o In revision 1.45 of exception.S we changed exception_restore to

unconditionally restore ar.k7 (kernel memory stack) and ar.k6
   (kernel register stack). I don't know what I was smoking then,
   but if you unconditionally restore ar.k6, you also want to
   compute its value unconditionally. By having the computation
   predicated and dependent on whether we return to user mode, we
   would end up writing junk (= invalid value for ar.bspstore) if
   we would return to kernel mode. But the whole point of the
   unconditional restoration was that there is a grey area where
   we still need to have ar.k6 restored. If we restore with a junk
   value, we would end up wedging the machine on the next interrupt.
   So, unconditionally calculate the value we unconditionally write
   to ar.k6.

o  The previous braino was found while making the following change:
   We used to clear the lower 9 bits of the value we write to ar.k6.
   The meaning being that we know that the kernel register stack is
   at least 512 byte aligned and simply clearing the lower 9 bits
   allows us to return to a context of which we don't have dirty
   registers on the kernel stack, even though the context that
   entered the kernel does have dirty registers on the kernel stack.
   By masking-off the lower bits, we correctly obtain the base of
   the register stack without having to worry that we didn't actually
   reached the base while unwinding it.
   The change is to mask off the lower 13 bits, knowing that the
   kernel register stack is always 8KB aligned. The advantage is that
   we don't have to worry anymore if there's more than 512 bytes of
   dirty registers on the kernel stack. A situation that frequently
   occurs. In exec_setregs() in machdep.c:1.147 or older, we had to
   deal with that situation by copying the active portion of the
   register stack down in multiples of 512 bytes. Now that we mask off
   the lower 13 bits we don't have to do that at all. Contemporary
   IPF processors have a register file that can hold up to 96 stacked
   registers (=784 bytes [incl. 2 NaT collections]). With no indication
   that register files grow beyond a couple of hundred registers, we
   should not have to worry about it anymore... and yes, 640KB is
   enough for everybody :-)
   This change helps setcontext(2) and cpu_set_upcall_kse() in that
   they can return to completely different contexts without having to
   mess with the kernel stack. Of course exec_setregs() doesn't need
   to do that anymore as well.

sys/ia64/ia64/exception.S

@@ -535,10 +535,10 @@ exception_restore_restart:
 	;;
 }
 {	.mmi
-(p15)	mov		r31=ar.bspstore
+	mov		r31=ar.bspstore
 	;;
 	mov		ar.bspstore=r20
-(p15)	dep		r31=0,r31,0,9
+	dep		r31=0,r31,0,13		// 8KB aligned
 	;;
 }
 {	.mmb

sys/ia64/ia64/syscall.S

@@ -486,7 +486,7 @@ epc_syscall_return:
 	mov		r30=ar.bspstore
 	;;
 	mov		r14=ar.k5
-	dep		r30=0,r30,0,9
+	dep		r30=0,r30,0,13		// 8KB aligned.
 	;;
 }
 {	.mmi
@@ -518,7 +518,7 @@ epc_syscall_setup_ia32:
 	mov		r30=ar.bspstore
 	;;
 	mov		ar.unat=r17
-	dep		r30=0,r30,0,9
+	dep		r30=0,r30,0,13		// 8KB aligned
 	;;
 }
 {	.mmi