tmem: Unify XEN_SYSCTL_TMEM_OP_[[SAVE_[BEGIN|END]|RESTORE_BEGIN]

return values. For success they used to be 1 ([SAVE,RESTORE]_BEGIN),
0 if guest did not have any tmem (but only for SAVE_BEGIN), and
-1 for any type of failure.

And SAVE_END (which you would think would mirror SAVE_BEGIN)
had 0 for success and -1 if guest did not any tmem enabled for it.

This is confusing. Now the code will return 0 if the operation was
success.  Various XEN_EXX values are returned if tmem is not enabled
or the operation could not performed.

The xc_tmem.c code only needs one place to check - where we use
SAVE_BEGIN. The place where RESTORE_BEGIN is used will have errno
with the proper error value and return will be -1, so will still
fail properly.

Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

tools/libxc/xc_tmem.c

@@ -214,15 +214,23 @@ int xc_tmem_save(xc_interface *xch,
                  int dom, int io_fd, int live, int field_marker)
 {
     int marker = field_marker;
-    int i, j;
+    int i, j, rc;
     uint32_t flags;
     uint32_t minusone = -1;
     uint32_t pool_id;
     struct tmem_handle *h;
     xen_tmem_client_t info;
 
-    if ( xc_tmem_control(xch,0,XEN_SYSCTL_TMEM_OP_SAVE_BEGIN,dom,live,0,NULL) <= 0 )
+    rc = xc_tmem_control(xch, 0, XEN_SYSCTL_TMEM_OP_SAVE_BEGIN,
-        return 0;
+                         dom, live, 0, NULL);
+    if ( rc )
+    {
+        /* Nothing to save - no tmem enabled. */
+        if ( errno == ENOENT )
+            return 0;
+
+        return rc;
+    }
 
     if ( write_exact(io_fd, &marker, sizeof(marker)) )
         return -1;

xen/common/tmem.c

@@ -1656,30 +1656,31 @@ static int tmemc_save_subop(int cli_id, uint32_t pool_id,
     struct client *client = tmem_client_from_cli_id(cli_id);
     uint32_t p;
     struct tmem_page_descriptor *pgp, *pgp2;
-    int rc = -1;
+    int rc = -ENOENT;
 
     switch(subop)
     {
     case XEN_SYSCTL_TMEM_OP_SAVE_BEGIN:
         if ( client == NULL )
-            return 0;
+            break;
         for (p = 0; p < MAX_POOLS_PER_DOMAIN; p++)
             if ( client->pools[p] != NULL )
                 break;
+
         if ( p == MAX_POOLS_PER_DOMAIN )
-        {
-            rc = 0;
             break;
-        }
+
         client->was_frozen = client->info.flags.u.frozen;
         client->info.flags.u.frozen = 1;
         if ( arg1 != 0 )
             client->info.flags.u.migrating = 1;
-        rc = 1;
+        rc = 0;
         break;
     case XEN_SYSCTL_TMEM_OP_RESTORE_BEGIN:
-        if ( client == NULL && (client = client_create(cli_id)) != NULL )
+        if ( client == NULL )
-            return 1;
+            rc = client_create(cli_id) ? 0 : -ENOMEM;
+        else
+            rc = -EEXIST;
         break;
     case XEN_SYSCTL_TMEM_OP_SAVE_END:
         if ( client == NULL )