No, I'm not doing this for fun, this is part of work. It turned out that nfsroot is a bit of a problem while you're hacking the core network stack (and everything breaks all the time). So I now attached an 18GB UW-SCSI disk to an old aic7xxx controller and plugged this into my ARM development board. Seems to work quite fine, as long as the aic7xxx_old driver is used. The new one apparently calls pci_alloc_consistent from interrupt context ?!?.