x86: fix determination of bit count for struct domain allocations

We can't just add in the hole shift value, as the hole may be at or
above the 44-bit boundary. Instead we need to determine the total bit
count until reaching 32 significant (not squashed out) bits in PFN
representations.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/xen/arch/x86/domain.c
+++ b/xen/arch/x86/domain.c
@@ -180,6 +180,19 @@ void dump_pageframe_info(struct domain *
     spin_unlock(&d->page_alloc_lock);
 }
 
+static unsigned int __init noinline _domain_struct_bits(void)
+{
+    unsigned int bits = 32 + PAGE_SHIFT;
+    unsigned int sig = hweight32(~pfn_hole_mask);
+    unsigned int mask = pfn_hole_mask >> 32;
+
+    for ( ; bits < BITS_PER_LONG && sig < 32; ++bits, mask >>= 1 )
+        if ( !(mask & 1) )
+            ++sig;
+
+    return bits;
+}
+
 struct domain *alloc_domain_struct(void)
 {
     struct domain *d;
@@ -187,7 +200,10 @@ struct domain *alloc_domain_struct(void)
      * We pack the PDX of the domain structure into a 32-bit field within
      * the page_info structure. Hence the MEMF_bits() restriction.
      */
-    unsigned int bits = 32 + PAGE_SHIFT + pfn_pdx_hole_shift;
+    static unsigned int __read_mostly bits;
+
+    if ( unlikely(!bits) )
+         bits = _domain_struct_bits();
 
     BUILD_BUG_ON(sizeof(*d) > PAGE_SIZE);
     d = alloc_xenheap_pages(0, MEMF_bits(bits));