vmbus: add prefetch to ring buffer iterator

When iterating over incoming ring elements from the host, prefetch
the next descriptor so that it is cache hot.

Signed-off-by: Stephen Hemminger <sthemmin@microsoft.com>
Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/hv/ring_buffer.c

@@ -357,11 +357,16 @@ static u32 hv_pkt_iter_avail(const struct hv_ring_buffer_info *rbi)
 struct vmpacket_descriptor *hv_pkt_iter_first(struct vmbus_channel *channel)
 {
 	struct hv_ring_buffer_info *rbi = &channel->inbound;
+	struct vmpacket_descriptor *desc;
 
 	if (hv_pkt_iter_avail(rbi) < sizeof(struct vmpacket_descriptor))
 		return NULL;
 
-	return hv_get_ring_buffer(rbi) + rbi->priv_read_index;
+	desc = hv_get_ring_buffer(rbi) + rbi->priv_read_index;
+	if (desc)
+		prefetch((char *)desc + (desc->len8 << 3));
+
+	return desc;
 }
 EXPORT_SYMBOL_GPL(hv_pkt_iter_first);