openstack/nova
Code Issues Proposed changes
Files
b68033ecc9d38dcbe339c993338bdca691f73b33
nova/doc/source/admin/configuration/hypervisor-kvm.rst
Boris Bobrov 1a88d35cb1 Also enable iommu for virtio controllers and video in libvirt 
Ie54fca066f33 added logic to libvirt/designer.py for enabling iommu
for certain devices where virtio is used.  This is required for AMD
SEV[0].  However it missed two cases.

Firstly, a SCSI controller can have the model as 'virtio-scsi', e.g.:

    <controller type='scsi' index='0' model='virtio-scsi'>

As with other virtio devices, here a child element needs to be added
to the config when SEV is enabled:

    <driver iommu="on" />

We do not need to cover the case of a controller with type
'virtio-serial' now, since even though it is supported by libvirt, it
is not currently used anywhere in Nova.

Secondly, a video device can be virtio, e.g. when vgpus are in use:

    <video>
        <model type='virtio'/>
    </video>

Also take this opportunity to clarify the corresponding documentation
around disk bus options.

[0] http://specs.openstack.org/openstack/nova-specs/specs/train/approved/amd-sev-libvirt-support.html#proposed-change

Partial-Bug: #1845986
Change-Id: I626c35d1653e6a25125320032d0a4a0c67ab8bcf
2019-12-10 11:01:08 +00:00

