[PATCH 2/6] 8xx: get rid of _PAGE_HWWRITE dependency in MMU.

[Joakim Tjernlund]

Benjamin Herrenschmidt <benh at kernel.crashing.org> wrote on 08/10/2009 00:20:17:

>
> On Thu, 2009-10-08 at 00:08 +0200, Joakim Tjernlund wrote:
> >
> > Benjamin Herrenschmidt <benh at kernel.crashing.org> wrote on 07/10/2009 23:14:52:
> > >
> > > On Wed, 2009-10-07 at 22:46 +0200, Joakim Tjernlund wrote:
> > >
> > > > +   andi.   r11, r10, _PAGE_USER | _PAGE_ACCESSED
> > > > +   cmpwi   cr0, r11, _PAGE_USER | _PAGE_ACCESSED
> > > > +   bne-   cr0, 2f
> > >
> > > Did you mean _PAGE_PRESENT | _PAGE_ACCESSED ?

YES! cut and paste error, will send a new much improved patch
with my new idea.

> > >
> > > > +2:
> > > > +   mfspr   r11, SRR1
> > > > +   rlwinm   r11, r11, 0, 5, 3 /* clear guarded */
> > > > +   mtspr   SRR1, r11
> > >
> > > What is the above for ?
> >
> > TLB Miss will set that bit unconditionally and that is
> > the same bit as protection error in TLB error.
>
> And ? Big deal :-) IE. Once you get to InstructionAccess, it doesn't
> matter if that bit is set, does it ?

Yes it does. If one adds HWEXEC it will fail, right?
Also this count as a read and you could easily end up
in the protection case(in 2.4 you do)

>
> > Lets start simple, shall we? :)
> > Anyhow, I looked some more at that and I don't the best thing is
> > to use shifts. All bits are correct if you invert RW and add an exception
> > for extended coding.
>
> Right, as long as you avoid doing a conditional branch :-)

hey, I think you have to show how then :) I am not
good at ppc shift, mask, rotate insn.

>
> > Because if you go to C with a protection fault, you are in trouble.
>
> Why ?

In 2.4 you end up in read protection fault an get a SEGV back :)

>
> > So deal with it here. Now, I got another idea too that will make this go away
> > if it work out
>
> I don't understand your point about protection faults.
>
> You should be able to go straight to C with -anything-, that's what I do
> for all other platforms.

Well, you don't force a tlb error like I do, however my new version
handles this better.

Now I only handle DIRTY and the rest in C. Figured it is
much faster and really simplie now, stay tuned.