Re: [PATCH 3/6] 8xx: invalidate non present TLBs

On Thu, 2009-10-08 at 17:36 -0700, Dan Malek wrote:

On Oct 8, 2009, at 3:23 PM, Benjamin Herrenschmidt wrote:

quoted

<<
• Reference and change bit updates—The MPC850 does not generate an  
exception for
  an R (reference) bit update. In fact, there is no entry for an R  
bit in the TLB.
  The change bit (C) is bit 23 in the level-two descriptor,  
described in Table 8-4.
  Software updates C (changed) bits, but hardware treats the C bit  
(negated) as a
  write-protect attribute. Therefore, attempting to write to a page  
marked unmodified
  invalidates that entry and causes an implementation-specific DTLB  
error exception.
  ^^^^^^^^^^^^^^^^^^^^^^
  If change bits are not needed, set the C bit to one by default in  
the PTEs.

How interesting....

I've looked at many 8xx docs and they all have the same text
(probably cut/paste :-))  I'd place some debug code in the C functions
to print out a few of the TLB Entry for various errors to see if this  
really
happens, and for other errors, too.  I guess I never stumbled into
this because I always thought I had to do everything from software,
so just made sure I did.

I'm not sure it's worth bothering :-) I'm happy to continue assuming we
need to tlbie and always make sure we do so.

Cheers,
Ben.