Just my two cents, but I think this is a case of people not seeing the
forest for the trees.

What you need to do when you change anything in a PTE is get the cached copy
of that PTE out of the TLB.  That is what the tlbie instruction does.  So
you must interpret the notation "tlbie(PTE)" as meaning call tlbie with
paramaters such that the referenced PTE is cleared out of the TLB.

Gregory


> -----Original Message-----
> From: Chuck Silvers [mailto:chuq@chuq.com]
> Sent: Wednesday, November 07, 2001 11:09 PM
> To: Matt Thomas
> Cc: Markus W Kilbinger; port-macppc@netbsd.org; matt@netbsd.org
> Subject: Re: Are there any known problems with uvm or pmap in -current
> as of about Oct. 16?
> 
> 
> On Wed, Nov 07, 2001 at 10:21:20AM -0800, Matt Thomas wrote:
> > At 09:10 AM 11/7/2001 -0800, Chuck Silvers wrote:
> > >ah, I was wrong, it is a pmap bug.  pmap_pte_clear() is calling
> > >TLBIE() on the address that contains the PTE rather than 
> the address
> > >that the PTE is mapping.  the attached diff fixes the problem.
> > 
> > That's intentional.  When only updating the R/C bits of the PTE,
> > the PPC 32-bit Programming Environment Manual.
> > 
> > >I don't know what document that reference about "As shown 
> in Section 
> > >7.6.3.2.2"
> > >comes from, is that document wrong or did you just 
> mis-read it, matt?
> > 
> > I doubt it's wrong.  Hmmm.  that sequence is only value for the REF
> > bit, not the CHG bit.  I think I need to think about this more.
> 
> executing tblie on the address of the PTE doesn't make any sense...
> we don't even use the TLB to access the PTEs, we use the BATs, right?
> in all the other examples in those pages of the manual they use
> "tlbie(old_EA)", why would this one be different?
> 
> 
> > Reading a bit closer, I think I need some lwarx/stwcx. instructions
> > when directly manipulating the PPC PTEs in order to garuantee
> > consistency of the REF|CHG bits as I read them:
> > 
> > To modify the WIMG or PP bits without overwriting an R or C 
> bit update 
> > being performed
> > by the processor, a sequence similar to the one shown above 
> can be used, 
> > except that the
> > second line is replaced by a loop containing an 
> lwarx/stwcx. instruction 
> > pair that emulates
> > an atomic compare and swap of the low-order word of the PTE.
> 
> that's in the "Modifying the Virtual Address" section, which uses
> "tlbie(old_EA)" instead of "tlbie(PTE)".  also, it doesn't say that
> you can use sequence to clear the R or C bits.
> 
> overall I think we're a lot safer if we use the algorithm 
> from the old pmap
> for updating the C bit.  if you want to play around with the 
> R bit that's
> fine, but we really need the C bit to be handled correctly.
> 
> -Chuck
>