xen/arm: Allow a privileged domain to map foreign page from DOMID_XEN

For auto-translated domain, the only way to map a page to itself is the
using the foreign map API. The current code does not allow mapping page from
special page (such as DOMID_XEN).

As xentrace buffers are shared using DOMID_XEN, it is not possible to use
tracing for Arm.

This could be solved by using the helper get_pg_owner(). This helper will
be able to get a reference on DOMID_XEN and therefore allow mapping for
privileged domain.

This patch replace the call to rcu_lock_domain_by_any_id() with
get_pg_owner(). For consistency, all the call to rcu_unlock_domain are
replaced by put_pg_owner().

Signed-off-by: Julien grall <julien.grall@arm.com>
Reviewed-by: Andrii Anisov <andrii_anisov@epam.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>

xen/arch/arm/mm.c

@@ -1233,20 +1233,20 @@ int xenmem_add_to_physmap_one(
         struct domain *od;
         p2m_type_t p2mt;
 
-        od = rcu_lock_domain_by_any_id(extra.foreign_domid);
+        od = get_pg_owner(extra.foreign_domid);
         if ( od == NULL )
             return -ESRCH;
 
         if ( od == d )
         {
-            rcu_unlock_domain(od);
+            put_pg_owner(od);
             return -EINVAL;
         }
 
         rc = xsm_map_gmfn_foreign(XSM_TARGET, d, od);
         if ( rc )
         {
-            rcu_unlock_domain(od);
+            put_pg_owner(od);
             return rc;
         }
 
@@ -1255,7 +1255,7 @@ int xenmem_add_to_physmap_one(
         page = get_page_from_gfn(od, idx, &p2mt, P2M_ALLOC);
         if ( !page )
         {
-            rcu_unlock_domain(od);
+            put_pg_owner(od);
             return -EINVAL;
         }
 
@@ -1264,13 +1264,13 @@ int xenmem_add_to_physmap_one(
         else
         {
             put_page(page);
-            rcu_unlock_domain(od);
+            put_pg_owner(od);
             return -EINVAL;
         }
 
         mfn = page_to_mfn(page);
 
-        rcu_unlock_domain(od);
+        put_pg_owner(od);
         break;
     }
     case XENMAPSPACE_dev_mmio: