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

Hi All,

During the last Wg-openstack meetup we briefly discussed a long-standing bug when using Xen+libvirt+OpenStack with Neutron networking


The bug was also discussed on this list with no resolution


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.


[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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