[PATCH] x86/mm: PGC_page_table is used by shadow code only

[Jan Beulich <jbeulich@xxxxxxxx>]

By defining the constant to zero when !SHADOW_PAGING we give compilers
the chance to eliminate a little more dead code elsewhere in the tree.
Plus, as a minor benefit, the general reference count can be one bit
wider. (To simplify things, have PGC_page_table change places with
PGC_extra.)

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
---
tboot.c's update_pagetable_mac() is suspicious: It effectively is a
no-op even prior to this change when !SHADOW_PAGING, which can't be
quite right. If (guest) page tables are relevant to include in the
verification, shouldn't this look for PGT_l<N>_page_table as well? How
to deal with HAP guests there is entirely unclear.

--- a/xen/arch/x86/include/asm/mm.h
+++ b/xen/arch/x86/include/asm/mm.h
@@ -70,9 +70,9 @@
  /* Page is Xen heap? */
 #define _PGC_xen_heap     PG_shift(2)
 #define PGC_xen_heap      PG_mask(1, 2)
- /* Set when is using a page as a page table */
-#define _PGC_page_table   PG_shift(3)
-#define PGC_page_table    PG_mask(1, 3)
+ /* Page is not reference counted */
+#define _PGC_extra        PG_shift(3)
+#define PGC_extra         PG_mask(1, 3)
  /* Page is broken? */
 #define _PGC_broken       PG_shift(4)
 #define PGC_broken        PG_mask(1, 4)
@@ -83,12 +83,20 @@
 #define PGC_state_offlined  PG_mask(2, 6)
 #define PGC_state_free      PG_mask(3, 6)
 #define page_state_is(pg, st) (((pg)->count_info&PGC_state) == PGC_state_##st)
-/* Page is not reference counted */
-#define _PGC_extra        PG_shift(7)
-#define PGC_extra         PG_mask(1, 7)
+#ifdef CONFIG_SHADOW_PAGING
+ /* Set when is using a page as a page table */
+#define _PGC_page_table   PG_shift(7)
+#define PGC_page_table    PG_mask(1, 7)
+#else
+#define PGC_page_table    0
+#endif
 
 /* Count of references to this frame. */
+#if PGC_page_table
 #define PGC_count_width   PG_shift(7)
+#else
+#define PGC_count_width   PG_shift(6)
+#endif
 #define PGC_count_mask    ((1UL<<PGC_count_width)-1)
 
 /*