openstack-manuals/doc/common/section_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. <note>
            <para>There are several sections about hypervisor
                selection in this document. If you are reading this
                document linearly, you do not want to load the KVM
                module prior to installing <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>
    <para>To enable KVM explicitly, add the following configuration
        options
        <filename>/etc/nova/nova.conf</filename>:<programlisting language="ini">compute_driver=libvirt.LibvirtDriver
libvirt_type=kvm</programlisting>
        The KVM hypervisor supports the following virtual machine
        image formats:<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>
    <para>The rest of this section describes how to enable KVM on your system. You may also wish to
        consult distribution-specific documentation:<itemizedlist>
            <listitem>
                <para><link
                        xlink:href="http://fedoraproject.org/wiki/Getting_started_with_virtualization"
                        >Fedora: Getting started with virtualization</link> from the Fedora project
                    wiki.</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"
                        >RHEL: Installing virtualization packages on
                        an existing Red Hat Enterprise Linux
                        system</link> from the Red Hat Enterprise
                    Linux Virtualization Host Configuration and Guest
                    Installation Guide.</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="http://doc.opensuse.org/products/draft/SLES/SLES-kvm_sd_draft/cha.kvm.requires.html#sec.kvm.requires.install"
                        >SLES: Installing KVM</link> from the SUSE
                    Linux Enterprise Server Virtualization with KVM
                    manual.</para>
            </listitem>
        </itemizedlist></para>
    <section xml:id="checking-kvm">
        <title>Checking for hardware virtualization support</title>
        <para>The processors of your compute host need to support virtualization technology (VT)
            (mainly Intel <emphasis role="italic">VT -x</emphasis> or AMD <emphasis role="italic"
                >AMD-v</emphasis> technologies) to use KVM.</para>
        <para>In order to check if your processor has VT support (which has to be enabled in the
            BIOS), issue as
            root:<screen os="ubuntu"><prompt>#</prompt> <userinput>apt-get install cpu-checker</userinput>
<prompt>#</prompt> <userinput>kvm-ok</userinput></screen>
                <screen os="rhel;fedoa;centos"><prompt>$</prompt> <userinput>egrep '(vmx|svm)' --color=always /proc/cpuinfo</userinput></screen>
            </para>
        <para>If KVM is supported, the output should look something like:<screen os="ubuntu">
<computeroutput>INFO: /dev/kvm exists
KVM acceleration can be used</computeroutput></screen>
            <screen os="rhel;fedora;centos"><computeroutput>flags       : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush dts acpi mmx fxsr sse sse2 ss ht tm pbe syscall nx rdtscp lm constant_tsc arch_perfmon pebs bts rep_good nopl xtopology nonstop_tsc aperfmperf pni dtes64 monitor ds_cpl vmx est tm2 ssse3 cx16 xtpr pdcm pcid sse4_1 sse4_2 popcnt lahf_lm arat dtherm tpr_shadow vnmi flexpriority ept vpid</computeroutput></screen>
        </para>
        <para os="ubuntu">If KVM is not supported, the output should look something
            like:<screen><computeroutput>INFO: Your CPU does not support KVM extensions
KVM acceleration can NOT be used</computeroutput></screen></para>
        <para os="rhel;fedora;centos">If KVM is not supported, you should get no output.</para>
        <note><para>Some systems require that you enable VT support in the system BIOS. If you believe
            your processor supports hardware acceleration but the above command produced no output,
            you may need to reboot your machine, enter the system BIOS, and enable the VT
            option.</para></note>

        <para>In the case that KVM acceleration is not supported, Compute should be configured to
            use a different hypervisor, such as <link linkend="qemu">QEMU</link> or <link
                linkend="introduction-to-xen">Xen</link>.</para>

    </section>

    <section xml:id="enabling-kvm">
        <title>Enabling KVM</title>
        <para>KVM requires the <literal>kvm</literal> and either <literal>kvm-intel</literal> or
                <literal>kvm-amd</literal> modules to be loaded. This may have been configured
            automatically on your distribution when KVM is installed.</para>
        <para>You can check that they have been loaded using <command>lsmod</command>, as follows,
            with expected output for Intel-based
            processors:<screen><prompt>$</prompt> <userinput>lsmod | grep kvm</userinput>
<computeroutput>kvm_intel             137721  9
kvm                   415459  1 kvm_intel</computeroutput></screen>The
            following sections describe how to load the kernel modules for Intel-based and AMD-based
            processors if they were not loaded automatically by your distribution's KVM installation
            process.</para>

