x86: expose XENMEM_get_pod_target to subject domain

Not having got any satisfactory suggestions on the inquiry on how to
determine the amount a PoD guest needs to balloon down by (see
http://lists.xenproject.org/archives/html/xen-devel/2014-01/msg01524.html
and the thread following it), expose XENMEM_get_pod_target such that
the guest can use it for this purpose.

Also leverage some cleanup potential resulting from this change.

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

--- a/xen/arch/x86/mm.c
+++ b/xen/arch/x86/mm.c
@@ -4800,7 +4800,6 @@ long arch_memory_op(int op, XEN_GUEST_HA
     {
         xen_pod_target_t target;
         struct domain *d;
-        struct p2m_domain *p2m;
 
         if ( copy_from_guest(&target, arg, 1) )
             return -EFAULT;
@@ -4810,23 +4809,17 @@ long arch_memory_op(int op, XEN_GUEST_HA
             return -ESRCH;
 
         if ( op == XENMEM_set_pod_target )
-            rc = xsm_set_pod_target(XSM_PRIV, d);
-        else
-            rc = xsm_get_pod_target(XSM_PRIV, d);
-
-        if ( rc != 0 )
-            goto pod_target_out_unlock;
-
-        if ( op == XENMEM_set_pod_target )
         {
-            if ( target.target_pages > d->max_pages )
-            {
+            rc = xsm_set_pod_target(XSM_PRIV, d);
+            if ( rc )
+                /* nothing */;
+            else if ( target.target_pages > d->max_pages )
                 rc = -EINVAL;
-                goto pod_target_out_unlock;
-            }
-            
-            rc = p2m_pod_set_mem_target(d, target.target_pages);
+            else
+                rc = p2m_pod_set_mem_target(d, target.target_pages);
         }
+        else
+            rc = xsm_get_pod_target(XSM_TARGET, d);
 
         if ( rc == -EAGAIN )
         {
@@ -4835,19 +4828,16 @@ long arch_memory_op(int op, XEN_GUEST_HA
         }
         else if ( rc >= 0 )
         {
-            p2m = p2m_get_hostp2m(d);
+            const struct p2m_domain *p2m = p2m_get_hostp2m(d);
+
             target.tot_pages       = d->tot_pages;
             target.pod_cache_pages = p2m->pod.count;
             target.pod_entries     = p2m->pod.entry_count;
 
             if ( __copy_to_guest(arg, &target, 1) )
-            {
                 rc= -EFAULT;
-                goto pod_target_out_unlock;
-            }
         }
         
-    pod_target_out_unlock:
         rcu_unlock_domain(d);
         return rc;
     }
--- a/xen/include/xsm/dummy.h
+++ b/xen/include/xsm/dummy.h
@@ -307,7 +307,7 @@ static XSM_INLINE char *xsm_show_securit
 
 static XSM_INLINE int xsm_get_pod_target(XSM_DEFAULT_ARG struct domain *d)
 {
-    XSM_ASSERT_ACTION(XSM_PRIV);
+    XSM_ASSERT_ACTION(XSM_TARGET);
     return xsm_default_action(action, current->domain, d);
 }