2 files changed, 41 insertions, 15 deletions

diff --git a/src/drivers/net/netfront.c b/src/drivers/net/netfront.c
index 2f4bbf2a..b6205542 100644
--- a/src/drivers/net/netfront.c
+++ b/src/drivers/net/netfront.c
@@ -511,15 +511,12 @@ static void netfront_refill_rx ( struct net_device *netdev ) {
 	struct xen_device *xendev = netfront->xendev;
 	struct io_buffer *iobuf;
 	struct netif_rx_request *request;
+	unsigned int refilled = 0;
 	int notify;
 	int rc;
 
-	/* Do nothing if ring is already full */
-	if ( netfront_ring_is_full ( &netfront->rx ) )
-		return;
-
 	/* Refill ring */
-	do {
+	while ( netfront_ring_fill ( &netfront->rx ) < NETFRONT_RX_FILL ) {
 
 		/* Allocate I/O buffer */
 		iobuf = alloc_iob ( PAGE_SIZE );
@@ -543,13 +540,17 @@ static void netfront_refill_rx ( struct net_device *netdev ) {
 
 		/* Move to next descriptor */
 		netfront->rx_fring.req_prod_pvt++;
+		refilled++;
 
-	} while ( ! netfront_ring_is_full ( &netfront->rx ) );
+	}
 
 	/* Push new descriptors and notify backend if applicable */
-	RING_PUSH_REQUESTS_AND_CHECK_NOTIFY ( &netfront->rx_fring, notify );
-	if ( notify )
-		netfront_send_event ( netfront );
+	if ( refilled ) {
+		RING_PUSH_REQUESTS_AND_CHECK_NOTIFY ( &netfront->rx_fring,
+						      notify );
+		if ( notify )
+			netfront_send_event ( netfront );
+	}
 }
 
 /**
diff --git a/src/drivers/net/netfront.h b/src/drivers/net/netfront.h
index 38fd0a77..c95ed264 100644
--- a/src/drivers/net/netfront.h
+++ b/src/drivers/net/netfront.h
@@ -16,7 +16,20 @@ FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
 #define NETFRONT_NUM_TX_DESC 16
 
 /** Number of receive ring entries */
-#define NETFRONT_NUM_RX_DESC 8
+#define NETFRONT_NUM_RX_DESC 32
+
+/** Receive ring fill level
+ *
+ * The xen-netback driver from kernels 3.18 to 4.2 inclusive have a
+ * bug (CA-163395) which prevents packet reception if fewer than 18
+ * receive descriptors are available.  This was fixed in upstream
+ * kernel commit d5d4852 ("xen-netback: require fewer guest Rx slots
+ * when not using GSO").
+ *
+ * We provide 18 receive descriptors to avoid unpleasant silent
+ * failures on these kernel versions.
+ */
+#define NETFRONT_RX_FILL 18
 
 /** Grant reference indices */
 enum netfront_ref_index {
@@ -89,6 +102,21 @@ netfront_init_ring ( struct netfront_ring *ring, const char *ref_key,
 }
 
 /**
+ * Calculate descriptor ring fill level
+ *
+ * @v ring		Descriptor ring
+ * @v fill		Fill level
+ */
+static inline __attribute__ (( always_inline )) unsigned int
+netfront_ring_fill ( struct netfront_ring *ring ) {
+	unsigned int fill_level;
+
+	fill_level = ( ring->id_prod - ring->id_cons );
+	assert ( fill_level <= ring->count );
+	return fill_level;
+}
+
+/**
  * Check whether or not descriptor ring is full
  *
  * @v ring		Descriptor ring
@@ -96,11 +124,8 @@ netfront_init_ring ( struct netfront_ring *ring, const char *ref_key,
  */
 static inline __attribute__ (( always_inline )) int
 netfront_ring_is_full ( struct netfront_ring *ring ) {
-	unsigned int fill_level;
 
-	fill_level = ( ring->id_prod - ring->id_cons );
-	assert ( fill_level <= ring->count );
-	return ( fill_level >= ring->count );
+	return ( netfront_ring_fill ( ring ) >= ring->count );
 }
 
 /**
@@ -112,7 +137,7 @@ netfront_ring_is_full ( struct netfront_ring *ring ) {
 static inline __attribute__ (( always_inline )) int
 netfront_ring_is_empty ( struct netfront_ring *ring ) {
 
-	return ( ring->id_prod == ring->id_cons );
+	return ( netfront_ring_fill ( ring ) == 0 );
 }
 
 /** A netfront NIC */