x86/p2m: limit cache flush in memory_type_changed()

Only do the cache flush when there's a p2m type change to propagate,
otherwise there's no change in the p2m effective caching attributes.

If the p2m memory_type_changed hook is not set p2m_memory_type_changed() is
a no-op, no recalculation of caching attributes is needed, nor flushing of
the previous cache contents.

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

xen/arch/x86/hvm/mtrr.c

@@ -783,9 +783,8 @@ HVM_REGISTER_SAVE_RESTORE(MTRR, hvm_save_mtrr_msr, NULL, hvm_load_mtrr_msr, 1,
 void memory_type_changed(struct domain *d)
 {
     if ( (is_iommu_enabled(d) || cache_flush_permitted(d)) &&
-         d->vcpu && d->vcpu[0] )
+         d->vcpu && d->vcpu[0] && p2m_memory_type_changed(d) )
     {
-        p2m_memory_type_changed(d);
         flush_all(FLUSH_CACHE);
     }
 }

xen/arch/x86/include/asm/p2m.h

@@ -700,7 +700,7 @@ void p2m_pod_dump_data(struct domain *d);
 #ifdef CONFIG_HVM
 
 /* Report a change affecting memory types. */
-void p2m_memory_type_changed(struct domain *d);
+bool p2m_memory_type_changed(struct domain *d);
 
 /* Called by p2m code when demand-populating a PoD page */
 bool

xen/arch/x86/mm/p2m.c

@@ -126,12 +126,21 @@ static void _memory_type_changed(struct p2m_domain *p2m)
 {
     if ( p2m->memory_type_changed )
         p2m->memory_type_changed(p2m);
+    else
+        ASSERT_UNREACHABLE();
 }
 
-void p2m_memory_type_changed(struct domain *d)
+bool p2m_memory_type_changed(struct domain *d)
 {
     struct p2m_domain *hostp2m = p2m_get_hostp2m(d);
 
+    /*
+     * The p2m memory_type_changed hook will be the same for the host p2m or
+     * the altp2ms, do the check early and return if not set.
+     */
+    if ( !hostp2m->memory_type_changed )
+        return false;
+
     p2m_lock(hostp2m);
 
     _memory_type_changed(hostp2m);
@@ -154,6 +163,8 @@ void p2m_memory_type_changed(struct domain *d)
     }
 
     p2m_unlock(hostp2m);
+
+    return true;
 }
 
 int p2m_set_ioreq_server(struct domain *d,