[Wg-openstack] tap device name for emulated NIC too long

[Jim Fehlig <jfehlig@xxxxxxxx>]

Hi All,

During the last Wg-openstack meetup we briefly discussed a long-standing bug 
when using Xen+libvirt+OpenStack with Neutron networking
https://bugs.launchpad.net/nova/+bug/1450465

The bug was also discussed on this list with no resolution

https://lists.xenproject.org/archives/html/xen-devel/2015-06/msg04116.html

To summarize: the tap device name for an emulated NIC is too long after libxl 
appends '-emu' to the name provided by Neutron. Some proposed fixes include
1. Shorten '-emu' to just '-e', avoiding IFNAMSIZ limit. But users are free to 
provide a name that already occupies the full IFNAMSIZ. Also, the user-provided 
name may be used in rules, filters, etc. elsewhere in the network, so modifying 
it at all seems questionable.
2. Change OpenStack to not exceed IFNAMSIZ-4 when specifying Xen vif name. This 
could be proposed to the Neutron devs, but IMO adding such Xen-specific hacks in 
OpenStack is undesirable.
3. Change the Xen default vif type from 'ioemu' to 'vif' (see 
docs/misc/xl-network-configuration.markdown), which avoids creating an emulated 
device. (Note: such a change could be made in Xen or libvirt.) But I think this 
is a no-go. I'd suspect it would result in a lot of broken configurations. E.g. 
a guest may not have PV drivers and is relying on the emulated device. Or the 
guest may be configured to network boot, in which case the emulated device would 
be needed for PXE [0].
We (the Wg-openstack folks) would like to hear your opinions on these proposals, 
or alternatives for fixing this bug.
Regards,
Jim

[0] iPXE claims support for Xen netfront devices, but I've not yet got it to 
work: http://lists.ipxe.org/pipermail/ipxe-devel/2014-July/003674.html
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