Solved 4.2BSD panic trap 9 problem on VAX 11/785

[cbush at RAND-UNIX.ARPA]

 "WARNING probe instructions may be hazardous to the health of your system."

>>>THANKS:

This is a thanks to all on quickly solving are problem.  No new crashes YET!!
Got 6 replies which all hit it on the nose or very close to it.

Restating the problem and solution may fill in the story for some.  I'm
somewhat suprised that at the very least a warning comment in not included
in locore.s if the bug has been know so so long.


>>>PROBLEM:

Need help with persistent, at least one a day, system crashes on our
new VAX 11/785 running 4.2BSD UNIX.  Always get the same panic messages;

	trap type 9, code = 80001400, pc = 80001400
	panic: Protection fault

My reading of the above and from looking at the kernal stack frames says,
in summary, the system was attempting to execute the instruction at location
80001400 while in user mode!!

>>>SOLUTION: ( Courtesy of Chris Torek <chris at maryland> )

    Sounds like you've managed to invoke the 780/785 CPU bug with prefetch.
    The probe instruction works by changing the CPU microstate to user mode.
    If a prefetch crosses a page boundary you can get a trap.

    The "fix" is to insert enough noops to push the probe ... in locore.s ...
    down past the start of a new page.  If that works, let us all know . .

I'm still puzzeled by the difference in rate of occurence on the 785
(~once/day) as compared to 780 (~once a month) but am leaving it as an
exercise for now.