Fast User Switching, a feature of Windows XP and later, enables multiple users to be logged onto the same machine. A particular user's desktop and any running applications persist from that user's logon session to his or her next.
Fast User Switching works by allowing multiple virtual display drivers to run at the same time. (Each virtual display driver is associated with a particular PDEV.) The video miniport driver, however, exists as a single instance. When one of the virtual display drivers calls a video miniport driver callback, serious problems ensue if the miniport driver attempts to access a passed-in memory address in the context of the display driver when that display driver instance is no longer the active kernel thread. A tenet of the display driver/video miniport driver architecture is that information should flow in one direction only: from the display driver to the video miniport driver.