x86/HVM: avoid memory_type_changed() invocations when possible

They're expensive, and nothing changes if MTRRs are disabled and any of
the ranges gets changed, or if fixed range MTRRs are disabled and any of
them gets changed.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>

xen/arch/x86/hvm/mtrr.c

@@ -472,7 +472,9 @@ bool_t mtrr_fix_range_msr_set(struct domain *d, struct mtrr_state *m,
                 return 0;
 
         fixed_range_base[row] = msr_content;
-        memory_type_changed(d);
+
+        if ( m->enabled && m->fixed_enabled )
+            memory_type_changed(d);
     }
 
     return 1;
@@ -515,7 +517,8 @@ bool_t mtrr_var_range_msr_set(
 
     m->overlapped = is_var_mtrr_overlapped(m);
 
-    memory_type_changed(d);
+    if ( m->enabled )
+        memory_type_changed(d);
 
     return 1;
 }