        <section xml:id="kvm-intel">
            <title>Intel-based processors</title>
        <para>If your compute host is Intel-based, run the following as root to load the kernel
                modules:<screen><prompt>#</prompt> modprobe kvm
<prompt>#</prompt> modprobe kvm-intel</screen>
                Add the following lines to <filename>/etc/modules</filename> so that these modules
                will load on reboot:<programlisting>kvm
kvm-intel</programlisting></para>
</section>
        <section xml:id="kvm-amd">
            <title>AMD-based processors</title>
            <para>If your compute host is AMD-based, run the following as root to load the kernel
                modules:<screen><prompt>#</prompt> modprobe kvm
<prompt>#</prompt> modprobe kvm-amd</screen>
                Add the following lines to <filename>/etc/modules</filename> so that these modules
                will load on reboot:<programlisting>kvm
kvm-amd</programlisting></para>
           </section>
    </section>
    <section xml:id="libvirt-xml-cpu-model">
        <title>Specifying the CPU model of KVM guests</title>
        <para>The Compute service allows you to control the guest CPU model that is exposed to KVM
            virtual machines. Use cases include:<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>
        <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. Examples of model names include:</para>
        <para><literal>"486", "pentium", "pentium2", "pentiumpro", "coreduo", "n270", "pentiumpro",
                "qemu32", "kvm32", "cpu64-rhel5", "cpu64-rhel5", "kvm64", "pentiumpro", "Conroe"
                "Penryn", "Nehalem", "Westmere", "pentiumpro", "cpu64-rhel5", "cpu64-rhel5",
                "Opteron_G1", "Opteron_G2", "Opteron_G3", "Opteron_G4"</literal></para>
        <para>These models are defined in  the file
                <filename>/usr/share/libvirt/cpu_map.xml</filename>. Check this file to determine
            which models are supported by your local installation.</para>
        <para>There are two Compute configuration options that determine the type of CPU model
            exposed to the hypervisor when using KVM, <literal>libvirt_cpu_mode</literal> and
                <literal>libvirt_cpu_model</literal>.</para>
        <para>The <literal>libvirt_cpu_mode</literal> option can take one of four 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> contains
                    <literal>libvirt_cpu_mode=host-model</literal>, libvirt will identify the CPU
                model in <filename>/usr/share/libvirt/cpu_map.xml</filename> which most closely
                matches the host, and then request additional CPU flags to complete the match. This
                should give close to maximum functionality/performance, which maintaining good
                reliability/compatibility if the guest is migrated to another host with slightly
                different host CPUs.</para>
        </simplesect>
        <simplesect>
            <title>Host passthrough</title>
            <para>If your <filename>nova.conf</filename> contains
                    <literal>libvirt_cpu_mode=host-passthrough</literal>, libvirt will tell KVM to
                passthrough 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 absolutely 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 an exactly matching host CPU.</para>
        </simplesect>
        <simplesect>
            <title>Custom</title>
            <para>If your <filename>nova.conf</filename> file contains <literal>libvirt_cpu_mode=custom</literal>, you can
                explicitly specify one of the supported named model using the libvirt_cpu_model
                configuration option. For example, to configure the KVM guests to expose Nehalem
                CPUs, your nova.conf should contain:</para>
            <para>
                <programlisting language="ini">libvirt_cpu_mode=custom
libvirt_cpu_model=Nehalem</programlisting>
            </para>
        </simplesect>
        <simplesect>
            <title>None (default for all libvirt-driven hypervisors other than KVM &amp;
                QEMU)</title>
            <para>If your <filename>nova.conf</filename> contains
                    <literal>libvirt_cpu_mode=none</literal>, then
                libvirt will not specify any CPU model at all. It will
                leave it up to the hypervisor to choose the default
                model. This setting is equivalent to the Compute
                service behavior prior to the Folsom release.</para>
          </simplesect>
    </section>
    <section xml:id="kvm-performance">
    <title>KVM Performance Tweaks</title>
    <para>A recommended resource to help you improve
    the performance of KVM is
    the <link xlink:href="http://www.linux-kvm.org/page/VhostNet">VHostNet</link>
    kernel module. This module improves network performance. To load the
     kernel module, as root:
     <screen><prompt>#</prompt> <userinput>modprobe vhost_net</userinput></screen></para>
    </section>
    <section xml:id="xml-troubleshooting">
        <title>Troubleshooting</title>
        <para>Trying to launch a new virtual machine instance fails
            with the <literal>ERROR</literal> state, and the following
            error appears in
                <filename>/var/log/nova/nova-compute.log</filename><screen><computeroutput>libvirtError: internal error no supported architecture for os type 'hvm'</computeroutput></screen>
            This is a symptom that the KVM kernel modules have not
            been loaded.</para>
        <para>If you cannot start VMs after installation without
            rebooting, it's possible the permissions are not correct.
            This can happen if you load the KVM module before you've
            installed <systemitem class="service">nova-compute</systemitem>. To check the permissions, run
                <userinput>ls -l /dev/kvm</userinput> to see whether
            the group is set to kvm. If not, run <userinput>sudo
                udevadm trigger</userinput>.</para>
    </section>
</section>