dom_cow is needed for mem-sharing only

A couple of adjustments are needed to code checking for dom_cow, but
since there are pretty few it is probably better to adjust those than
to set up and keep around a never used domain.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Julien Grall <julien.grall@arm.com>

xen/arch/x86/mm.c

@@ -966,8 +966,8 @@ get_page_from_l1e(
         return flip;
     }
 
-    if ( unlikely( (real_pg_owner != pg_owner) &&
-                   (real_pg_owner != dom_cow) ) )
+    if ( unlikely((real_pg_owner != pg_owner) &&
+                  (!dom_cow || (real_pg_owner != dom_cow))) )
     {
         /*
          * Let privileged domains transfer the right to map their target

xen/arch/x86/mm/p2m.c

@@ -568,7 +568,8 @@ struct page_info *p2m_get_page_from_gfn(
             }
             else if ( !get_page(page, p2m->domain) &&
                       /* Page could be shared */
-                      (!p2m_is_shared(*t) || !get_page(page, dom_cow)) )
+                      (!dom_cow || !p2m_is_shared(*t) ||
+                       !get_page(page, dom_cow)) )
                 page = NULL;
         }
         p2m_read_unlock(p2m);
@@ -941,7 +942,8 @@ guest_physmap_add_entry(struct domain *d, gfn_t gfn, mfn_t mfn,
     /* Then, look for m->p mappings for this range and deal with them */
     for ( i = 0; i < (1UL << page_order); i++ )
     {
-        if ( page_get_owner(mfn_to_page(mfn_add(mfn, i))) == dom_cow )
+        if ( dom_cow &&
+             page_get_owner(mfn_to_page(mfn_add(mfn, i))) == dom_cow )
         {
             /* This is no way to add a shared page to your physmap! */
             gdprintk(XENLOG_ERR, "Adding shared mfn %lx directly to dom%d physmap not allowed.\n",

xen/common/domain.c

@@ -74,7 +74,9 @@ integer_param("hardware_dom", hardware_domid);
 /* Private domain structs for DOMID_XEN, DOMID_IO, etc. */
 struct domain *__read_mostly dom_xen;
 struct domain *__read_mostly dom_io;
+#ifdef CONFIG_HAS_MEM_SHARING
 struct domain *__read_mostly dom_cow;
+#endif
 
 struct vcpu *idle_vcpu[NR_CPUS] __read_mostly;
 
@@ -547,6 +549,7 @@ void __init setup_system_domains(void)
     if ( IS_ERR(dom_io) )
         panic("Failed to create d[IO]: %ld\n", PTR_ERR(dom_io));
 
+#ifdef CONFIG_HAS_MEM_SHARING
     /*
      * Initialise our COW domain.
      * This domain owns sharable pages.
@@ -554,6 +557,7 @@ void __init setup_system_domains(void)
     dom_cow = domain_create(DOMID_COW, NULL, false);
     if ( IS_ERR(dom_cow) )
         panic("Failed to create d[COW]: %ld\n", PTR_ERR(dom_cow));
+#endif
 }
 
 void domain_update_node_affinity(struct domain *d)

xen/common/grant_table.c

@@ -1085,7 +1085,7 @@ map_grant_ref(
             host_map_created = true;
         }
     }
-    else if ( owner == rd || owner == dom_cow )
+    else if ( owner == rd || (dom_cow && owner == dom_cow) )
     {
         if ( (op->flags & GNTMAP_device_map) && !(op->flags & GNTMAP_readonly) )
         {

xen/include/xen/mm.h

@@ -274,7 +274,12 @@ struct npfec {
 #endif
 
 /* Private domain structs for DOMID_XEN, DOMID_IO, etc. */
-extern struct domain *dom_xen, *dom_io, *dom_cow;
+extern struct domain *dom_xen, *dom_io;
+#ifdef CONFIG_HAS_MEM_SHARING
+extern struct domain *dom_cow;
+#else
+# define dom_cow NULL
+#endif
 
 #define page_list_entry list_head