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Description of problem:
I have two virtual QEMU/KVM virtual machines on a Fedora 7 x86_64 machine.
Both are running "hardware-accelerated ?" (QEMU + KVM).  The 'virbr0'
virtual network is started by default by libvirtd et. al. (?) on
machine startup.

I can run both machines separately, i.e., one at a time, but cannot
run both together.  Whenever trying to start the second virtual
machine I get a particular error which I'll describe later.

One virtual machine is Fedora Core 7 running x86_64 (QEMU + KVM).
The other virtual machine is Windows XP Pro SP 2 (SP 2 from install,
not applied post-install), running on i686 (QEMU + KVM, I'm supposing
hardware KVM works on x86_64 for i686 since the virtual machine runs
fine in i686 other than this networking issue).


Version-Release number of selected component (if applicable):
qemu-0.9.0-2.fc7 libvirt-0.2.3-1.fc7 kvm-24-1 (libvirt-python-0.2.3-1.fc7 ??)

How reproducible:
Always


Steps to Reproduce:
[1] Boot machine into *.fc7 (not *.fc7xen) kernel, along with
    'qemu' and 'libvirtd' daemons started at beginning

[2] virtual network 'virbr0' started automatically, config file
    shown below

[3] start either virtual machine 'BogatzaFedora7M01' or
    'BogatzaWinXpProM02', both are configured to attach to
    'virbr0' (this step succeeds) -- NOTE:  I used virt-manager
    to create/install both virtual machines, and also to
    start/stop the virtual machines

[4] while first virtual machine is one, try to start the
    other virtual machine, which also attaches to 'virbr0'
    virtual network (start from virt-manager) --
    THIS STEP FAILS WITH ERROR SHOW IN THE "Actual Results"
    section


Actual Results:
In every case where I try to start second virtual machine I get
a virt-manager error dialog box exposing an error seen at the Python
level (must be virt-manager impl lang?).  The dialog box has as
heading:

  virDomainCreate() failed internal error Failed to add tap
  interface 'vnet0' to bridge 'virbr0':  Device or resource busy

... there is a Details "button" allowing one to see more error
details, the details read:

Unable to start virtual machine '<class 'libvirt.libvirtError'> virDomainCreate() failed internal error Failed to add tap interface 'vnet0' to bridge
'virbr0' : Device or resource busy
Traceback (most recent call last):
  File "/usr/share/virt-manager/virtManager/console.py", line 348, in control_vm_run
    self.vm.startup()
  File "/usr/share/virt-manager/virtManager/domain.py", line 361, in startup
    self.vm.create()
  File "/usr/lib64/python2.5/site-packages/libvirt.py", line 217, in create
    if ret == -1: raise libvirtError ('virDomainCreate() failed', dom=self)
libvirtError: virDomainCreate() failed internal error Failed to add tap interface 'vnet0' to bridge 'virbr0' : Device or resource busy
'

There is no workaround that I can see since it's necessary and
essential to attach multiple machines to the same virtual
network.

Expected Results:
virt-manager/libvirt, etc., should start second virtual machine without
complaining and without problem.

(And I assume the 'veth0' is a virtual-machine-visible interface
name, and that the 'veth0' apparently started by the two machines
should not conflict in namespace with each other.  In any case,
I'll probably try to state an explicit 'veth1' for the Windows XP
virtual machine definition, because (a) it's missing; and (b) it
might resolve the problem by avoiding namespace/perceived-device
clash.)

(I might also try creating a second virtual network 'virbr1' and
attaching the two machines to independent networks or to the
second network and see what happens there.)

Additional info:

--- DIRECTORY LISTING FOR:  /etc/libvirt/qemu (config dir for QEMU virt env)

.:
BogatzaFedora7m01.xml
BogatzaWinXpProM02.xml
networks

./networks:
autostart
default.xml

./networks/autostart:
default.xml

--- config for Fedora 7 virt machine, untouched/unedited, as created/maintained
    by virt-manager

<domain type='kvm'>
  <name>BogatzaFedora7m01</name>
  <uuid>b1b4fa10-927b-fde1-c6de-eabfbbd8ecf0</uuid>
  <memory>1024000</memory>
  <currentMemory>1024000</currentMemory>
  <vcpu>1</vcpu>
  <os>
    <type>hvm</type>
    <boot dev='hd'/>
  </os>
  <features>
    <acpi/>
  </features>
  <on_poweroff>destroy</on_poweroff>
  <on_reboot>restart</on_reboot>
  <on_crash>destroy</on_crash>
  <devices>
    <emulator>/usr/bin/qemu-kvm</emulator>
    <disk type='block' device='disk'>
      <source dev='/dev/NoRaidVolGroupB/BogatzaFedora7m01_RootFs'/>
      <target dev='hda'/>
    </disk>
    <interface type='network'>
      <mac address='00:16:3e:56:4b:5a'/>
      <source network='default'/>
      <target dev='vnet0'/>
    </interface>
    <graphics type='vnc' port='-1'/>
  </devices>
</domain>

--- config for in XP virt machine, untouched/unedited, as created/maintained
    by virt-manager

<domain type='kvm'>
  <name>BogatzaWinXpProM02</name>
  <uuid>2414b75d-bee1-b91d-bb64-93f3f318edeb</uuid>
  <memory>512000</memory>
  <currentMemory>512000</currentMemory>
  <vcpu>1</vcpu>
  <os>
    <type>hvm</type>
    <boot dev='hd'/>
  </os>
  <features>
    <acpi/>
  </features>
  <on_poweroff>destroy</on_poweroff>
  <on_reboot>restart</on_reboot>
  <on_crash>destroy</on_crash>
  <devices>
    <emulator>/usr/bin/qemu-kvm</emulator>
    <disk type='block' device='disk'>
      <source dev='/dev/NoRaidVolGroupB/BogatzaWinXpProM02_RootFs'/>
      <target dev='hda'/>
    </disk>
    <interface type='network'>
      <mac address='00:16:3e:63:42:f4'/>
      <source network='default'/>
    </interface>
    <graphics type='vnc' port='-1'/>
  </devices>
</domain>

--- config for 'default' network 'virbr0', edited only to change the
    DHCP range (and the problem occurred before ever changing this
    DHCP range by hand and re-booting)

<network>
  <name>default</name>
  <uuid>d83670ee-a32a-4d07-9939-e83071852405</uuid>
  <bridge name="virbr0" />
  <forward/>
  <ip address="192.168.122.1" netmask="255.255.255.0">
    <dhcp>
      <range start="192.168.122.100" end="192.168.122.110" />
    </dhcp>
  </ip>
</network>