[This is a repost of an earlier patch now that I've had time to finish it and 
test it more thoroughly.]

This patch adds support for sending a SIGBUS to a userspace application 
using /proc/bus/pci to drive a device if an I/O error occurs.  We're using 
this in house for the X server's BIOS emulator and it seems to be working 
well.

The idea is to track mmaped /proc/bus/pci regions so that the machine check 
handler is able to properly determine which process is responsible for any 
faults that occur (ia64 is interesting in that the error may not occur in the 
process context that actually generated the bad reference).  If a match is 
found, a SIGBUS is sent to the process, along with the address that caused 
the fault.  The machine check record is then cleared and recovery takes place 
(the assumption is that the signal to userspace is a sufficient record of the 
error).

The patch also special cases memory mapping of legacy space, which is the 
first 64k of I/O space and the first megabyte of memory space.  Sub platforms 
can optionally remap their bridge to the target bus and setup legacy handling 
in the callout.

Comments?  Given that this is working well for us, I'd like to get it upstream 
sometime soon.  I also expect that it could be used to deal with more types 
of I/O errors, perhaps allowing the kernel to call a driver shutdown routine 
if an I/O error occurs in a kernel driver.

Thanks,
Jesse