SW TLB MMU rework and SMP issues (pte read/write)

[Kumar Gala]

>> * 64-bit PTEs and reader vs writer hazards.  How do we ensure that  
>> the
>> TLB miss handler samples a consistent view of the pte.  pte_updates
>> seem ok since we only update the flag word.  However set_pte_at seems
>> like it could be problematic.
>
> eieio on the writer and a data dependency on the reader. segher
> suggested a nice way to do it on the reader side, by doing a subf of  
> the
> value from the pointer and then a lwxz using that value as an offset.
>
> ie. something like that, with r3 containing the PTE pointer:
>
> 	lwz	r10,4(r3)
> 	subf	r4,r10,r3  <-- you can use r3,r10,r3 if clobber is safe
> 	lwzx	r11,r10,r4 <-- in which case you use r3 here too
>
> That ensures that the top half is loaded after the bottom half, which
> is what you want if you do the set_pte_at() that way:
>
> 	stw	r11,0(r3)  <-- write top half first
> 	eieio	           <-- maitain order to coherency domain
>        stw	r10,4(r3)  <-- write bottom half last
>
> In fact, in the reader case, while at it, you can interleave that with
> the testing of the present bit. Assuming _PAGE_PRESENT is in the low
> bits and you can clobber r3, you get something like:
>
> 	lwz	r10,4(r3)
> 	<-- can't do much here unless you can do unrelated things -->
> 	andi.	r0,r10,_PAGE_PRESENT
> 	subf	r3,r10,r3
> 	beq	page_fault
> 	lwzx	r11,r10,r3

This makes sense.  I think we need to order the stores in set_pte_at  
regardless of CONFIG_SMP.  Also, I think we should change pte_clear to  
use pte_update() so we only clear the low-order flag bits.  Patch will  
be sent shortly for review.

- k