README for XFree86 3.3.2 on OpenBSD : Kernel Support for X : Aperture Driver
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## 8.1. Aperture Driver

By default OpenBSD includes the BSD 4.4 kernel security feature that disable access to the `/dev/mem` device when in multi-users mode. But XFree86 servers can take advantage (or require) linear access to the display memory.

The P9000, Mach64 and AGX servers require linear memory access, other accelerated servers can take advantage of it, but do not require it. Some drivers in the SVGA server require linear memory access too, notably the Matrox driver.

There are two ways to allow XFree86 to access linear memory:

1. Disable the kernel security feature by adding `option INSECURE' in the kernel configuration file and build a new kernel. In OpenBSD 2.2 and later, you will also need to comment out the line initializing `securelevel` to 1 in `/etc/rc.securelevel`.
2. Install the aperture driver:
   1. The first step is highly dependent from your exact operating system version:
      - OpenBSD 2.0

        Use the aperture driver from /usr/lkm:

        add the following lines to the end of `/etc/rc.local`:

        > `

        >      KERNDIR=/usr/lkm
        >      if [ -f ${KERNDIR}/ap.o ]; then
        >        modload -o ${KERNDIR}/ap -e ap -p ${KERNDIR}/apinstall ${KERNDIR}/ap.o
        >      fi

        > `

      - OpenBSD 2.1, 2.2

        Uncomment the lines loading the aperture driver from `/etc/rc.securelevel`

      - OpenBSD-current

        In addition to the loadable kernel module, you can now use an in-kernel aperture driver. Add 'option APERTURE' to your kernel configuration file, build and install the new kernel and run `./MAKEDEV std` in `/dev`. Edit `/etc/sysctl.conf` to set the variable machdep.allowaperture to 1.

   2. Reboot your system. XFree86 will auto-detect the aperture driver if available.
   Warning: if you boot another kernel than `/bsd`, loadable kernel modules can crash your system. Always boot in single user mode when you want to run another kernel.

   > Caveat: the aperture driver only allows one access at a time (so that the system is in the same security state once X is launched). This means that if you run multiple servers on multiples VT, only the first one will have linear memory access. Use 'option INSECURE' if you need more that one X server at a time.

README for XFree86 3.3.2 on OpenBSD : Kernel Support for X : Aperture Driver
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Next: MIT-SHM