[RFC] Dynamic creation of VFs in a network definition containing an SRIOV device

Context: Libvirt can already detect the active VFs of an SRIOV PF device specified in a network definition and automatically assign these VFs to guests via an <interface> entry referring to that network in the domain definition. This functionality, however, depends on the system administrator having activated in advance the desired number of VFs outside of libvirt (either manually or through system scripts). It would be more convenient if the VFs activation could also be managed inside libvirt so that the whole management of the VF pool is done exclusively by libvirt and in only one place (the network definition) rather than spread in different components of the system. Proposal: We can extend the existing network definition by adding a new tag <vf> as a child of the tag <pf> in order to allow the user to specify how many VFs they wish to have activated for the corresponding SRIOV device when the network is started. That would look like the following: <network> <name>sriov-pool</name> <forward mode='hostdev' managed='yes'> <pf dev='eth1'> <vf num='10'/> </pf> </forward> </network> At xml definition time nothing gets changed on the system, as it is today. When the network is started with 'virth net-start sriov-pool' then libvirt will activate the desired number of VFs as specified in the tag <vf> of the network definition. The operation might require resetting 'sriov_numvfs' to zero first in case the number of VFs currently active differs from the desired value. In order to avoid the situation where the user tries to start the network when a VF is already assigned to a running guest, the implementation will have to ensure all existing VFs of the target PF are not in use, otherwise VFs would be inadvertently hot-unplugged from guests upon network start. In such cases, trying to start the network will then result in an error. Stopping the network with 'virsh net-destroy' will cause all VFs to be removed. Similarly to when starting the network, the implementation will also need to verify for running guests in order to prevent inadvertent hot-unplugging. Is the functionality proposed above desirable? -- Thanks and best regards, Paulo de Rezende Pinatti