Did Arch Linux eat your KVM?

Mon 08 April 2013 by Lars Kellogg-Stedman Tags archlinux virtualization kvm

A recent update to Arch Linux replaced the qemu-kvm package with an updated version of qemu. A side effect of this change is that the qemu-kvm binary is no longer available, and any libvirt guests on your system utilizing that binary will no longer operate. As is typical with Arch, there is no announcement about this incompatible change, and queries to #archlinux will be met with the knowledge, grace and decorum you would expect of that channel:

2013-04-08T18:00 < gtmanfred> USE --enable-kvm for fucks sake
2013-04-08T18:00 < gtmanfred> DO I HAVE TO SAY IT AGAIN?

The emulator binary is hardcoded into your domain in the <emulator> emulator, and typically looks something like this:

<emulator>/usr/bin/qemu-kvm</emulator>

In order to get your guests working again after the upgrade you'll need to replace this path with an appropriate selection from one of the other binaries provided by the qemu package, which include qemu-system-i386 and qemu-system-x86_64. You'll want to select the one appropriate for your guest architecture. You can do this manually running virsh edit for each affected guest, but if you have more than a couple that rapidly becomes annoying.

We can use XSLT to write a transformation that will set the <emulator> to an appropriate value, and we can set things up to run this automatically across all of our guests. The following stylesheet will replace the <emulator> tag with a path to an appropriate qemu (by extracting the arch attribute of the domain/os/type element:

<?xml version="1.0"?>
<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">

  <!-- copy all elements verbatim... -->
  <xsl:template match="@*|node()">
    <xsl:copy>
      <xsl:apply-templates select="@*|node()"/>
    </xsl:copy>
  </xsl:template>

  <!-- ...except for the 'emulator' element. -->
  <xsl:template match="emulator">
    <emulator>/usr/bin/qemu-system-<xsl:value-of select="/*/os/type/@arch"/></emulator>
  </xsl:template>

</xsl:stylesheet>

We're going to apply this to all of our (inactive) guests via the virsh edit subcommand. This command runs an editor (selected based on your VISUAL or EDITOR environment variables) on your domain XML. We need to create an "editor" that will apply the above transformation to its input file. Something like this will work:

#!/bin/sh

tmpfile=$(mktemp "$1.patched.XXXXXX")
xsltproc -o "$tmpfile" patch-emulator.xsl "$1"
mv "$tmpfile" "$1"

Assuming the above script has been saved as "patch-emulator.sh" (and made executable), we can run this across all of our inactive guests like this:

#!/bin/sh

VISUAL=./patch-emulator.sh
export VISUAL

virsh list --inactive --name | while read vm; do
        [ "$vm" ] || continue
        virsh edit $vm
done

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