[Qemu-devel] [V2 PATCH] rtl8139: check the buffer availiability

[Jason Wang <jasowang@redhat.com>]

Reduce spurious packet drops on RX ring empty when in c+ mode by verifying that
we have at least 1 buffer ahead of the time.

Change from v1:
Fix style comments from Stefan.

Signed-off-by: Jason Wang <jasowang@redhat.com>
---
 hw/rtl8139.c |   44 ++++++++++++++++++++++++++++++--------------
 1 files changed, 30 insertions(+), 14 deletions(-)

diff --git a/hw/rtl8139.c b/hw/rtl8139.c
index 3753950..bcbc5e3 100644
--- a/hw/rtl8139.c
+++ b/hw/rtl8139.c
@@ -84,6 +84,19 @@
 #define VLAN_TCI_LEN 2
 #define VLAN_HLEN (ETHER_TYPE_LEN + VLAN_TCI_LEN)
 
+/* w0 ownership flag */
+#define CP_RX_OWN (1<<31)
+/* w0 end of ring flag */
+#define CP_RX_EOR (1<<30)
+/* w0 bits 0...12 : buffer size */
+#define CP_RX_BUFFER_SIZE_MASK ((1<<13) - 1)
+/* w1 tag available flag */
+#define CP_RX_TAVA (1<<16)
+/* w1 bits 0...15 : VLAN tag */
+#define CP_RX_VLAN_TAG_MASK ((1<<16) - 1)
+/* w2 low  32bit of Rx buffer ptr */
+/* w3 high 32bit of Rx buffer ptr */
+
 #if defined (DEBUG_RTL8139)
 #  define DPRINTF(fmt, ...) \
     do { fprintf(stderr, "RTL8139: " fmt, ## __VA_ARGS__); } while (0)
@@ -805,6 +818,22 @@ static inline target_phys_addr_t rtl8139_addr64(uint32_t low, uint32_t high)
 #endif
 }
 
+/* Verify that we have at least one available rx buffer */
+static int rtl8139_cp_has_rxbuf(RTL8139State *s)
+{
+    uint32_t val, rxdw0;
+    target_phys_addr_t cplus_rx_ring_desc = rtl8139_addr64(s->RxRingAddrLO,
+                                                           s->RxRingAddrHI);
+    cplus_rx_ring_desc += 16 * s->currCPlusRxDesc;
+    cpu_physical_memory_read(cplus_rx_ring_desc, &val, 4);
+    rxdw0 = le32_to_cpu(val);
+    if (rxdw0 & CP_RX_OWN) {
+        return 1;
+    } else {
+        return 0;
+    }
+}
+
 static int rtl8139_can_receive(VLANClientState *nc)
 {
     RTL8139State *s = DO_UPCAST(NICState, nc, nc)->opaque;
@@ -819,7 +848,7 @@ static int rtl8139_can_receive(VLANClientState *nc)
     if (rtl8139_cp_receiver_enabled(s)) {
         /* ??? Flow control not implemented in c+ mode.
            This is a hack to work around slirp deficiencies anyway.  */
-        return 1;
+        return rtl8139_cp_has_rxbuf(s);
     } else {
         avail = MOD2(s->RxBufferSize + s->RxBufPtr - s->RxBufAddr,
                      s->RxBufferSize);
@@ -965,19 +994,6 @@ static ssize_t rtl8139_do_receive(VLANClientState *nc, const uint8_t *buf, size_
 
         /* begin C+ receiver mode */
 
-/* w0 ownership flag */
-#define CP_RX_OWN (1<<31)
-/* w0 end of ring flag */
-#define CP_RX_EOR (1<<30)
-/* w0 bits 0...12 : buffer size */
-#define CP_RX_BUFFER_SIZE_MASK ((1<<13) - 1)
-/* w1 tag available flag */
-#define CP_RX_TAVA (1<<16)
-/* w1 bits 0...15 : VLAN tag */
-#define CP_RX_VLAN_TAG_MASK ((1<<16) - 1)
-/* w2 low  32bit of Rx buffer ptr */
-/* w3 high 32bit of Rx buffer ptr */
-
         int descriptor = s->currCPlusRxDesc;
         target_phys_addr_t cplus_rx_ring_desc;