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imx_fec: Change queue flushing heuristics

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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.

To prevent the problem, change the code to always flush packet queue
when ENET_RDAR transitions 0 -> ENET_RDAR_RDAR.

Cc: Peter Maydell <peter.maydell@linaro.org>
Cc: Jason Wang <jasowang@redhat.com>
Cc: Philippe Mathieu-Daudé <f4bug@amsat.org>
Cc: qemu-devel@nongnu.org
Cc: qemu-arm@nongnu.org
Cc: yurovsky@gmail.com
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
This commit is contained in:
Andrey Smirnov 2018-01-11 13:25:35 +00:00 committed by Peter Maydell
parent a6383e99ff
commit b2b012afdd
1 changed files with 6 additions and 6 deletions
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12
hw/net/imx_fec.c
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@ -533,7 +533,7 @@ static void imx_eth_do_tx(IMXFECState *s)
} }
} }
static void imx_eth_enable_rx(IMXFECState *s) static void imx_eth_enable_rx(IMXFECState *s, bool flush)
{ {
IMXFECBufDesc bd; IMXFECBufDesc bd;
bool rx_ring_full; bool rx_ring_full;
@ -544,7 +544,7 @@ static void imx_eth_enable_rx(IMXFECState *s)
if (rx_ring_full) { if (rx_ring_full) {
FEC_PRINTF("RX buffer full\n"); FEC_PRINTF("RX buffer full\n");
} else if (!s->regs[ENET_RDAR]) { } else if (flush) {
qemu_flush_queued_packets(qemu_get_queue(s->nic)); qemu_flush_queued_packets(qemu_get_queue(s->nic));
} }
@ -807,7 +807,7 @@ static void imx_eth_write(void *opaque, hwaddr offset, uint64_t value,
if (s->regs[ENET_ECR] & ENET_ECR_ETHEREN) { if (s->regs[ENET_ECR] & ENET_ECR_ETHEREN) {
if (!s->regs[index]) { if (!s->regs[index]) {
s->regs[index] = ENET_RDAR_RDAR; s->regs[index] = ENET_RDAR_RDAR;
imx_eth_enable_rx(s); imx_eth_enable_rx(s, true);
} }
} else { } else {
s->regs[index] = 0; s->regs[index] = 0;
@ -930,7 +930,7 @@ static int imx_eth_can_receive(NetClientState *nc)
FEC_PRINTF("\n"); FEC_PRINTF("\n");
return s->regs[ENET_RDAR] ? 1 : 0; return !!s->regs[ENET_RDAR];
} }
static ssize_t imx_fec_receive(NetClientState *nc, const uint8_t *buf, static ssize_t imx_fec_receive(NetClientState *nc, const uint8_t *buf,
@ -1020,7 +1020,7 @@ static ssize_t imx_fec_receive(NetClientState *nc, const uint8_t *buf,
} }
} }
s->rx_descriptor = addr; s->rx_descriptor = addr;
imx_eth_enable_rx(s); imx_eth_enable_rx(s, false);
imx_eth_update(s); imx_eth_update(s);
return len; return len;
} }
@ -1116,7 +1116,7 @@ static ssize_t imx_enet_receive(NetClientState *nc, const uint8_t *buf,
} }
} }
s->rx_descriptor = addr; s->rx_descriptor = addr;
imx_eth_enable_rx(s); imx_eth_enable_rx(s, false);
imx_eth_update(s); imx_eth_update(s);
return len; return len;
} }