x86/HVM: limit flushing on cache attribute pinning adjustments

Avoid cache flush on EPT when removing a UC- range, since when used
this type gets converted to UC anyway (there's no UC- among the types
valid in MTRRs and hence EPT's emt field).

We might further wwant to consider only forcing write buffer flushes
when removing WC ranges.

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

--- a/xen/arch/x86/hvm/mtrr.c
+++ b/xen/arch/x86/hvm/mtrr.c
@@ -611,8 +611,21 @@ int32_t hvm_set_mem_pinned_cacheattr(
                 type = range->type;
                 call_rcu(&range->rcu, free_pinned_cacheattr_entry);
                 p2m_memory_type_changed(d);
-                if ( type != PAT_TYPE_UNCACHABLE )
+                switch ( type )
+                {
+                case PAT_TYPE_UC_MINUS:
+                    /*
+                     * For EPT we can also avoid the flush in this case;
+                     * see epte_get_entry_emt().
+                     */
+                    if ( hap_enabled(d) && cpu_has_vmx )
+                case PAT_TYPE_UNCACHABLE:
+                        break;
+                    /* fall through */
+                default:
                     flush_all(FLUSH_CACHE);
+                    break;
+                }
                 return 0;
             }
         rcu_read_unlock(&pinned_cacheattr_rcu_lock);