[PATCH] 8xx: avoid "dcbst" misbehaviour with unpopulated TLB

The proposed _tlbie call at update_mmu_cache() is safe because:

Addresses for which update_mmu_cache() gets invocated are never inside the
static kernel virtual mapping, meaning that there is no risk for the
_tlbie() here to be thrashing the pinned entry, as Dan suspected.

The intermediate TLB state in which this bug can be triggered is not
visible by userspace or any other contexts, except the page fault handling
path.  So there is no need to worry about userspace dcbxxx users.

The other solution to this is to avoid dcbst misbehaviour in the first
place, which involves changing in-kernel "dcbst" callers to use 8xx
specific SPR's.

Summary:

On 8xx, cache control instructions (particularly "dcbst" from
flush_dcache_icache) fault as write operation if there is an unpopulated
TLB entry for the address in question.  To workaround that, we invalidate
the TLB here, thus avoiding dcbst misbehaviour.
Signed-off-by: Marcelo Tosatti <marcelo.tosatti@cyclades.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

arch/ppc/mm/init.c

@@ -606,9 +606,19 @@ void update_mmu_cache(struct vm_area_struct *vma, unsigned long address,
 		struct page *page = pfn_to_page(pfn);
 		if (!PageReserved(page)
 		    && !test_bit(PG_arch_1, &page->flags)) {
-			if (vma->vm_mm == current->active_mm)
+			if (vma->vm_mm == current->active_mm) {
+#ifdef CONFIG_8xx
+			/* On 8xx, cache control instructions (particularly 
+		 	 * "dcbst" from flush_dcache_icache) fault as write 
+			 * operation if there is an unpopulated TLB entry 
+			 * for the address in question. To workaround that, 
+			 * we invalidate the TLB here, thus avoiding dcbst 
+			 * misbehaviour.
+			 */
+				_tlbie(address);
+#endif
 				__flush_dcache_icache((void *) address);
-			else
+			} else
 				flush_dcache_icache_page(page);
 			set_bit(PG_arch_1, &page->flags);
 		}