> > > A process running at ring 0 can still disable interrupts, DMA into
> > > kernel memory, etc...
> >
> > But the Xserver isn't running in the kernel mode.
>
> I may have missed something, but the difference between iopl 3 and "kernel
> mode" is...?
Hmm... It's enormous. The ring-0 code can do anything to your machine
including memory management, LDT/GDT/IDT management, virtual memory management
- everything your CPU and your hardware is capable of. The IOPL 3 merely
(yeah...) allows the application to output to any of the ports of your
hardware. That can crash the system, but usually the application are not
allowed to access ALL ports - the IOPL at the end of the TSS is used to
enable/disable access to the hardware ports. So the graphics application is
allowed to trash only the video card registers - that can only hurt your
display (unless your video card is buggy and freezes the bus ;-))
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