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===
KVM
===
.. todo:: Some of this is installation guide material and should probably be
moved.
KVM is configured as the default hypervisor for Compute.
.. note::
This document contains several sections about hypervisor selection. If you
are reading this document linearly, you do not want to load the KVM module
before you install ``nova-compute``. The ``nova-compute`` service depends
on qemu-kvm, which installs ``/lib/udev/rules.d/45-qemu-kvm.rules``, which
sets the correct permissions on the ``/dev/kvm`` device node.
To enable KVM explicitly, add the following configuration options to the
``/etc/nova/nova.conf`` file:
.. code-block:: ini
compute_driver = libvirt.LibvirtDriver
[libvirt]
virt_type = kvm
The KVM hypervisor supports the following virtual machine image formats:
* Raw
* QEMU Copy-on-write (QCOW2)
* QED Qemu Enhanced Disk
* VMware virtual machine disk format (vmdk)
This section describes how to enable KVM on your system. For more information,
see the following distribution-specific documentation:
* `Fedora: Virtualization Getting Started Guide <http://docs.fedoraproject.org/
en-US/Fedora/22/html/Virtualization_Getting_Started_Guide/index.html>`_
from the Fedora 22 documentation.
* `Ubuntu: KVM/Installation <https://help.ubuntu.com/community/KVM/
Installation>`_ from the Community Ubuntu documentation.
* `Debian: Virtualization with KVM <http://static.debian-handbook.info/browse/
stable/sect.virtualization.html#idp11279352>`_ from the Debian handbook.
* `Red Hat Enterprise Linux: Installing virtualization packages on an existing
Red Hat Enterprise Linux system <http://docs.redhat.com/docs/en-US/
Red_Hat_Enterprise_Linux/6/html/Virtualization_Host_Configuration_and_Guest_
Installation_Guide/sect-Virtualization_Host_Configuration_and_Guest_Installa
tion_Guide-Host_Installation-Installing_KVM_packages_on_an_existing_Red_Hat_
Enterprise_Linux_system.html>`_ from the ``Red Hat Enterprise Linux
Virtualization Host Configuration and Guest Installation Guide``.
* `openSUSE: Installing KVM <http://doc.opensuse.org/documentation/html/
openSUSE/opensuse-kvm/cha.kvm.requires.html#sec.kvm.requires.install>`_
from the openSUSE Virtualization with KVM manual.
* `SLES: Installing KVM <https://www.suse.com/documentation/sles-12/book_virt/
data/sec_vt_installation_kvm.html>`_ from the SUSE Linux Enterprise Server
``Virtualization Guide``.
.. _enable-kvm:
Enable KVM
~~~~~~~~~~
The following sections outline how to enable KVM based hardware virtualization
on different architectures and platforms. To perform these steps, you must be
logged in as the ``root`` user.
For x86 based systems
---------------------
#. To determine whether the ``svm`` or ``vmx`` CPU extensions are present, run
this command:
.. code-block:: console
# grep -E 'svm|vmx' /proc/cpuinfo
This command generates output if the CPU is capable of
hardware-virtualization. Even if output is shown, you might still need to
enable virtualization in the system BIOS for full support.
If no output appears, consult your system documentation to ensure that your
CPU and motherboard support hardware virtualization. Verify that any
relevant hardware virtualization options are enabled in the system BIOS.
The BIOS for each manufacturer is different. If you must enable
virtualization in the BIOS, look for an option containing the words
``virtualization``, ``VT``, ``VMX``, or ``SVM``.
#. To list the loaded kernel modules and verify that the ``kvm`` modules are
loaded, run this command:
.. code-block:: console
# lsmod | grep kvm
If the output includes ``kvm_intel`` or ``kvm_amd``, the ``kvm`` hardware
virtualization modules are loaded and your kernel meets the module
requirements for OpenStack Compute.
If the output does not show that the ``kvm`` module is loaded, run this
command to load it:
.. code-block:: console
# modprobe -a kvm
Run the command for your CPU. For Intel, run this command:
.. code-block:: console
# modprobe -a kvm-intel
For AMD, run this command:
.. code-block:: console
# modprobe -a kvm-amd
Because a KVM installation can change user group membership, you might need
to log in again for changes to take effect.
If the kernel modules do not load automatically, use the procedures listed
in these subsections.
If the checks indicate that required hardware virtualization support or kernel
modules are disabled or unavailable, you must either enable this support on the
system or find a system with this support.
.. note::
Some systems require that you enable VT support in the system BIOS. If you
believe your processor supports hardware acceleration but the previous
command did not produce output, reboot your machine, enter the system BIOS,
and enable the VT option.
If KVM acceleration is not supported, configure Compute to use a different
hypervisor, such as ``QEMU`` or ``Xen``. See :ref:`compute_qemu` or
:ref:`compute_xen_api` for details.
These procedures help you load the kernel modules for Intel-based and AMD-based
processors if they do not load automatically during KVM installation.
.. rubric:: Intel-based processors
If your compute host is Intel-based, run these commands as root to load the
kernel modules:
.. code-block:: console
# modprobe kvm
# modprobe kvm-intel
Add these lines to the ``/etc/modules`` file so that these modules load on
