For the QEMU / KVM driver, if the user doesn't specify an explicit name for
the TAP device associated with a virtual NIC, we auto-generate one with a
name vnetXXX. You can see this if you dump XML for a running QEMU guest.
Unfortunately if you dump XML, make a change and then feed it back in with
the define XML API, you have now persisted this auto-generated VIF name.
Do this for several domains at varying times and you'll eventually get
2 domains which have persisted the same auto-generated vnetXXX device
name. You can now not start both of these VMs at once.
The fix for this is simple - simply strip any TAP device name starting with
the string 'vnet' when defining a new VM. It will thus get assigned a new
automatically generate name which doesn't clash. The patch also strips out
hardcoded vnetXXX names when starting a VM to proactively deal with any
existing VMs whose config has been broken in this way.
Dan.
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