IA64 nested_dtlb handler fix up comments

Add some comments that hopefully help to understand the code better

Signed-off-by: Christoph Lameter <clameter@sgi.com>

Index: linux-2.6.18-mm3/arch/ia64/kernel/ivt.S
===================================================================
--- linux-2.6.18-mm3.orig/arch/ia64/kernel/ivt.S	2006-10-08 17:59:23.652378637 -0700
+++ linux-2.6.18-mm3/arch/ia64/kernel/ivt.S	2006-10-08 18:15:26.611662500 -0700
@@ -437,30 +437,30 @@ ENTRY(nested_dtlb_miss)
 	 */
 	rsm psr.dt				// switch to using physical data addressing
 	mov r19=IA64_KR(PT_BASE)		// get the page table base address
-	shl r21=r16,3				// shift bit 60 into sign bit
+	shl r21=r16,3				// Clear region bits
 	mov r18=cr.itir
 	;;
 	shr.u r17=r16,61			// get the region number into r17
 	extr.u r18=r18,2,6			// get the faulting page size
 	;;
 	cmp.eq p6,p7=5,r17			// is faulting address in region 5?
-	add r22=-PAGE_SHIFT,r18			// adjustment for hugetlb address
-	add r18=PGDIR_SHIFT-PAGE_SHIFT+3,r18
+	add r22=-PAGE_SHIFT,r18			// page order
+	add r18=PGDIR_SHIFT-PAGE_SHIFT+3,r18	// Shift to get to PGD pointer number.
 	;;
-	shr.u r22=r16,r22
-	shr.u r18=r21,r18
+	shr.u r22=r16,r22			// address >> page order
+	shr.u r18=r21,r18			// PGD pointer number
 (p7)	dep r17=r17,r19,(PAGE_SHIFT-3),3	// put region number bits in place
 
 	srlz.d
 	LOAD_PHYSICAL(p6, r19, swapper_pg_dir)	// region 5 is rooted at swapper_pg_dir
 
 	.pred.rel "mutex", p6, p7
-(p6)	shr.u r21=r21,PGDIR_SHIFT+PAGE_SHIFT
-(p7)	shr.u r21=r21,PGDIR_SHIFT+PAGE_SHIFT-3
+(p6)	shr.u r21=r21,PGDIR_SHIFT+PAGE_SHIFT	// Kernel can use full size
+(p7)	shr.u r21=r21,PGDIR_SHIFT+PAGE_SHIFT-3	// User space requires region bits
 	;;
 (p6)	dep r17=r18,r19,3,(PAGE_SHIFT-3)	// r17=pgd_offset for region 5
 (p7)	dep r17=r18,r17,3,(PAGE_SHIFT-6)	// r17=pgd_offset for region[0-4]
-	cmp.eq p7,p6=0,r21			// unused address bits all zeroes?
+	cmp.eq p7,p6=r0,r21			// unused address bits all zeroes?
 #ifdef CONFIG_PGTABLE_4
 	shr.u r18=r22,PUD_SHIFT			// shift pud index into position
 #else