cpu_mode`` configuration
to one of the following options:

Host pass through
In this mode, nested virtualization is automatically enabled once
the KVM kernel module is loaded with nesting support.

.. code-block:: ini

cpu_mode = host-passthrough

However, do consider the other implications that `Host pass
through`_ mode has on compute functionality.

Host model
In this mode, nested virtualization is automatically enabled once
the KVM kernel module is loaded with nesting support, **if** the
matching CPU model exposes the ``vmx`` feature flag to guests by
default (you can verify this with ``virsh capabilities`` on your
compute node).

.. code-block:: ini

cpu_mode = host-model

Again, consider the other implications that apply to the `Host model
(default for KVM & Qemu)`_ mode.

Nested guest support limitations

When enabling nested guests, you should be aware of (and inform your
users about) certain limitations that are currently inherent to nested
KVM virtualization. Most importantly, guests using nested
virtualization will, *while nested guests are running*,

* fail to complete live migration;
* fail to resume from suspend.

See `the KVM documentation
<>`_ for more
information on these limitations.

Guest agent support

Use guest agents to enable optional access between compute nodes and guests
through a socket, using the QMP protocol.