Re: [libvirt PATCH 12/12] conf/qemu: new <driver> attribute "useBackupMAC"

Current virtio-net drivers that support the failover feature match up the virtio backup device with its corresponding hostdev device by looking for an interface with a matching MAC address. Since libvirt will assign a different random MAC address to each interface that isn't given an explicit MAC address, this means that the configuration for any failover pairs must include explicit matching MAC addresses. To make life easier, we could have libvirt populate the XML config with the same auto-generated MAC address for both interfaces when it detects a failover pair that have no MAC addresses provided (a failover pair can be detected by matching <alias name='blah'/> of the virtio interface with <driver backupAlias='blah'/> of the hostdev interface), but then we would be stuck with that behavior even if the virtio guest driver later eliminated the requirement that mac addresses match. Additionally, some management software uses the MAC address as the primary index for its list of network devices, and can't properly deal with two interfaces having the same MAC address (oVirt). Even libvirt's own virsh utility uses MAC address (combined with interface type) to uniquely identify interfaces for the virsh detach-interface command (in this case, fortunately the runtime interface type is used, so one of the interfaces will always be of type='hostdev' and the other type='something-else", so it doesn't currently cause any problem). In order to remove the necessity of explicitly setting interface MAC addresses, as well as permit the two interfaces of a failover pair to each have a unique index while still fulfilling the current guest driver requirement that the MAC addresses matchin the guest, this patch adds a new attribute "useBackupMAC" that is set on the hostdev interface of the pair. When useBackupMAC='yes', the setup for the hostdev interface will find the virtio failover interface (using backupAlias) and use that interface's MAC address to initialize the MAC address of the hostdev interface; the MAC address in the hostdev interface config remains unchanged, it just isnt used for device initialization. I made this patch to followup on https://bugzilla.redhat.com/show_bug.cgi?id=1693587#c12 (where I suggested this attribute as a possible remedy to oVirt's requirement that each network device have a unique MAC address). Truthfully, I'm not convinced that I want it though, as it seems "a bit" hackish. In particular, I've thought for a long time that the "hostdev manager" code in util/virhostdev.c should really live in the node device driver and be called from the hypervisors via a public API (so that there is one central place on the host that maintains the list of in-use PCI devices and their status), but this patch adds an obstacle to that goal by adding a virDomainDefPtr to more of the APIs in that library - if this was turned into a public API, then entire virDomainDef would need to be serialized and sent in the API call, then parsed at the other end - yuck :-/. NB: there are already functions in virhostdev.h that take a virDomainDefPtr, so maybe I'm being too sensitive. On the upside, it solves a problem, and default bevahior is unchanged.