There are several threads hanging out of the "drivers in userspace" tangle.
One of them is filesystems, which are a very logical thing to do in userspace. Peter's work doesn't address that at all. Years ago (back in 2.0) Jeremy Fitzharding (sp?) from Sydney did userfs, which let you do things like mount an ftp site. Some obvious fs candidates such as imapfs involve very long strings and according to (a) people who really know what they are doing and (b) my bumbling experiments, 64k strings are a *really* bad idea in kernel modules.
Another one is virtualisation. Read Jeff Dike on pushing User Mode Linux into kernel space for some clues on where this might be going. Jeff articulates advanced architecture and design better than most, so it is worthwhile looking for his stuff. His idea in this case is to port UML from the current libc interfaces at the backend to kernel interfaces, something like a module. Think of it like IBM's VM with OSs running underneath that, and the kernel of each OS is in true kernel space but within that applications (drivers, in this case) are completely protected.
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