Understanding Linux Network Internals

Virtual Devices

In the section "Virtual Devices" in Chapter 5, we saw how virtual devices differ from real ones with regard to initialization. As far as registration is concerned, virtual devices need to be registered and enabled just like real ones, to be used. However, there are differences:

• Virtual devices sometimes call register_netdevice and unregister_netdevice rather than their wrappers, and take care of locking by themselves. They may need to handle locking to keep the lock for a little longer than a real device does. With this approach, the lock could also be misused and hold longer than needed, by making it protect additional pieces of code (besides register_netdev) that could be protected in other ways.

• Real devices cannot be unregistered (i.e., destroyed) with user commands; they can only be disabled. Real devices are unregistered at the time their drivers are unloaded (when loaded as modules, of course). Virtual devices, in contrast, may be created and unregistered with user commands, too. Whether this is possible depends on the virtual device driver's design.

We also saw in the sections "register_netdevice Function" and "Device Unregistration" that virtual devices, unlike most real ones, use dev->init, dev->uninit, and dev->destructor. Because most virtual devices implement some kind of more or less complex logic on top of real devices, they use dev->init and dev->uninit to take care of extra initialization and cleanup. dev->destructor is often initialized to free_netdev ...