[Qemu-devel] [PATCH 04/17] imx_fec: Change queue flushing heuristics

[Andrey Smirnov <andrew.smirnov@gmail.com>]

In current implementation, packet queue flushing logic seem to suffer
from a deadlock like scenario if a packet is received by the interface
before before Rx ring is initialized by Guest's driver. Consider the
following sequence of events:

	1. A QEMU instance is started against a TAP device on Linux
	   host, running Linux guest, e. g., something to the effect
	   of:

	   qemu-system-arm \
	      -net nic,model=imx.fec,netdev=lan0 \
	      netdev tap,id=lan0,ifname=tap0,script=no,downscript=no \
	      ... rest of the arguments ...

	2. Once QEMU starts, but before guest reaches the point where
	   FEC deriver is done initializing the HW, Guest, via TAP
	   interface, receives a number of multicast MDNS packets from
	   Host (not necessarily true for every OS, but it happens at
	   least on Fedora 25)

	3. Recieving a packet in such a state results in
	   imx_eth_can_receive() returning '0', which in turn causes
	   tap_send() to disable corresponding event (tap.c:203)

	4. Once Guest's driver reaches the point where it is ready to
	   recieve packets it prepares Rx ring descriptors and writes
	   ENET_RDAR_RDAR to ENET_RDAR register to indicate to HW that
	   more descriptors are ready. And at this points emulation
	   layer does this:

	   	 s->regs[index] = ENET_RDAR_RDAR;
                 imx_eth_enable_rx(s);

	   which, combined with:

	   	  if (!s->regs[ENET_RDAR]) {
		     qemu_flush_queued_packets(qemu_get_queue(s->nic));
		  }

	   results in Rx queue never being flushed and corresponding
	   I/O event beign disabled.

Change the code to remember the fact that can_receive callback was
called before Rx ring was ready and use it to make a decision if
receive queue needs to be flushed.

Cc: Peter Maydell <peter.maydell@linaro.org>
Cc: Jason Wang <jasowang@redhat.com>
Cc: qemu-devel@nongnu.org
Cc: qemu-arm@nongnu.org
Cc: yurovsky@gmail.com
Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
---
 hw/net/imx_fec.c         | 6 ++++--
 include/hw/net/imx_fec.h | 1 +
 2 files changed, 5 insertions(+), 2 deletions(-)

diff --git a/hw/net/imx_fec.c b/hw/net/imx_fec.c
index 84085afe09..767402909d 100644
--- a/hw/net/imx_fec.c
+++ b/hw/net/imx_fec.c
@@ -544,8 +544,9 @@ static void imx_eth_enable_rx(IMXFECState *s)
 
     if (rx_ring_full) {
         FEC_PRINTF("RX buffer full\n");
-    } else if (!s->regs[ENET_RDAR]) {
+    } else if (s->needs_flush) {
         qemu_flush_queued_packets(qemu_get_queue(s->nic));
+        s->needs_flush = false;
     }
 
     s->regs[ENET_RDAR] = rx_ring_full ? 0 : ENET_RDAR_RDAR;
@@ -930,7 +931,8 @@ static int imx_eth_can_receive(NetClientState *nc)
 
     FEC_PRINTF("\n");
 
-    return s->regs[ENET_RDAR] ? 1 : 0;
+    s->needs_flush = !s->regs[ENET_RDAR];
+    return !!s->regs[ENET_RDAR];
 }
 
 static ssize_t imx_fec_receive(NetClientState *nc, const uint8_t *buf,
diff --git a/include/hw/net/imx_fec.h b/include/hw/net/imx_fec.h
index 62ad473b05..4bc8f03ec2 100644
--- a/include/hw/net/imx_fec.h
+++ b/include/hw/net/imx_fec.h
@@ -252,6 +252,7 @@ typedef struct IMXFECState {
     uint32_t phy_int_mask;
 
     bool is_fec;
+    bool needs_flush;
 } IMXFECState;
 
 #endif
-- 
2.13.5