gnttab: eliminate several explicit version checks

By having nr_grant_entries() return zero when the grant table version
is still unset we can reduce the number of error paths and at once fix
grant_map_exists() running into the being removed ASSERT() when called
for a page owned by a domain not having its grant table set up yet.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
---
Said ASSERT() was pointless in the grant_map_exists() case, since the
function only looks at active entries, which are version independent.
Hence this wasn't a security issue.

--- a/xen/common/grant_table.c
+++ b/xen/common/grant_table.c
@@ -319,11 +319,15 @@ get_maptrack_handle(
 /* Number of grant table entries. Caller must hold d's grant table lock. */
 static unsigned int nr_grant_entries(struct grant_table *gt)
 {
-    ASSERT(gt->gt_version != 0);
-    if (gt->gt_version == 1)
+    switch ( gt->gt_version )
+    {
+    case 1:
         return (nr_grant_frames(gt) << PAGE_SHIFT) / sizeof(grant_entry_v1_t);
-    else
+    case 2:
         return (nr_grant_frames(gt) << PAGE_SHIFT) / sizeof(grant_entry_v2_t);
+    }
+
+    return 0;
 }
 
 static int _set_status_v1(domid_t  domid,
@@ -631,10 +635,6 @@ __gnttab_map_grant_ref(
     rgt = rd->grant_table;
     spin_lock(&rgt->lock);
 
-    if ( rgt->gt_version == 0 )
-        PIN_FAIL(unlock_out, GNTST_general_error,
-                 "remote grant table not yet set up\n");
-
     /* Bounds check on the grant ref */
     if ( unlikely(op->ref >= nr_grant_entries(rgt)))
         PIN_FAIL(unlock_out, GNTST_bad_gntref, "Bad ref (%d).\n", op->ref);
@@ -1502,14 +1502,6 @@ gnttab_prepare_for_transfer(
 
     spin_lock(&rgt->lock);
 
-    if ( rgt->gt_version == 0 )
-    {
-        gdprintk(XENLOG_INFO,
-                 "Grant table not ready for transfer to domain(%d).\n",
-                 rd->domain_id);
-        goto fail;
-    }
-
     if ( unlikely(ref >= nr_grant_entries(rgt)) )
     {
         gdprintk(XENLOG_INFO,
@@ -1905,10 +1897,6 @@ __acquire_grant_for_copy(
 
     spin_lock(&rgt->lock);
 
-    if ( rgt->gt_version == 0 )
-        PIN_FAIL(unlock_out, GNTST_general_error,
-                 "remote grant table not ready\n");
-
     if ( unlikely(gref >= nr_grant_entries(rgt)) )
         PIN_FAIL(unlock_out, GNTST_bad_gntref,
                  "Bad grant reference %ld\n", gref);
@@ -2395,20 +2383,16 @@ gnttab_set_version(XEN_GUEST_HANDLE_PARA
        change the version number, except for the first 8 entries which
        are allowed to be in use (xenstore/xenconsole keeps them mapped).
        (You need to change the version number for e.g. kexec.) */
-    if ( gt->gt_version != 0 )
+    for ( i = GNTTAB_NR_RESERVED_ENTRIES; i < nr_grant_entries(gt); i++ )
     {
-        for ( i = GNTTAB_NR_RESERVED_ENTRIES; i < nr_grant_entries(gt); i++ )
+        act = &active_entry(gt, i);
+        if ( act->pin != 0 )
         {
-            act = &active_entry(gt, i);
-            if ( act->pin != 0 )
-            {
-                gdprintk(XENLOG_WARNING,
-                         "tried to change grant table version from %d to %d, but some grant entries still in use\n",
-                         gt->gt_version,
-                         op.version);
-                res = -EBUSY;
-                goto out_unlock;
-            }
+            gdprintk(XENLOG_WARNING,
+                     "tried to change grant table version from %d to %d, but some grant entries still in use\n",
+                     gt->gt_version, op.version);
+            res = -EBUSY;
+            goto out_unlock;
         }
     }
 
@@ -2592,9 +2576,6 @@ __gnttab_swap_grant_ref(grant_ref_t ref_
 
     spin_lock(&gt->lock);
 
-    if ( gt->gt_version == 0 )
-        PIN_FAIL(out, GNTST_general_error, "grant table not yet set up\n");
-
     /* Bounds check on the grant refs */
     if ( unlikely(ref_a >= nr_grant_entries(d->grant_table)))
         PIN_FAIL(out, GNTST_bad_gntref, "Bad ref-a (%d).\n", ref_a);
@@ -3165,9 +3146,6 @@ static void gnttab_usage_print(struct do
 
     spin_lock(&gt->lock);
 
-    if ( gt->gt_version == 0 )
-        goto out;
-
     for ( ref = 0; ref != nr_grant_entries(gt); ref++ )
     {
         struct active_grant_entry *act;
@@ -3211,7 +3189,6 @@ static void gnttab_usage_print(struct do
                sha->domid, frame, status);
     }
 
- out:
     spin_unlock(&gt->lock);
 
     if ( first )