Re: Receiving delayed packets from RTL8139 card in KVM

[Avi Kivity <avi@redhat.com>]

  On 08/20/2010 04:01 AM, Karthik Vadambacheri Manian wrote:
> 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.
>

There may be a missing wakeup in the networking path.  You can try 
adding printf()s in RTL8139's receive function, and in 
kvm_set_irq_level() to see if there's a problem in interrupt flow.

-- 
error compiling committee.c: too many arguments to function