reboot:
.. code-block:: console
kvm
kvm-intel
.. rubric:: AMD-based processors
If your compute host is AMD-based, run these commands as root to load the
kernel modules:
.. code-block:: console
# modprobe kvm
# modprobe kvm-amd
Add these lines to ``/etc/modules`` file so that these modules load on reboot:
.. code-block:: console
kvm
kvm-amd
For POWER based systems
-----------------------
KVM as a hypervisor is supported on POWER system's PowerNV platform.
#. To determine if your POWER platform supports KVM based virtualization run
the following command:
.. code-block:: console
# cat /proc/cpuinfo | grep PowerNV
If the previous command generates the following output, then CPU supports
KVM based virtualization.
.. code-block:: console
platform: PowerNV
If no output is displayed, then your POWER platform does not support KVM
based hardware virtualization.
#. To list the loaded kernel modules and verify that the ``kvm`` modules are
loaded, run the following command:
.. code-block:: console
# lsmod | grep kvm
If the output includes ``kvm_hv``, the ``kvm`` hardware virtualization
modules are loaded and your kernel meets the module requirements for
OpenStack Compute.
If the output does not show that the ``kvm`` module is loaded, run the
following command to load it:
.. code-block:: console
# modprobe -a kvm
For PowerNV platform, run the following command:
.. code-block:: console
# modprobe -a kvm-hv
Because a KVM installation can change user group membership, you might need
to log in again for changes to take effect.
Configure Compute backing storage
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Backing Storage is the storage used to provide the expanded operating system
image, and any ephemeral storage. Inside the virtual machine, this is normally
presented as two virtual hard disks (for example, ``/dev/vda`` and ``/dev/vdb``
respectively). However, inside OpenStack, this can be derived from one of these
methods: ``lvm``, ``qcow``, ``rbd`` or ``flat``, chosen using the
:oslo.config:option:`libvirt.images_type` option in ``nova.conf`` on the
compute node.
.. note::
The option ``raw`` is acceptable but deprecated in favor of ``flat``. The
Flat back end uses either raw or QCOW2 storage. It never uses a backing
store, so when using QCOW2 it copies an image rather than creating an
overlay. By default, it creates raw files but will use QCOW2 when creating a
disk from a QCOW2 if :oslo.config:option:`force_raw_images` is not set in
configuration.
QCOW is the default backing store. It uses a copy-on-write philosophy to delay
allocation of storage until it is actually needed. This means that the space
required for the backing of an image can be significantly less on the real disk
than what seems available in the virtual machine operating system.
Flat creates files without any sort of file formatting, effectively creating
files with the plain binary one would normally see on a real disk. This can
increase performance, but means that the entire size of the virtual disk is
reserved on the physical disk.
Local `LVM volumes
<https://en.wikipedia.org/wiki/Logical_Volume_Manager_(Linux)>`__ can also be
used. Set the :oslo.config:option:`libvirt.images_volume_group` configuration
option to the name of the LVM group you have created.
Specify the CPU model of KVM guests
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The Compute service enables you to control the guest CPU model that is exposed
to KVM virtual machines. Use cases include:
* To maximize performance of virtual machines by exposing new host CPU features
to the guest
* To ensure a consistent default CPU across all machines, removing reliance of
variable QEMU defaults
In libvirt, the CPU is specified by providing a base CPU model name (which is a
shorthand for a set of feature flags), a set of additional feature flags, and
the topology (sockets/cores/threads). The libvirt KVM driver provides a number
of standard CPU model names. These models are defined in the
``/usr/share/libvirt/cpu_map.xml`` file for libvirt prior to version 4.7.0 or
``/usr/share/libvirt/cpu_map/*.xml`` files thereafter. Make a check to
determine which models are supported by your local installation.
Two Compute configuration options in the :oslo.config:group:`libvirt` group
of ``nova.conf`` define which type of CPU model is exposed to the hypervisor
when using KVM: :oslo.config:option:`libvirt.cpu_mode` and
:oslo.config:option:`libvirt.cpu_models`.
The :oslo.config:option:`libvirt.cpu_mode` option can take one of the following
values: ``none``, ``host-passthrough``, ``host-model``, and ``custom``.
See `Effective Virtual CPU configuration in Nova`_ for a recorded presentation
about this topic.
.. _Effective Virtual CPU configuration in Nova: https://www.openstack.org/videos/summits/berlin-2018/effective-virtual-cpu-configuration-in-nova
Host model (default for KVM & QEMU)
-----------------------------------
If your ``nova.conf`` file contains ``cpu_mode=host-model``, libvirt identifies
the CPU model in ``/usr/share/libvirt/cpu_map.xml`` for version prior to 4.7.0
or ``/usr/share/libvirt/cpu_map/*.xml`` for version 4.7.0 and higher 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.
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
