[PATCH 3/6] 8xx: get rid of _PAGE_HWWRITE dependency in MMU.

[Joakim Tjernlund]

Benjamin Herrenschmidt <benh at kernel.crashing.org> wrote on 06/10/2009 02:34:15:
>
> On Tue, 2009-10-06 at 01:35 +0200, Joakim Tjernlund wrote:
> >
> > > Well, if the HW has the ability to enforce trap when store with !
> > DIRTY,
> >
> > Yes, provided that the kernel invalidates the TLB too so the next
> > access
> > will provoke a TLB Miss, which will then provoke a TLB error. The TLB
> > error routine checks VALID, RW and USER(if not a kernel access), then
> > sets
> > ACCESSED & DIRTY and writes the TLB(RPN reg).
> >
> > Perhaps the missing invalidate is haunting us here?
>
> No, the kernel will invalidate when clearing dirty or accessed, I don't
> think that's our problem.
>
> This is still all inefficient, we end up basically with two traps.

Yes, but once the 2 traps is over, it gets much cheaper plus I don't
get a choice, see below.

>
> 8xx provides backup GPRs when doing TLB misses ? What does it cost to
> jump out of a TLB miss back into "normal" context ?

Nope, there is just one TLB scratch register. I have been meaning to
ask you about SPRG2, it seems unused?
There is a leftover from 2.4 that inits G2 to something but the
it appears unused otherwise.

>
> IE. What I do on 440 is I set a mask of required bits, basically
> _PAGE_PRESENT | _PAGE_ACCESSED is the base. The DTLB miss also sticks
> in _PAGE_RW | _PAGE_DIRTY when it's a store fault.

Yes, I would too but TLB Miss knows nothing about load/store, protection etc.
because DSISR isn't set. So I cannot see any other way than the TLB Error way.

>
> Then, I andc. the PTE value off that mask, and if the result is non-0
> (which means one of the required bits is missing), I get out of the TLB
> miss immediately and go to the data (or instruction) access interrupt.
>
> Once you've done that, you should be able to have data and instruction
> access go straight to C. Missing _PAGE_ACCESSED and _PAGE_DIRTY are
> going to be fixed up by generic code.
>
> > > then that's fine, just map it that way, but you shouldn't have to
> > handle
> > > it in the DTLB error neither, the kernel will fix it up for you in
> > > handle_pte_fault().
> >
> > Does not all ppc have the Changed bit?
>
> No. BookE doesn't.

But I guess BookE knows if it is a load or store in TLB Miss?
Then it can emulate changed bit I guess.