Add a comment explaining the need of a global temporary variable

cpu_xirr is used only as a temporary location for the OPAL call in
PIC_DISPATCH().

Requested by:	nwhitehorn

sys/powerpc/pseries/xics.c

@@ -140,6 +140,15 @@ static driver_t xics_driver = {
 };
 
 #ifdef POWERNV
+/* We can only pass physical addresses into OPAL.  Kernel stacks are in the KVA,
+ * not in the direct map, so we need to somehow extract the physical address.
+ * However, pmap_kextract() takes locks, which is forbidden in a critical region
+ * (which PMAP_DISPATCH() operates in).  The kernel is mapped into the Direct
+ * Map (0xc000....), and the CPU implicitly drops the top two bits when doing
+ * real address by nature that the bus width is smaller than 64-bits.  Placing
+ * cpu_xirr into the DMAP lets us take advantage of this and avoids the
+ * pmap_kextract() that would otherwise be needed if using the stack variable.
+ */
 static uint32_t cpu_xirr[MAXCPU];
 #endif