-----------------
If your ``nova.conf`` file contains ``cpu_mode=host-passthrough``, libvirt
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.
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.
Custom
------
If :file:`nova.conf` contains :oslo.config:option:`libvirt.cpu_mode`\ =custom,
you can explicitly specify an ordered list of supported named models using
the :oslo.config:option:`libvirt.cpu_models` configuration option. It is
expected that the list is ordered so that the more common and less advanced cpu
models are listed earlier.
An end user can specify required CPU features through traits. When specified,
the libvirt driver will select the first cpu model in the
:oslo.config:option:`libvirt.cpu_models` list that can provide the requested
feature traits. If no CPU feature traits are specified then the instance will
be configured with the first cpu model in the list.
For example, if specifying CPU features ``avx`` and ``avx2`` as follows:
.. code-block:: console
$ openstack flavor set FLAVOR_ID --property trait:HW_CPU_X86_AVX=required \
--property trait:HW_CPU_X86_AVX2=required
and :oslo.config:option:`libvirt.cpu_models` is configured like this:
.. code-block:: ini
[libvirt]
cpu_mode = custom
cpu_models = Penryn,IvyBridge,Haswell,Broadwell,Skylake-Client
Then ``Haswell``, the first cpu model supporting both ``avx`` and ``avx2``,
will be chosen by libvirt.
In selecting the ``custom`` mode, along with a
:oslo.config:option:`libvirt.cpu_models` 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
:oslo.config:option:`libvirt.cpu_models` you select passes the correct CPU
feature flags to the guest.
If you need to further tweak your CPU feature flags in the ``custom``
mode, see `Set CPU feature flags`_.
.. note::
If :oslo.config:option:`libvirt.cpu_models` is configured,
the CPU models in the list needs to be compatible with the host CPU. Also, if
:oslo.config:option:`libvirt.cpu_model_extra_flags` is configured, all flags
needs to be compatible with the host CPU. If incompatible CPU models or flags
are specified, nova service will raise an error and fail to start.
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 :oslo.config:option:`libvirt.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
[libvirt]
cpu_mode = custom
cpu_models = 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
content:
.. 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
:oslo.config:option:`libvirt.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
[libvirt]
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). If your CPU model does not pass in the ``vmx`` flag,
you can force it with :oslo.config:option:`libvirt.cpu_model_extra_flags`:
.. code-block:: ini
[libvirt]
cpu_mode = host-model
cpu_model_extra_flags = vmx
Again, consider the other implications that apply to the `Host model
(default for KVM & Qemu)`_ mode.
Custom
In custom mode, the same considerations apply as in host-model mode,
but you may *additionally* want to ensure that libvirt passes not only
the ``vmx``, but also the ``pcid`` flag to its guests:
.. code-block:: ini
[libvirt]
cpu_mode = custom
cpu_models = IvyBridge
cpu_model_extra_flags = vmx,pcid
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
<https://www.linux-kvm.org/page/Nested_Guests#Limitations>`_ for more
information on these limitations.
.. _amd-sev:
AMD SEV (Secure Encrypted Virtualization)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
`Secure Encrypted Virtualization (SEV)`__ is a technology from AMD which
enables the memory for a VM to be encrypted with a key unique to the VM.
SEV is particularly applicable to cloud computing since it can reduce the
amount of trust VMs need to place in the hypervisor and administrator of
their host system.
__ https://developer.amd.com/sev/
Nova supports SEV from the Train release onwards.
Requirements for SEV
--------------------
First the operator will need to ensure the following prerequisites are met:
- At least one of the Nova compute hosts must be AMD hardware capable
of supporting SEV. It is entirely possible for the compute plane to
be a mix of hardware which can and cannot support SEV, although as
per the section on `Permanent limitations`_ below, the maximum
number of simultaneously running guests with SEV will be limited by
the quantity and quality of SEV-capable hardware available.
- An appropriately configured software stack on those compute hosts,
so that the various layers are all SEV ready:
- kernel >= 4.16
- QEMU >= 2.12
- libvirt >= 4.5
- ovmf >= commit 75b7aa9528bd 2018-07-06
.. _deploying-sev-capable-infrastructure:
Deploying SEV-capable infrastructure
------------------------------------
In order for users to be able to use SEV, the operator will need to
perform the following steps:
- Ensure that sufficient memory is reserved on the SEV compute hosts
for host-level services to function correctly at all times. This is
particularly important when hosting SEV-enabled guests, since they
pin pages in RAM, preventing any memory overcommit which may be in
normal operation on other compute hosts.
It is `recommended`__ to achieve this by configuring an ``rlimit`` at
the ``/machine.slice`` top-level ``cgroup`` on the host, with all VMs
placed inside that. (For extreme detail, see `this discussion on the
