openstack-manuals/doc/config-reference/compute/section_hypervisor_kvm.xml

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  xml:id="kvm">
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    <title>KVM</title>
    <para>KVM is configured as the default hypervisor for Compute.</para>
    <note>
        <para>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
                <systemitem class="service">nova-compute</systemitem>. The <systemitem
                class="service">nova-compute</systemitem> service depends on qemu-kvm, which
            installs <filename>/lib/udev/rules.d/45-qemu-kvm.rules</filename>, which sets the
            correct permissions on the /dev/kvm device node.</para>
    </note>
    <para>To enable KVM explicitly, add the following configuration options to the
            <filename>/etc/nova/nova.conf</filename> file:</para>
    <programlisting language="ini">compute_driver = libvirt.LibvirtDriver

[libvirt]
virt_type = kvm</programlisting>
    <para>The KVM hypervisor supports the following virtual machine image formats:</para>
    <itemizedlist>
        <listitem>
            <para>Raw</para>
        </listitem>
        <listitem>
            <para>QEMU Copy-on-write (qcow2)</para>
        </listitem>
        <listitem>
            <para>QED Qemu Enhanced Disk</para>
        </listitem>
        <listitem>
            <para>VMware virtual machine disk format (vmdk)</para>
        </listitem>
    </itemizedlist>
    <para>This section describes how to enable KVM on your system. For more information, see the
        following distribution-specific documentation:</para>
    <itemizedlist>
        <listitem>
            <para><link
                    xlink:href="http://docs.fedoraproject.org/en-US/Fedora/22/html/Virtualization_Getting_Started_Guide/index.html"
                    >Fedora: Virtualization Getting Started Guide</link> from the Fedora 22
                documentation.</para>
        </listitem>
        <listitem>
            <para><link xlink:href="https://help.ubuntu.com/community/KVM/Installation">Ubuntu:
                    KVM/Installation</link> from the Community Ubuntu documentation.</para>
        </listitem>
        <listitem>
            <para><link
                    xlink:href="http://static.debian-handbook.info/browse/stable/sect.virtualization.html#idp11279352"
                    >Debian: Virtualization with KVM</link> from the Debian handbook.</para>
        </listitem>
        <listitem>
            <para><link
                    xlink:href="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_Installation_Guide-Host_Installation-Installing_KVM_packages_on_an_existing_Red_Hat_Enterprise_Linux_system.html"
                    >Red Hat Enterprise Linux: Installing virtualization packages on an existing Red
                    Hat Enterprise Linux system</link> from the <citetitle>Red Hat Enterprise Linux
                    Virtualization Host Configuration and Guest Installation
                Guide</citetitle>.</para>
        </listitem>
        <listitem>
            <para><link
                    xlink:href="http://doc.opensuse.org/documentation/html/openSUSE/opensuse-kvm/cha.kvm.requires.html#sec.kvm.requires.install"
                    >openSUSE: Installing KVM</link> from the openSUSE Virtualization with KVM
                manual.</para>
        </listitem>
        <listitem>
            <para><link
                    xlink:href="https://www.suse.com/documentation/sles-12/book_virt/data/sec_vt_installation_kvm.html"
                    >SLES: Installing KVM</link> from the SUSE Linux Enterprise Server
                <citetitle>Virtualization Guide</citetitle>.</para>
        </listitem>
    </itemizedlist>

