On Jul 16, 2008, at 3:57 PM, Kumar Gala wrote:
* 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.
Ok, so I realized that we don't actually need to order the stores
since the sequential programming model will ensure the right things
happens.
- k
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