[PATCH] netback: allow arbitrary mtu size until frontend connects

[Olaf Hering <olaf@aepfle.de>]

Allow arbitrary mtu size until the frontend is connected.  Once the
connection is established, adjust mtu by checking if the backend
supports the 'feature-sg'.  If the backend does support it, keep the
current mtu. Otherwise set it to the default value, which is 1500.

This helps the vif-bridge hotplug script to set the mtu size to 9000
while bringing up the guest.

Signed-off-by: Olaf Hering <olaf@aepfle.de>

---
 drivers/xen/netback/common.h    |    8 ++++++++
 drivers/xen/netback/interface.c |    8 +++++++-
 drivers/xen/netback/xenbus.c    |    9 +++++++++
 3 files changed, 24 insertions(+), 1 deletion(-)

--- linux-2.6.18-xen.hg.orig/drivers/xen/netback/common.h
+++ linux-2.6.18-xen.hg/drivers/xen/netback/common.h
@@ -82,6 +82,8 @@ typedef struct netif_st {
 	u8 can_queue:1;	/* can queue packets for receiver? */
 	u8 copying_receiver:1;	/* copy packets to receiver?       */
 
+	u8 frontend_connected:1; /* is frontend already connected? */
+
 	/* Allow netif_be_start_xmit() to peek ahead in the rx request ring. */
 	RING_IDX rx_req_cons_peek;
 
@@ -217,4 +219,10 @@ static inline int netbk_can_sg(struct ne
 	return netif->features & NETIF_F_SG;
 }
 
+static inline int netbk_frontend_connected(struct net_device *dev)
+{
+	netif_t *netif = netdev_priv(dev);
+	return !!netif->frontend_connected;
+}
+
 #endif /* __NETIF__BACKEND__COMMON_H__ */
--- linux-2.6.18-xen.hg.orig/drivers/xen/netback/interface.c
+++ linux-2.6.18-xen.hg/drivers/xen/netback/interface.c
@@ -83,9 +83,13 @@ static int net_close(struct net_device *
 	return 0;
 }
 
+/* accept the max value if frontend is not yet connected */
 static int netbk_change_mtu(struct net_device *dev, int mtu)
 {
-	int max = netbk_can_sg(dev) ? 65535 - ETH_HLEN : ETH_DATA_LEN;
+	int max = 65535 - ETH_HLEN;
+
+	if (netbk_frontend_connected(dev))
+		max = netbk_can_sg(dev) ? max : ETH_DATA_LEN;
 
 	if (mtu > max)
 		return -EINVAL;
@@ -359,6 +363,8 @@ err_rx:
 
 void netif_disconnect(netif_t *netif)
 {
+	netif->frontend_connected = 0;
+
 	if (netback_carrier_ok(netif)) {
 		rtnl_lock();
 		netback_carrier_off(netif);
--- linux-2.6.18-xen.hg.orig/drivers/xen/netback/xenbus.c
+++ linux-2.6.18-xen.hg/drivers/xen/netback/xenbus.c
@@ -329,6 +329,13 @@ static int xen_net_read_mac(struct xenbu
 	return 0;
 }
 
+static void xen_net_adjust_mtu(netif_t *netif)
+{
+	if (!netif->dev->change_mtu)
+		return;
+	netif->dev->change_mtu(netif->dev, netif->dev->mtu);
+}
+
 static void connect(struct backend_info *be)
 {
 	int err;
@@ -338,6 +345,8 @@ static void connect(struct backend_info
 	if (err)
 		return;
 
+	be->netif->frontend_connected = 1;
+	xen_net_adjust_mtu(be->netif);
 	err = xen_net_read_mac(dev, be->netif->fe_dev_addr);
 	if (err) {
 		xenbus_dev_fatal(dev, err, "parsing %s/mac", dev->nodename);