(II) AIGLX: Screen 0 is not DRI2 capable
(II) AIGLX: Screen 0 is not DRI capable

So it had fallen back to a software 3D renderer, which is pretty crap. So to make a long story short, it was because my ‘Virtual’ screen size was too big. I typed xrandr, and got the following:

$ xrandr
Screen 0: minimum 320 x 200, current 1280 x 960, maximum 2048 x 2048

Because of various technical reasons, when the Virtual size is too big (which, evidently, 2048×2048 is), DRI gets disabled. So, to re-enable it, I put this into my xorg.conf:

Section "Screen"
        Identifier "Main Screen"
        Device "Radeon 7500"
        Monitor "eMac CRT"
        SubSection "Display"
                Virtual 1280 960 # put the highest resolution you intend to use here
        EndSubSection
EndSection

Obviously, edit the values to suit.

The setup is very neat, and performance is surprisingly good. For all but the most graphically and processor-intensive applications, the fact two people are logged on to the machine is not even noticeable, from a performance point of view.

Some helpful guides on setting such a system up are:

• Build a Six-headed, Six-user Linux System
• MultiseatX — Community Ubuntu Documentation
• Multiseat X Under X11R6.9/7.0

However, there are a few gotchas when setting it up. Hopefully this list will be of help.

• Multiseat is very buggy. Test one X server at a time (by manually launching it with the right parameters), and do it from a remote computer over an SSH session, because your display will probably get corrupted, making your console temporarily unusable if you make even the slightest mistake.
• If the display is corrupted, and you are restarting X several times, and it doesn’t look like it’s going to work, it’s probably a good idea to reboot (do it safely, by either remotely rebooting, hitting Ctrl+Alt+Delete, or by using the Raising Skinny Elephants trick), because there’s a good chance your problems will be solved after a reboot.
• If you are using NVIDIA cards with the nv or nouveau driver, while I commend you for supporting free software, I have never got those drivers working with multiseat. I recommend running the proprietary driver (and different driver versions have different glitches with multiseat — if you are having problems, try an older — or newer — driver).
• Don’t mix-and-match video cards of different brands. A 3Dfx and NVIDIA card won’t play nicely, for example, but an NVIDIA and NVIDIA card may. Also, there are weird bugs with certain models. For example, my GeForce 7600GT and GeForce 6200 don’t work well together, but the GeForce Go 6150 and GeForce 6200 work fine.
• If you are using an ATI Radeon, then use the Xephyr method. Radeons cannot support the use of the Screen keyword in separate layouts.
• If you are getting issues with the screen scrolling or rotating of sorts (like a sideways badly tuned old-style television), then try adding no-scroll to your kernel boot parameters.
• When launching X.Org (or in your gdm.conf configuration), the -novtswitch and -sharevts options that are described in all multiseat tutorials can sometimes cause more harm than good, and may even be unnecessary. For example, if you use -sharevts, your keystrokes may silently be duplicated to a hidden login prompt which could wreak havoc on your system.
• In the [daemon] section of gdm.conf, set VTAllocation=false to stop GDM from appending its own arguments to the X.Org command line. Specify vt7 (or whatever VT you want X.Org to run on) manually on the X command line — and make sure both X servers are set to the same VT.
• Add Option "GrabDevice" "yes" to your input devices in xorg.conf to fix a bug where the display scrolls randomly or when you press enter.
• Sound is a pain to get working. Make sure PulseAudio is either running system-wide, or make PulseAudio use dmix:0 instead of hw:0 as a device (yes, I know Lennart would cringe, but at least it’s more reliable). If you created a second user to test with, don’t forget to add him/her to the audio group!

Whenever, I load this page in Firefox 3.0.4, which is a perfectly innocuous page on the OSx86 wiki, my X server crashes, just as if I hit Ctrl+Alt+Backspace.

I have an NVIDIA GeForce 7600GT, running on Linux 2.6.27-gentoo, with both the open source nv driver, as well as the proprietary driver version 177.82 (also occurs on 177.80).

Does it happen to anybody else? I strongly suspect a video driver bug, as it doesn’t occur on my Eee 901, which is GMA950–based.