x86: fix map_domain_page() last resort fallback

Guests with vCPU count not divisible by 4 have unused bits in the last
word of their inuse bitmap, and the garbage collection code therefore
would get mislead believing that some entries were actually recoverable
for use.

Also use an earlier established local variable in mapcache_vcpu_init()
instead of re-calculating the value (noticed while investigating the
generally better option of setting those overhanging bits once during
setup - this didn't work out in a simple enough fashion because the
mapping getting established there isn't in the current address space,
and hence the bitmap isn't directly accessible there).

Reported-by: Konrad Wilk <konrad.wilk@xxxxxxxxxx>
Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/xen/arch/x86/domain_page.c
+++ b/xen/arch/x86/domain_page.c
@@ -111,16 +111,17 @@ void *map_domain_page(unsigned long mfn)
     idx = find_next_zero_bit(dcache->inuse, dcache->entries, dcache->cursor);
     if ( unlikely(idx >= dcache->entries) )
     {
-        unsigned long accum = 0;
+        unsigned long accum = 0, prev = 0;
 
         /* /First/, clean the garbage map and update the inuse list. */
         for ( i = 0; i < BITS_TO_LONGS(dcache->entries); i++ )
         {
+            accum |= prev;
             dcache->inuse[i] &= ~xchg(&dcache->garbage[i], 0);
-            accum |= ~dcache->inuse[i];
+            prev = ~dcache->inuse[i];
         }
 
-        if ( accum )
+        if ( accum | (prev & BITMAP_LAST_WORD_MASK(dcache->entries)) )
             idx = find_first_zero_bit(dcache->inuse, dcache->entries);
         else
         {
@@ -280,8 +281,7 @@ int mapcache_vcpu_init(struct vcpu *v)
     if ( ents > dcache->entries )
     {
         /* Populate page tables. */
-        int rc = create_perdomain_mapping(d, MAPCACHE_VIRT_START,
-                                          d->max_vcpus * MAPCACHE_VCPU_ENTRIES,
+        int rc = create_perdomain_mapping(d, MAPCACHE_VIRT_START, ents,
                                           NIL(l1_pgentry_t *), NULL);
 
         /* Populate bit maps. */