net: smc91c111: gate can_receive() on rx FIFO having a slot

Return false from can_receive() when the FIFO doesn't have a free RX
slot. This fixes a bug in the current code where the allocated buffer
is freed before the fifo pop, triggering a premature flush of queued RX
packets. It also will handle a corner case, where the guest manually
frees the allocated buffer before popping the rx FIFO (hence it is not
enough to just delay the flush_queued_packets()).
Reported-by: NRichard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: NPeter Crosthwaite <crosthwaite.peter@gmail.com>
Reviewed-by: NFam Zheng <famz@redhat.com>
Tested-by: NRichard Purdie <richard.purdie@linuxfoundation.org>
Message-id: 97bfdfc5cbce0bd5e0cbbbff35ce7a1bf6f8603d.1441873621.git.crosthwaite.peter@gmail.com
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>

hw/net/smc91c111.c

@@ -129,7 +129,8 @@ static int smc91c111_can_receive(smc91c111_state *s)
     if ((s->rcr & RCR_RXEN) == 0 || (s->rcr & RCR_SOFT_RST)) {
         return 1;
     }
-    if (s->allocated == (1 << NUM_PACKETS) - 1) {
+    if (s->allocated == (1 << NUM_PACKETS) - 1 ||
+        s->rx_fifo_len == NUM_PACKETS) {
         return 0;
     }
     return 1;
@@ -182,6 +183,7 @@ static void smc91c111_pop_rx_fifo(smc91c111_state *s)
     } else {
         s->int_level &= ~INT_RCV;
     }
+    smc91c111_flush_queued_packets(s);
     smc91c111_update(s);
 }