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:
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 from the Fedora 22 documentation.
Ubuntu: KVM/Installation from the Community Ubuntu documentation.
Debian: Virtualization with KVM from the Debian handbook.
Red Hat Enterprise Linux: Installing virtualization packages on an existing Red Hat Enterprise Linux system from the
Red Hat Enterprise Linux Virtualization Host Configuration and Guest Installation Guide
.openSUSE: Installing KVM from the openSUSE Virtualization with KVM manual.
SLES: Installing KVM from the SUSE Linux Enterprise Server
Virtualization Guide
.
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
orvmx
CPU extensions are present, run this command:# 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
, orSVM
.To list the loaded kernel modules and verify that the
kvm
modules are loaded, run this command:# lsmod | grep kvm
If the output includes
kvm_intel
orkvm_amd
, thekvm
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:# modprobe -a kvm
Run the command for your CPU. For Intel, run this command:
# modprobe -a kvm-intel
For AMD, run this command:
# 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 QEMU or
XenServer (and other XAPI based Xen variants) 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.
Intel-based processors
If your compute host is Intel-based, run these commands as root to load the kernel modules:
# modprobe kvm
# modprobe kvm-intel
Add these lines to the /etc/modules
file so that these modules load on
reboot:
kvm
kvm-intel
AMD-based processors
If your compute host is AMD-based, run these commands as root to load the kernel modules:
# modprobe kvm
# modprobe kvm-amd
Add these lines to /etc/modules
file so that these modules load on reboot:
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:
# cat /proc/cpuinfo | grep PowerNV
If the previous command generates the following output, then CPU supports KVM based virtualization.
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:# lsmod | grep kvm
If the output includes
kvm_hv
, thekvm
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:# modprobe -a kvm
For PowerNV platform, run the following command:
# 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
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 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 can also be
used. Set the 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 libvirt
group
of nova.conf
define which type of CPU model is exposed to the hypervisor
when using KVM: libvirt.cpu_mode
and
libvirt.cpu_models
.
The 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.
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 nova.conf
contains libvirt.cpu_mode
=custom,
you can explicitly specify an ordered list of supported named models using
the 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
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:
$ openstack flavor set FLAVOR_ID --property trait:HW_CPU_X86_AVX=required \
--property trait:HW_CPU_X86_AVX2=required
and libvirt.cpu_models
is configured like this:
[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
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
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 libvirt.cpu_models
is configured,
the CPU models in the list needs to be compatible with the host CPU. Also, if
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 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:
[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:
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
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.
[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 withvirsh capabilities
on your compute node). If your CPU model does not pass in thevmx
flag, you can force it withlibvirt.cpu_model_extra_flags
:[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 thepcid
flag to its guests:[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 for more information on these limitations.
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.
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¶
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-levelcgroup
on the host, with all VMs placed inside that. (For extreme detail, see this discussion on the spec.)An alternative approach is to configure the
reserved_host_memory_mb
option in the[DEFAULT]
section ofnova.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.See the Memory Locking and Accounting section of the AMD SEV spec and previous discussion for further details.
A cloud administrator will need to define one or more SEV-enabled flavors as described in the user guide, unless it is sufficient for users to define SEV-enabled images.
Additionally the cloud operator should consider the following optional steps:
Configure the
libvirt.num_memory_encrypted_guests
option innova.conf
to represent the number of guests an SEV compute node can host concurrently with memory encrypted at the hardware level. For example:[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
libvirt.hw_machine_type
on all SEV-capable compute hosts to includex86_64=q35
, so that all x86_64 images use theq35
machine type by default. (Currently Nova defaults to thepc
machine type for thex86_64
architecture, although it is expected that this will change in the future.)Changing the default from
pc
toq35
makes the creation and configuration of images by users more convenient by removing the need for thehw_machine_type
property to be set toq35
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 withpc
, 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.
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 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.
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 propertyhw_disk_bus=virtio
). Valid alternatives for the disk include usinghw_disk_bus=scsi
withhw_scsi_model=virtio-scsi
, orhw_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
libvirt.num_memory_encrypted_guests
configuration option described above.
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).
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)
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 runningAttaching any volumes, as long as they do not require attaching via an IDE bus
Use of spice / VNC / serial / RDP consoles
For further technical details, see the nova spec for SEV support.
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 kernel module improves network performance. To load the kernel module, run the following command as root:
# 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:
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:
# ls -l /dev/kvm
If it is not set to kvm
, run:
# udevadm trigger