x86/PoD: skip eager reclaim when possible

Reclaiming pages is pointless when the cache can already satisfy all
outstanding PoD entries, and doing reclaims in that case can be very
harmful to performance when that memory gets used by the guest, but
only to store zeroes there.

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

--- a/xen/arch/x86/mm/p2m-pod.c
+++ b/xen/arch/x86/mm/p2m-pod.c
@@ -1027,7 +1027,6 @@ static void pod_eager_record(struct p2m_
 {
     struct pod_mrp_list *mrp = &p2m->pod.mrp;
 
-    ASSERT(mrp->list[mrp->idx] == INVALID_GFN);
     ASSERT(gfn != INVALID_GFN);
 
     mrp->list[mrp->idx++] =
@@ -1075,7 +1074,9 @@ p2m_pod_demand_populate(struct p2m_domai
         return 0;
     }
 
-    pod_eager_reclaim(p2m);
+    /* Only reclaim if we're in actual need of more cache. */
+    if ( p2m->pod.entry_count > p2m->pod.count )
+        pod_eager_reclaim(p2m);
 
     /* Only sweep if we're actually out of memory.  Doing anything else
      * causes unnecessary time and fragmentation of superpages in the p2m. */