spec`__.)
__ http://specs.openstack.org/openstack/nova-specs/specs/train/approved/amd-sev-libvirt-support.html#memory-reservation-solutions
__ https://review.opendev.org/#/c/641994/2/specs/train/approved/amd-sev-libvirt-support.rst@167
An alternative approach is to configure the
:oslo.config:option:`reserved_host_memory_mb` option in the
``[DEFAULT]`` section of :file:`nova.conf`, based on the expected
maximum number of SEV guests simultaneously running on the host, and
the details provided in `an earlier version of the AMD SEV spec`__
regarding memory region sizes, which cover how to calculate it
correctly.
__ https://specs.openstack.org/openstack/nova-specs/specs/stein/approved/amd-sev-libvirt-support.html#proposed-change
See `the Memory Locking and Accounting section of the AMD SEV spec`__
and `previous discussion for further details`__.
__ http://specs.openstack.org/openstack/nova-specs/specs/train/approved/amd-sev-libvirt-support.html#memory-locking-and-accounting
__ https://review.opendev.org/#/c/641994/2/specs/train/approved/amd-sev-libvirt-support.rst@167
- A cloud administrator will need to define one or more SEV-enabled
flavors :ref:`as described in the user guide
<extra-specs-memory-encryption>`, unless it is sufficient for users
to define SEV-enabled images.
Additionally the cloud operator should consider the following optional
steps:
.. _num_memory_encrypted_guests:
- Configure the :oslo.config:option:`libvirt.num_memory_encrypted_guests`
option in :file:`nova.conf` to represent the number of guests an SEV
compute node can host concurrently with memory encrypted at the
hardware level. For example:
.. code-block:: ini
[libvirt]
num_memory_encrypted_guests = 15
This option exists because on AMD SEV-capable hardware, the memory
controller has a fixed number of slots for holding encryption keys,
one per guest. For example, at the time of writing, earlier
generations of hardware only have 15 slots, thereby limiting the
number of SEV guests which can be run concurrently to 15. Nova
needs to track how many slots are available and used in order to
avoid attempting to exceed that limit in the hardware.
At the time of writing (September 2019), work is in progress to
allow QEMU and libvirt to expose the number of slots available on
SEV hardware; however until this is finished and released, it will
not be possible for Nova to programmatically detect the correct
value.
So this configuration option serves as a stop-gap, allowing the
cloud operator the option of providing this value manually. It may
later be demoted to a fallback value for cases where the limit
cannot be detected programmatically, or even removed altogether when
Nova's minimum QEMU version guarantees that it can always be
detected.
.. note::
When deciding whether to use the default of ``None`` or manually
impose a limit, operators should carefully weigh the benefits
vs. the risk. The benefits of using the default are a) immediate
convenience since nothing needs to be done now, and b) convenience
later when upgrading compute hosts to future versions of Nova,
since again nothing will need to be done for the correct limit to
be automatically imposed. However the risk is that until
auto-detection is implemented, users may be able to attempt to
launch guests with encrypted memory on hosts which have already
reached the maximum number of guests simultaneously running with
encrypted memory. This risk may be mitigated by other limitations
which operators can impose, for example if the smallest RAM
footprint of any flavor imposes a maximum number of simultaneously
running guests which is less than or equal to the SEV limit.
- Configure :oslo.config:option:`libvirt.hw_machine_type` on all
SEV-capable compute hosts to include ``x86_64=q35``, so that all
x86_64 images use the ``q35`` machine type by default. (Currently
Nova defaults to the ``pc`` machine type for the ``x86_64``
architecture, although `it is expected that this will change in the
future`__.)
Changing the default from ``pc`` to ``q35`` makes the creation and
configuration of images by users more convenient by removing the
need for the ``hw_machine_type`` property to be set to ``q35`` on
every image for which SEV booting is desired.
.. caution::
Consider carefully whether to set this option. It is
particularly important since a limitation of the implementation
prevents the user from receiving an error message with a helpful
explanation if they try to boot an SEV guest when neither this
configuration option nor the image property are set to select
a ``q35`` machine type.
On the other hand, setting it to ``q35`` may have other
undesirable side-effects on other images which were expecting to
be booted with ``pc``, so it is suggested to set it on a single
compute node or aggregate, and perform careful testing of typical
images before rolling out the setting to all SEV-capable compute
hosts.
__ https://bugs.launchpad.net/nova/+bug/1780138
Launching SEV instances
-----------------------
Once an operator has covered the above steps, users can launch SEV
instances either by requesting a flavor for which the operator set the
``hw:mem_encryption`` extra spec to ``True``, or by using an image
with the ``hw_mem_encryption`` property set to ``True``.
