Receiving delayed packets from RTL8139 card in KVM

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Hi All,

I am running a light weight kernel named kitten in QEMU-KVM which uses
LWIP (Light Weight TCP/IP) stack for networking. I am trying to fine
tune this LWIP but I am now facing an issue regarding delayed packet
reception. Basically I wrote a small pingpong program where master
sends a data to a slave and slave returns it back. But I see some
retransmissions due to missing acknowledgement. When I debugged
further I came to know the following:

1) Master sends some data to Slave
2) Slave receives it and sends an acknowledgement to Master. Slave
then returns the same data to Master
3) Master receives both acknowledgement and the data

The problem occurs in the 3 step. The master receives a single
interrupt for both the acknowledgement packet and data packet. So the
driver tries to read and reads the acknowledgement packet first and
then checks again whether there is more data in the buffer and it
reads the data packet. But sometimes the data packet is available in
the buffer after 90-100 microseconds after acknowledgement packet,
hence the driver thinks there is no more packet to receive. So the
receive is failed. This causes the slave to resend the packet which
leads to great performance loss. And from the debug messages it is
sure that there is no delay from the slave side in sending the data
packet.

When I run the same program in QEMU everything is running perfectly. I
think it is because the emulation itself causes a long delay within
which the packet arrives at the buffer and hence the scenario is not
reproduced. Hence I would like to know whether KVM has issues with the
RTL8139 card? and how to proceed further.

Thanks in advance for your time,
karthik
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