This module was originally written to help work around the security
feature of NetBSD that prevents read/write access to /dev/mem. It also
provide Pentium II and AMD K6 MTRR control in order to control
write-combining to the framebuffer memory.

XFree86 can take advantage of having direct access to video
memory (especially with VLB and PCI cards) and even requires it for
the most recent PCI or AGP video boards.

This driver works like the standard /dev/mem driver. It just allows
mapping of the VGA framebuffer and BIOS even if kernel security level
is > 0.  The driver only implements the open(), close(), ioctl() and
mmap()  calls. In order not to defeat kernel security, only one open()
at a  time is allowed and only a process with effective user id of 0 can
perform it. So while you're running XFree86, no other process will be
allowed to open /dev/xf86.