    <xi:include href="../../common/section_kvm_enable.xml"/>

    <section xml:id="libvirt-xml-cpu-model">
        <title>Specify the CPU model of KVM guests</title>
        <para>The Compute service enables you to control the guest CPU model that is exposed to KVM
            virtual machines. Use cases include:</para>
        <itemizedlist>
            <listitem>
                <para>To maximize performance of virtual machines by exposing new host CPU features
                    to the guest</para>
            </listitem>
            <listitem>
                <para>To ensure a consistent default CPU across all machines, removing reliance of
                    variable QEMU defaults</para>
            </listitem>
        </itemizedlist>
        <para>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
                <filename>/usr/share/libvirt/cpu_map.xml</filename> file. Check this file to
            determine which models are supported by your local installation.</para>
        <para>Two Compute configuration options in the <literal>[libvirt]</literal> group of
                <filename>nova.conf</filename> define which type of CPU model is exposed to the
            hypervisor when using KVM: <literal>cpu_mode</literal> and
            <literal>cpu_model</literal>.</para>
        <para>The <literal>cpu_mode</literal> option can take one of the following values:
                <literal>none</literal>, <literal>host-passthrough</literal>,
                <literal>host-model</literal>, and <literal>custom</literal>.</para>
        <simplesect>
            <title>Host model (default for KVM &amp; QEMU)</title>
            <para>If your <filename>nova.conf</filename> file contains
                    <literal>cpu_mode=host-model</literal>, libvirt identifies the CPU model in
                    <filename>/usr/share/libvirt/cpu_map.xml</filename> 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.</para>
        </simplesect>
        <simplesect>
            <title>Host pass through</title>
            <para>If your <filename>nova.conf</filename> file contains
                    <literal>cpu_mode=host-passthrough</literal>, 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. The guest can only be
                migrated to a matching host CPU.</para>
        </simplesect>
        <simplesect>
            <title>Custom</title>
            <para>If your <filename>nova.conf</filename> file contains
                    <literal>cpu_mode=custom</literal>, you can explicitly specify one of the
                supported named models using the cpu_model configuration option. For example, to
                configure the KVM guests to expose Nehalem CPUs, your <filename>nova.conf</filename>
                file should contain:</para>
            <programlisting language="ini">[libvirt]
cpu_mode = custom
cpu_model = Nehalem</programlisting>
        </simplesect>
        <simplesect>
            <title>None (default for all libvirt-driven hypervisors other than KVM &amp;
                QEMU)</title>
            <para>If your <filename>nova.conf</filename> file contains
                    <literal>cpu_mode=none</literal>, libvirt does not specify a CPU model. Instead,
                the hypervisor chooses the default model.</para>
        </simplesect>
    </section>
    <section xml:id="kvm-guest-agent-support">
        <title>Guest agent support</title>
        <para>Use guest agents to enable optional access between compute nodes and guests through a
            socket, using the QMP protocol.</para>
        <para>To enable this feature, you must set <literal>hw_qemu_guest_agent=yes</literal> 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
                <literal>hw_qemu_guest_agent=no</literal> in the image metadata.</para>
    </section>
    <section xml:id="kvm-performance">
        <title>KVM performance tweaks</title>
        <para>The <link xlink:href="http://www.linux-kvm.org/page/VhostNet">VHostNet</link> kernel
            module improves network performance. To load the kernel module, run the following
            command as root:</para>
        <screen><prompt>#</prompt> <userinput>modprobe vhost_net</userinput></screen>
    </section>
    <section xml:id="kvm-troubleshooting">
        <title>Troubleshoot KVM</title>
        <para>Trying to launch a new virtual machine instance fails with the
                <literal>ERROR</literal>state, and the following error appears in the
                <filename>/var/log/nova/nova-compute.log</filename> file:</para>
        <screen><computeroutput>libvirtError: internal error no supported architecture for os type 'hvm'</computeroutput></screen>
        <para>This message indicates that the KVM kernel modules were not loaded.</para>
        <para>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
                <systemitem class="service">nova-compute</systemitem>. To check whether the group is
            set to <systemitem>kvm</systemitem>, run:</para>
        <screen><prompt>#</prompt> <userinput>ls -l /dev/kvm</userinput></screen>
        <para>If it is not set to <systemitem>kvm</systemitem>, run:</para>
        <screen><prompt>#</prompt> <userinput>udevadm trigger</userinput></screen>
    </section>
</section>