rtl8139: avoid clobbering tx descriptor bits

The device turns the Tx Descriptor into a Tx Status descriptor after
fully reading the descriptor. This involves clearing Tx Own (bit 31) to
indicate that the driver has ownership of the descriptor again as well
as several other bits.

The code keeps the first dword of the Tx Descriptor in the txdw0 local
variable. txdw0 is reused to build the first word of the Tx Status
descriptor. Later on the code uses txdw0 again, incorrectly assuming
that it still contains the first dword of the Tx Descriptor. The tx
offloading code misbehaves because it sees bogus bits in txdw0.

Use a separate local variable for Tx Status and preserve Tx Descriptor
in txdw0.

Acked-by: Jason Wang <jasowang@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-Id: <20221117165554.1773409-2-stefanha@redhat.com>

1 files changed, 10 insertions, 7 deletions

diff --git a/hw/net/rtl8139.c b/hw/net/rtl8139.c
index e6643e3..ffef378 100644
--- a/hw/net/rtl8139.c
+++ b/hw/net/rtl8139.c
@@ -2027,18 +2027,21 @@ static int rtl8139_cplus_transmit_one(RTL8139State *s)
             s->currCPlusTxDesc = 0;
     }
 
+    /* Build the Tx Status Descriptor */
+    uint32_t tx_status = txdw0;
+
     /* transfer ownership to target */
-    txdw0 &= ~CP_TX_OWN;
+    tx_status &= ~CP_TX_OWN;
 
     /* reset error indicator bits */
-    txdw0 &= ~CP_TX_STATUS_UNF;
-    txdw0 &= ~CP_TX_STATUS_TES;
-    txdw0 &= ~CP_TX_STATUS_OWC;
-    txdw0 &= ~CP_TX_STATUS_LNKF;
-    txdw0 &= ~CP_TX_STATUS_EXC;
+    tx_status &= ~CP_TX_STATUS_UNF;
+    tx_status &= ~CP_TX_STATUS_TES;
+    tx_status &= ~CP_TX_STATUS_OWC;
+    tx_status &= ~CP_TX_STATUS_LNKF;
+    tx_status &= ~CP_TX_STATUS_EXC;
 
     /* update ring data */
-    val = cpu_to_le32(txdw0);
+    val = cpu_to_le32(tx_status);
     pci_dma_write(d, cplus_tx_ring_desc, (uint8_t *)&val, 4);
 
     /* Now decide if descriptor being processed is holding the last segment of packet */