hw/net/npcm_gmac.c: Send the right data for second packet in a row

The transmit loop in gmac_try_send_next_packet() is constructed in a
way that means it will send incorrect data if it it sends more than
one packet.

The function assembles the outbound data in a dynamically allocated
block of memory which is pointed to by tx_send_buffer.  We track the
first point in this block of memory which is not yet used with the
prev_buf_size offset, initially zero.  We track the size of the
packet we're sending with the length variable, also initially zero.

As we read chunks of data out of guest memory, we write them to
tx_send_buffer[prev_buf_size], and then increment both prev_buf_size
and length.  (We might dynamically reallocate the buffer if needed.)

When we send a packet, we checksum and send length bytes, starting at
tx_send_buffer, and then we reset length to 0.  This gives the right
data for the first packet.  But we don't reset prev_buf_size.  This
means that if we process more descriptors with further data for the
next packet, that data will continue to accumulate at offset
prev_buf_size, i.e.  after the data for the first packet.  But when
we transmit that second packet, we send length bytes from
tx_send_buffer, so we will send a packet which has the length of the
second packet but the data of the first one.

The fix for this is to also clear prev_buf_size after the packet has
been sent -- we never need the data from packet one after we've sent
it, so we can write packet two's data starting at the beginning of
the buffer.

Cc: [email protected]
Signed-off-by: Peter Maydell <[email protected]>
Signed-off-by: Jason Wang <[email protected]>
(cherry picked from commit 871a6e5b339f0b5e71925ec7d3f452944a1c82d3)
Signed-off-by: Michael Tokarev <[email protected]>

Author: pm215

hw/net/npcm_gmac.c

@@ -615,6 +615,7 @@ static void gmac_try_send_next_packet(NPCMGMACState *gmac)
             trace_npcm_gmac_packet_sent(DEVICE(gmac)->canonical_path, length);
             buf = tx_send_buffer;
             length = 0;
+            prev_buf_size = 0;
         }
 
         /* step 6 */