[libvirt] opening tap devices that are created in a container

Hi, Opening tap devices, such as macvtap, that are created in containers is problematic because the interface for opening tap devices is via /dev/tapNN and devtmpfs is not typically mounted inside a container as its not namespace aware. It is possible to do a mknod() in the container, once the tap devices are created, however, since the tap devices are created dynamically its not possible to apriori allow access to certain major/minor numbers, since we don't know what these are going to be. In addition, its desirable to not allow the mknod capability in containers. This behavior, I think is somewhat inconsistent with the tuntap driver where one can create tuntap devices inside a container by first opening /dev/net/tun and then using them by supplying the tuntap device name via the ioctl(TUNSETIFF). And since TUNSETIFF validates the network namespace, one is limited to opening network devices that belong to your current network namespace. Here are some options to this issue, that I wanted to get feedback about, and just wondering if anybody else has run into this. 1) Don't create the tap device, such as macvtap in the container. Instead, create the tap device outside of the container and then move it into the desired container network namespace. In addition, do a mknod() for the corresponding /dev/tapNN device from outside the container before doing chroot(). This solution still doesn't allow tap devices to be created inside the container. Thus, in the case of kubevirt, which runs libvirtd inside of a container, it would mean changing libvirtd to open existing tap devices (as opposed to the current behavior of creating new ones). This would not require any kernel changes, but as mentioned seems inconsistent with the tuntap interface. 2) Add a new kernel interface for tap devices similar to how /dev/net/tun currently works. It might be nice to use TUNSETIFF for tap devices, but because tap devices have different fops they can't be easily switched after open(). So the suggestion is a new ioctl (TUNGETFDBYNAME?), where the tap device name is supplied and a new fd (distinct from the fd returned by the open of /dev/net/tun) is returned as an output field as part of the new ioctl parameter. It may not make sense to have this new ioctl call for /dev/net/tun since its really about opening a tap device, so it may make sense to introduce it as part of a new device, such as /dev/net/tap. This new ioctl could be used for macvtap and ipvtap (or any tap device). I think it might also improve performance for tuntap devices themselves, if they are opened this way since currently all tun operations such as read() and write() take a reference count on the underlying tuntap device, since it can be changed via TUNSETIFF. I tested this interface out, so I can provide the kernel changes if that's helpful for clarification. Thanks, -Jason