README for X11R6.8.2 on OpenBSD : Kernel Support for X
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6. Kernel Support for X

OpenBSD's GENERIC kernels have all support for running X enabled.

6.1. Console drivers

The server supports wscons, the standard OpenBSD/i386 console driver.

6.2. Aperture Driver

By default OpenBSD includes the BSD 4.4 kernel security feature that disables access to the /dev/mem device when in multi-user mode. But the X server requires linear access to the display memory in most cases.

OpenBSD requires the aperture driver to be enabled for all X servers, because the aperture driver also controls access to the I/O ports of the video boards.

To enable the aperture driver, once included in the kernel, set

machdep.allowaperture=2     
in /etc/sysctl.conf. See the xf86(4) manual page for details.

Another (less recommended) way to enable linear memory and I/O ports access is to disable the kernel security feature by initializing securelevel to -1 in /etc/rc.securelevel.

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 multiple virtual terminals, only the first one will have linear memory access. Set securelevel to -1 if you need more that one X server at a time.

6.3. MIT-SHM

OpenBSD supports System V shared memory. If X detects this support in your kernel, it will support the MIT-SHM extension.


README for X11R6.8.2 on OpenBSD : Kernel Support for X
Previous: Running X
Next: Rebuilding the X Distribution