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QEMU notes

Enabled features and how they are expressed in XML

kvm=off option to the -cpu command

QEMU 2.1 added support for the kvm=off option to the -cpu command, allowing the KVM hypervisor signature to be hidden from the guest. This enables disabling of some paravirualization features in the guest as well as allowing certain drivers which test for the hypervisor to load. Domain XML syntax is as follows:

<domain type='kvm'>
  ...
  <features>
    ...
    <kvm>
      <hidden state='on'/>
    </kvm>
  </features>

CPU mode for host-passthrough syntax

Syntax for cpu mode has to be specific when using domain type qemu

VM start fails with an error

unsupported configuration: CPU mode 'host-passthrough' is only supported with kvm
<domain type='qemu' xmlns:qemu='http://libvirt.org/schemas/domain/qemu/1.0'>

when cpu mode is specified as :

<cpu mode='host-passthrough'></cpu>

Have to specify as

<cpu>
  <mode>host-passthrough</mode>
</cpu>

Fix for rombar errors

VM start fails with error

pci-assign: Cannot read from host /sys/bus/pci/devices/0000:02:00.0/rom
Device option ROM contents are probably invalid (check dmesg).
Skip option ROM probe with rombar=0, or load from file with romfile=

To fix add in xml file

<hostdev ...>
  ...
  <rom bar='off'/>
</hostdev>