xen-netback: use jiffies_64 value to calculate credit timeout

time_after_eq() only works if the delta is < MAX_ULONG/2.

For a 32bit Dom0, if netfront sends packets at a very low rate, the time
between subsequent calls to tx_credit_exceeded() may exceed MAX_ULONG/2
and the test for timer_after_eq() will be incorrect. Credit will not be
replenished and the guest may become unable to send packets (e.g., if
prior to the long gap, all credit was exhausted).

Use jiffies_64 variant to mitigate this problem for 32bit Dom0.

Suggested-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Reviewed-by: David Vrabel <david.vrabel@citrix.com>
Cc: Ian Campbell <ian.campbell@citrix.com>
Cc: Jason Luan <jianhai.luan@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/xen-netback/common.h

@@ -163,6 +163,7 @@ struct xenvif {
 	unsigned long   credit_usec;
 	unsigned long   remaining_credit;
 	struct timer_list credit_timeout;
+	u64 credit_window_start;
 
 	/* Statistics */
 	unsigned long rx_gso_checksum_fixup;

drivers/net/xen-netback/interface.c

@@ -312,8 +312,7 @@ struct xenvif *xenvif_alloc(struct device *parent, domid_t domid,
 	vif->credit_bytes = vif->remaining_credit = ~0UL;
 	vif->credit_usec  = 0UL;
 	init_timer(&vif->credit_timeout);
-	/* Initialize 'expires' now: it's used to track the credit window. */
+	vif->credit_window_start = get_jiffies_64();
-	vif->credit_timeout.expires = jiffies;
 
 	dev->netdev_ops	= &xenvif_netdev_ops;
 	dev->hw_features = NETIF_F_SG | NETIF_F_IP_CSUM | NETIF_F_TSO;

drivers/net/xen-netback/netback.c

@@ -1185,9 +1185,8 @@ out:
 
 static bool tx_credit_exceeded(struct xenvif *vif, unsigned size)
 {
-	unsigned long now = jiffies;
+	u64 now = get_jiffies_64();
-	unsigned long next_credit =
+	u64 next_credit = vif->credit_window_start +
-		vif->credit_timeout.expires +
 		msecs_to_jiffies(vif->credit_usec / 1000);
 
 	/* Timer could already be pending in rare cases. */
@@ -1195,8 +1194,8 @@ static bool tx_credit_exceeded(struct xenvif *vif, unsigned size)
 		return true;
 
 	/* Passed the point where we can replenish credit? */
-	if (time_after_eq(now, next_credit)) {
+	if (time_after_eq64(now, next_credit)) {
-		vif->credit_timeout.expires = now;
+		vif->credit_window_start = now;
 		tx_add_credit(vif);
 	}
 
@@ -1208,6 +1207,7 @@ static bool tx_credit_exceeded(struct xenvif *vif, unsigned size)
 			tx_credit_callback;
 		mod_timer(&vif->credit_timeout,
 			  next_credit);
+		vif->credit_window_start = next_credit;
 
 		return true;
 	}