These do not inherently cause a preference for SEV-capable hardware,
but for now SEV is the only way of fulfilling the requirement for
memory encryption. However in the future, support for other
hardware-level guest memory encryption technology such as Intel MKTME
may be added. If a guest specifically needs to be booted using SEV
rather than any other memory encryption technology, it is possible to
ensure this by adding ``trait:HW_CPU_X86_AMD_SEV=required`` to the
flavor extra specs or image properties.
In all cases, SEV instances can only be booted from images which have
the ``hw_firmware_type`` property set to ``uefi``, and only when the
machine type is set to ``q35``. This can be set per image by setting
the image property ``hw_machine_type=q35``, or per compute node by
the operator via :oslo.config:option:`libvirt.hw_machine_type` as
explained above.
Impermanent limitations
-----------------------
The following limitations may be removed in the future as the
hardware, firmware, and various layers of software receive new
features:
- SEV-encrypted VMs cannot yet be live-migrated or suspended,
therefore they will need to be fully shut down before migrating off
an SEV host, e.g. if maintenance is required on the host.
- SEV-encrypted VMs cannot contain directly accessible host devices
(PCI passthrough). So for example mdev vGPU support will not
currently work. However technologies based on `vhost-user`__ should
work fine.
__ https://wiki.qemu.org/Features/VirtioVhostUser
- The boot disk of SEV-encrypted VMs can only be ``virtio``.
(``virtio-blk`` is typically the default for libvirt disks on x86,
but can also be explicitly set e.g. via the image property
``hw_disk_bus=virtio``). Valid alternatives for the disk
include using ``hw_disk_bus=scsi`` with
``hw_scsi_model=virtio-scsi`` , or ``hw_disk_bus=sata``.
- QEMU and libvirt cannot yet expose the number of slots available for
encrypted guests in the memory controller on SEV hardware. Until
this is implemented, it is not possible for Nova to programmatically
detect the correct value. As a short-term workaround, operators can
optionally manually specify the upper limit of SEV guests for each
compute host, via the new
:oslo.config:option:`libvirt.num_memory_encrypted_guests`
configuration option :ref:`described above
<num_memory_encrypted_guests>`.
Permanent limitations
---------------------
The following limitations are expected long-term:
- The number of SEV guests allowed to run concurrently will always be
limited. `On the first generation of EPYC machines it will be
limited to 15 guests`__; however this limit becomes much higher with
the second generation (Rome).
__ https://www.redhat.com/archives/libvir-list/2019-January/msg00652.html
- The operating system running in an encrypted virtual machine must
contain SEV support.
Non-limitations
---------------
For the sake of eliminating any doubt, the following actions are *not*
expected to be limited when SEV encryption is used:
- Cold migration or shelve, since they power off the VM before the
operation at which point there is no encrypted memory (although this
could change since there is work underway to add support for `PMEM
<https://pmem.io/>`_)
- Snapshot, since it only snapshots the disk
- ``nova evacuate`` (despite the name, more akin to resurrection than
evacuation), since this is only initiated when the VM is no longer
running
- Attaching any volumes, as long as they do not require attaching via
an IDE bus
- Use of spice / VNC / serial / RDP consoles
- `VM guest virtual NUMA (a.k.a. vNUMA)
<https://www.suse.com/documentation/sles-12/singlehtml/article_vt_best_practices/article_vt_best_practices.html#sec.vt.best.perf.numa.vmguest>`_
For further technical details, see `the nova spec for SEV support`__.
__ http://specs.openstack.org/openstack/nova-specs/specs/train/approved/amd-sev-libvirt-support.html
Guest agent support
~~~~~~~~~~~~~~~~~~~
Use guest agents to enable optional access between compute nodes and guests
through a socket, using the QMP protocol.
To enable this feature, you must set ``hw_qemu_guest_agent=yes`` as a metadata
parameter on the image you wish to use to create the guest-agent-capable
instances from. You can explicitly disable the feature by setting
``hw_qemu_guest_agent=no`` in the image metadata.
KVM performance tweaks
~~~~~~~~~~~~~~~~~~~~~~
The `VHostNet <http://www.linux-kvm.org/page/VhostNet>`_ kernel module improves
network performance. To load the kernel module, run the following command as
root:
.. code-block:: console
# modprobe vhost_net
Troubleshoot KVM
~~~~~~~~~~~~~~~~
Trying to launch a new virtual machine instance fails with the ``ERROR`` state,
and the following error appears in the ``/var/log/nova/nova-compute.log`` file:
.. code-block:: console
libvirtError: internal error no supported architecture for os type 'hvm'
This message indicates that the KVM kernel modules were not loaded.
If you cannot start VMs after installation without rebooting, the permissions
might not be set correctly. This can happen if you load the KVM module before
you install ``nova-compute``. To check whether the group is set to ``kvm``,
run:
.. code-block:: console
# ls -l /dev/kvm
If it is not set to ``kvm``, run:
.. code-block:: console
# udevadm trigger
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