netdev - Re: user space virtio-net exits with "truncating packet" error

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Date:	Thu, 14 May 2009 08:34:02 -0700
From:	Sridhar Samudrala <sri@...ibm.com>
To:	Rusty Russell <rusty@...tcorp.com.au>
Cc:	Or Gerlitz <ogerlitz@...taire.com>, Avi Kivity <avi@...hat.com>,
	netdev@...r.kernel.org, Gregory Haskins <ghaskins@...ell.com>,
	Anthony Liguori <anthony@...emonkey.ws>
Subject: Re: user space virtio-net exits with "truncating packet" error

On Thu, 2009-05-14 at 21:28 +0930, Rusty Russell wrote:
> On Thu, 14 May 2009 07:37:42 pm Or Gerlitz wrote:
> > Hi,
> >
> > When running with jumbo frames (i.e set tap0 and guest nic mtu to 9k)
> > and using 8k sized packets with iperf, the qemu process exits with
> > "virtio-net truncating packet" which I see in the code of
> > qemu/hw/virtio-net.c
> >
> > :: virtio_net_receive(). This happens only when the VM is receiving, if I
> >
> > send 8K packets from the VM things go fine.
> >
> > I use virtio based NIC in the VM and Linux 2.6.29.1 in both the VM and the
> > host. Qemu is the one provided by kvm release 84 - whose sources don't
> > point me to a specific git tree nor a maintainer, so I hope you can help
> > me...
> 
> Maintainer Cc'd.  The answer is that virtio_net by default only supports 1500 
> MTU; I've not tried larger MTUs.


I am able to get virtio-net to work with an MTU upto 65521 between host and guest 
using qemu-kvm git tree and linux-2.6.30-rc5 running on the host and guest.
I didn't run into any issues while running iperf or netperf  with larger than 8K
message sizes.

I changed the mtu of the bridge and tap device on the host and the guest 
virtio-net device to 65521.

Thanks
Sridhar

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