x86/bigmem: eliminate struct domain address width restriction

PDX-es are 64 bits wide in that case, and hence no limit needs to be
enforced.

Reported-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
Reviewed-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
v2: Use "const unsigned int bits = 0" instead of #define / #undef
    (as suggested by IanC).

--- unstable.orig/xen/arch/x86/domain.c	2015-09-22 12:54:38.000000000 +0200
+++ unstable/xen/arch/x86/domain.c	2015-08-26 09:24:28.000000000 +0200
@@ -204,6 +204,7 @@ smap_check_policy_t smap_policy_change(s
     return old_policy;
 }
 
+#ifndef CONFIG_BIGMEM
 /*
  * The hole may be at or above the 44-bit boundary, so we need to determine
  * the total bit count until reaching 32 significant (not squashed out) bits
@@ -225,10 +226,14 @@ static unsigned int __init noinline _dom
 
     return bits;
 }
+#endif
 
 struct domain *alloc_domain_struct(void)
 {
     struct domain *d;
+#ifdef CONFIG_BIGMEM
+    const unsigned int bits = 0;
+#else
     /*
      * We pack the PDX of the domain structure into a 32-bit field within
      * the page_info structure. Hence the MEMF_bits() restriction.
@@ -237,6 +242,7 @@ struct domain *alloc_domain_struct(void)
 
     if ( unlikely(!bits) )
          bits = _domain_struct_bits();
+#endif
 
     BUILD_BUG_ON(sizeof(*d) > PAGE_SIZE);
     d = alloc_xenheap_pages(0, MEMF_bits(bits));