PV devices with the ioemu attribute set.
sys/dev/xen/netfront/netfront.c:
o If a mac address for the interface cannot be found
in the front-side XenStore tree, look for an entry
in the back-side tree. With ioemu devices, the
emulator does not populate the front side tree and
neither does Xend.
o Return an error rather than panic when an attach
attempt fails.
Reported by: Janne Snabb (fix inspired by patch provided)
PR: kern/154302
Approved by: re
ffa06904b5