xen/netfront: set default upper limit of tx/rx queues to 8

The default for the number of tx/rx queues of one interface is the
number of vcpus of the system today. As each queue pair reserves 512
grant pages this default consumes a ridiculous number of grants for
large guests.

Limit the queue number to 8 as default. This value can be modified
via a module parameter if required.

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>

drivers/net/xen-netfront.c

@@ -57,6 +57,7 @@
 #include <xen/interface/grant_table.h>
 
 /* Module parameters */
+#define MAX_QUEUES_DEFAULT 8
 static unsigned int xennet_max_queues;
 module_param_named(max_queues, xennet_max_queues, uint, 0644);
 MODULE_PARM_DESC(max_queues,
@@ -2164,11 +2165,12 @@ static int __init netif_init(void)
 
 	pr_info("Initialising Xen virtual ethernet driver\n");
 
-	/* Allow as many queues as there are CPUs if user has not
+	/* Allow as many queues as there are CPUs inut max. 8 if user has not
 	 * specified a value.
 	 */
 	if (xennet_max_queues == 0)
-		xennet_max_queues = num_online_cpus();
+		xennet_max_queues = min_t(unsigned int, MAX_QUEUES_DEFAULT,
+					  num_online_cpus());
 
 	return xenbus_register_frontend(&netfront_driver);
 }