[PATCH v3] gnttab: defer allocation of status frame tracking array

[Jan Beulich <jbeulich@xxxxxxxx>]

This array can be large when many grant frames are permitted; avoid
allocating it when it's not going to be used anyway, by doing this only
in gnttab_populate_status_frames().

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
---
v3: Drop smp_wmb(). Re-base.
v2: Defer allocation to when a domain actually switches to the v2 grant
    API.

--- a/xen/common/grant_table.c
+++ b/xen/common/grant_table.c
@@ -1747,6 +1747,17 @@ gnttab_populate_status_frames(struct dom
     /* Make sure, prior version checks are architectural visible */
     block_speculation();
 
+    if ( gt->status == ZERO_BLOCK_PTR )
+    {
+        gt->status = xzalloc_array(grant_status_t *,
+                                   
grant_to_status_frames(gt->max_grant_frames));
+        if ( !gt->status )
+        {
+            gt->status = ZERO_BLOCK_PTR;
+            return -ENOMEM;
+        }
+    }
+
     for ( i = nr_status_frames(gt); i < req_status_frames; i++ )
     {
         if ( (gt->status[i] = alloc_xenheap_page()) == NULL )
@@ -1767,18 +1778,23 @@ status_alloc_failed:
         free_xenheap_page(gt->status[i]);
         gt->status[i] = NULL;
     }
+    if ( !nr_status_frames(gt) )
+    {
+        xfree(gt->status);
+        gt->status = ZERO_BLOCK_PTR;
+    }
     return -ENOMEM;
 }
 
 static int
 gnttab_unpopulate_status_frames(struct domain *d, struct grant_table *gt)
 {
-    unsigned int i;
+    unsigned int i, n = nr_status_frames(gt);
 
     /* Make sure, prior version checks are architectural visible */
     block_speculation();
 
-    for ( i = 0; i < nr_status_frames(gt); i++ )
+    for ( i = 0; i < n; i++ )
     {
         struct page_info *pg = virt_to_page(gt->status[i]);
         gfn_t gfn = gnttab_get_frame_gfn(gt, true, i);
@@ -1833,12 +1849,11 @@ gnttab_unpopulate_status_frames(struct d
         page_set_owner(pg, NULL);
     }
 
-    for ( i = 0; i < nr_status_frames(gt); i++ )
-    {
-        free_xenheap_page(gt->status[i]);
-        gt->status[i] = NULL;
-    }
     gt->nr_status_frames = 0;
+    for ( i = 0; i < n; i++ )
+        free_xenheap_page(gt->status[i]);
+    xfree(gt->status);
+    gt->status = ZERO_BLOCK_PTR;
 
     return 0;
 }
@@ -1969,11 +1984,11 @@ int grant_table_init(struct domain *d, i
     if ( gt->shared_raw == NULL )
         goto out;
 
-    /* Status pages for grant table - for version 2 */
-    gt->status = xzalloc_array(grant_status_t *,
-                               grant_to_status_frames(gt->max_grant_frames));
-    if ( gt->status == NULL )
-        goto out;
+    /*
+     * Status page tracking array for v2 gets allocated on demand. But don't
+     * leave a NULL pointer there.
+     */
+    gt->status = ZERO_BLOCK_PTR;
 
     grant_write_lock(gt);
 
@@ -4047,11 +4062,12 @@ int gnttab_acquire_resource(
         if ( gt->gt_version != 2 )
             break;
 
+        rc = gnttab_get_status_frame_mfn(d, final_frame, &tmp);
+
         /* Check that void ** is a suitable representation for gt->status. */
         BUILD_BUG_ON(!__builtin_types_compatible_p(
                          typeof(gt->status), grant_status_t **));
         vaddrs = (void **)gt->status;
-        rc = gnttab_get_status_frame_mfn(d, final_frame, &tmp);
         break;
     }