[libvirt] Problem with virInterfaceCreate(), IFF_UP, and NetworkManager

Due to "a checkered past" (a myriad of minor issues, changing over time), libvirt's semi-official position on the virInterface*() APIs and NetworkManager is that virInterface*() is only supported if NM is disabled. We do still attempt to make it work as well as possible, but normally I only test those APIs on systems that have NM disabled and use the network service (RHEL/Fedora/CentOS systems here) instead. On a seemingly unrelated note, a few months ago mprivozn pushed a patch that makes it an error to call virInterfaceCreate() (i.e. "ifup") for an interface that is already active. (the "active" state of an interface is determined by looking at an interface's IFF_UP flag (and also IFF_RUNNING, if the interface isn't a bridge device). Previously, this was allowed, as it is common practive to ifup an interface to make new config take effect. Last week, I happened to test the "virsh iface-bridge" command on a system with NM enabled. That command gave an error about the interface being already active, so I tried again, this time ifdowning the interface in advance - I *still* got the error. Further investigation and questioning of NM developers led me to the realization that when NM is enabled, all interfaces *always* have IFF_UP and IFF_RUNNING set, even if they are ifdowned. Further, if NM is active there is no way to determine an interface's "active" status via iotctl() or netlink; instead, must query to determine if NM is active, and if it is you must call a NM API instead (I got this much information from NM developers directly; haven't investigated yet exactly what the API is). NM developers say that this pinning-up of the IFF_UP flag has been done for a long time, and is necessary to do interface auto-config. I think it is violating a long-standing assumption (if not a standard) about the meaning of IFF_UP, and I'm not convinced that it really is a necessity (certainly once a config file is present for an interface, it shouldn't be needed), but then I haven't spent as much time in that problem space as they have. In the meantime, the virInterfaceCreate() API fails 100% of the time on any system that has NM enabled. My dilemma now is whether to attempt to affect change in NM's use of IFF_UP so that it once again can be used as an indicator of whether or not an interface is active, or to just give in and 1) officially declare that virInterface*() isn't supported if NM is enabled until 2) we add code to netcf that detects when NM is active and learns how to query interface status from NM instead of the standard ioctl(SIOCGIFFLAGS). And if the latter is preferred, should we in the meantime perhaps revert the patch that made virInterfaceCreate() an error if the interface was active? Or just leave it completely broken? Any opinions?