hw/net/spapr_llan: Provide counter with dropped rx frames to the guest

The last 8 bytes of the receive buffer list page (that has been supplied
by the guest with the H_REGISTER_LOGICAL_LAN call) contain a counter
for frames that have been dropped because there was no suitable receive
buffer available. This patch introduces code to use this field to
provide the information about dropped rx packets to the guest.
There it can be queried with "ethtool -S eth0 | grep rx_no_buffer".
Signed-off-by: NThomas Huth <thuth@redhat.com>
Signed-off-by: NDavid Gibson <david@gibson.dropbear.id.au>

hw/net/spapr_llan.c

@@ -122,6 +122,21 @@ static int spapr_vlan_can_receive(NetClientState *nc)
     return (dev->isopen && dev->rx_bufs > 0);
 }
 
+/**
+ * The last 8 bytes of the receive buffer list page (that has been
+ * supplied by the guest with the H_REGISTER_LOGICAL_LAN call) contain
+ * a counter for frames that have been dropped because there was no
+ * suitable receive buffer available. This function is used to increase
+ * this counter by one.
+ */
+static void spapr_vlan_record_dropped_rx_frame(VIOsPAPRVLANDevice *dev)
+{
+    uint64_t cnt;
+
+    cnt = vio_ldq(&dev->sdev, dev->buf_list + 4096 - 8);
+    vio_stq(&dev->sdev, dev->buf_list + 4096 - 8, cnt + 1);
+}
+
 /**
  * Get buffer descriptor from one of our receive buffer pools
  */
@@ -207,6 +222,7 @@ static ssize_t spapr_vlan_receive(NetClientState *nc, const uint8_t *buf,
     }
 
     if (!dev->rx_bufs) {
+        spapr_vlan_record_dropped_rx_frame(dev);
         return 0;
     }
 
@@ -216,6 +232,7 @@ static ssize_t spapr_vlan_receive(NetClientState *nc, const uint8_t *buf,
         bd = spapr_vlan_get_rx_bd_from_page(dev, size);
     }
     if (!bd) {
+        spapr_vlan_record_dropped_rx_frame(dev);
         return 0;
     }