Re: [Qemu-devel] [PATCH] rtl8139: flush queued packets when RxBufPtr is written

[Oliver Francke]

Well,

Am 27.05.2013 um 08:15 schrieb Peter Lieven <address@hidden>:

> Hi all,
> 
> I ocassionally have seen a probably related problem in the past. It mainly 
> happend with rtl8139 under
> WinXP where we most likely use rtl8139 due to lack of shipped e1000 drivers.
> 
> My question is if you see increasing dropped packets on the tap device if 
> this problem occurs?
> 
> tap36     Link encap:Ethernet  HWaddr b2:84:23:c0:e2:c0
>          inet6 addr: fe80::b084:23ff:fec0:e2c0/64 Scope:Link
>          UP BROADCAST RUNNING PROMISC MULTICAST  MTU:1500  Metric:1
>          RX packets:5816096 errors:0 dropped:0 overruns:0 frame:0
>          TX packets:3878744 errors:0 dropped:13775 overruns:0 carrier:0
>          collisions:0 txqueuelen:500
>          RX bytes:5161769434 (5.1 GB)  TX bytes:380415916 (380.4 MB)
> 
> In my case as well the only option to recover without shutting down the whole 
> vServer is Live Migration
> to another Node.
> 

ACK, tried it and every network-devices might have been re-created into a 
defined state qemu-wise.

> However, I also see this problem under qemu-kvm-1.2.0 while Oliver reported 
> it does not happen there.
> 

Neither me nor any  affected customers have ever seen such failures in 
qemu-1.2.0, so this was my last-known-good ;)

Oliver.

> Thank you,
> Peter
> 
> On 22.05.2013 14:50, Stefan Hajnoczi wrote:
>> Net queues support efficient "receive disable".  For example, tap's file
>> descriptor will not be polled while its peer has receive disabled.  This
>> saves CPU cycles for needlessly copying and then dropping packets which
>> the peer cannot receive.
>> 
>> rtl8139 is missing the qemu_flush_queued_packets() call that wakes the
>> queue up when receive becomes possible again.
>> 
>> As a result, the Windows 7 guest driver reaches a state where the
>> rtl8139 cannot receive packets.  The driver has actually refilled the
>> receive buffer but we never resume reception.
>> 
>> The bug can be reproduced by running a large FTP 'get' inside a Windows
>> 7 guest:
>> 
>>   $ qemu -netdev tap,id=tap0,...
>>          -device rtl8139,netdev=tap0
>> 
>> The Linux guest driver does not trigger the bug, probably due to a
>> different buffer management strategy.
>> 
>> Reported-by: Oliver Francke <address@hidden>
>> Signed-off-by: Stefan Hajnoczi <address@hidden>
>> ---
>>  hw/net/rtl8139.c | 3 +++
>>  1 file changed, 3 insertions(+)
>> 
>> diff --git a/hw/net/rtl8139.c b/hw/net/rtl8139.c
>> index 9369507..7993f9f 100644
>> --- a/hw/net/rtl8139.c
>> +++ b/hw/net/rtl8139.c
>> @@ -2575,6 +2575,9 @@ static void rtl8139_RxBufPtr_write(RTL8139State *s, 
>> uint32_t val)
>>      /* this value is off by 16 */
>>      s->RxBufPtr = MOD2(val + 0x10, s->RxBufferSize);
>>  +    /* more buffer space may be available so try to receive */
>> +    qemu_flush_queued_packets(qemu_get_queue(s->nic));
>> +
>>      DPRINTF(" CAPR write: rx buffer length %d head 0x%04x read 0x%04x\n",
>>          s->RxBufferSize, s->RxBufAddr, s->RxBufPtr);
>>  }
>