I wrote:
 > During the /etc/daily run, with lots of "find"-ing going on, the same
 > DMA errors came back.  Back to the drawing board ... will try disabling
 > tagged queuing next.

 (Last message everyone, promise)

 Built a new kernel with the previous siop changes and tagged queuing disabled.
 While trying to make a whole new kernel (changed config file), it happened
 again:

 DMA IRQ: bus fault dma fifo empty, DSP=0x3b0 DSA=0xc006c7e0: last msg_in=0x0 status=0xff
 siop0: scsi bus reset
 cmd 0x1a3e340 (target 0:0) in reset list
 cmd 0x1a3e340 (status 2) about to be processed
 siop0: target 0 not synchronous at 20.0Mhz, offset 16
 DMA IRQ: bus fault, DSP=0x3e0 DSA=0xc006c8dc: last msg_in=0x4 status=0xff

 It then hung here for some time, finally followed by a

 sd0(siop0:0:0): command timeout

 And now it's hung completely.  I'm going back to my stable 1.5-based kernel ...

Grab the latest version of iommu.c  It fixes a dvma bug.

Eduado