xen-netback: use skb to determine number of required guest Rx requests

Using the MTU or GSO size to determine the number of required guest Rx
requests for an skb was subtly broken since these value may change at
runtime.

After 1650d545 (xen-netback: always
fully coalesce guest Rx packets) we always fully pack a packet into
its guest Rx slots.  Calculating the number of required slots from the
packet length is then easy.
Signed-off-by: David Vrabel <david.vrabel@citrix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/xen-netback/netback.c

@@ -149,20 +149,19 @@ static inline pending_ring_idx_t pending_index(unsigned i)
 	return i & (MAX_PENDING_REQS-1);
 }
 
-static int xenvif_rx_ring_slots_needed(struct xenvif *vif)
-{
-	if (vif->gso_mask)
-		return DIV_ROUND_UP(vif->dev->gso_max_size, XEN_PAGE_SIZE) + 1;
-	else
-		return DIV_ROUND_UP(vif->dev->mtu, XEN_PAGE_SIZE);
-}
-
 static bool xenvif_rx_ring_slots_available(struct xenvif_queue *queue)
 {
 	RING_IDX prod, cons;
+	struct sk_buff *skb;
 	int needed;
 
-	needed = xenvif_rx_ring_slots_needed(queue->vif);
+	skb = skb_peek(&queue->rx_queue);
+	if (!skb)
+		return false;
+
+	needed = DIV_ROUND_UP(skb->len, XEN_PAGE_SIZE);
+	if (skb_is_gso(skb))
+		needed++;
 
 	do {
 		prod = queue->rx.sring->req_prod;
@@ -2005,8 +2004,7 @@ static bool xenvif_rx_queue_ready(struct xenvif_queue *queue)
 
 static bool xenvif_have_rx_work(struct xenvif_queue *queue)
 {
-	return (!skb_queue_empty(&queue->rx_queue)
-		&& xenvif_rx_ring_slots_available(queue))
+	return xenvif_rx_ring_slots_available(queue)
 		|| (queue->vif->stall_timeout &&
 		    (xenvif_rx_queue_stalled(queue)
 		     || xenvif_rx_queue_ready(queue)))