netdev - Re: [Qemu-devel] tap devices not receiving packets from a bridge

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Date:	Fri, 23 Nov 2012 10:41:21 +0100
From:	Peter Lieven <pl@...net.de>
To:	Stefan Hajnoczi <stefanha@...il.com>
Cc:	qemu-devel@...gnu.org, netdev@...r.kernel.org, mst@...hat.com
Subject: Re: [Qemu-devel] tap devices not receiving packets from a bridge

Am 23.11.2012 um 08:02 schrieb Stefan Hajnoczi:

> On Thu, Nov 22, 2012 at 03:29:52PM +0100, Peter Lieven wrote:
>> is anyone aware of a problem with the linux network bridge that in very rare circumstances stops
>> a bridge from sending pakets to a tap device?
>> 
>> My problem occurs in conjunction with vanilla qemu-kvm-1.2.0 and Ubuntu Kernel 3.2.0-34.53
>> which is based on Linux 3.2.33.
>> 
>> I was not yet able to reproduce the issue, it happens in really rare cases. The symptom is that
>> the tap does not have any TX packets. RX is working fine. I see the packets coming in at
>> the physical interface on the host, but they are not forwarded to the tap interface.
>> The bridge itself has learnt the mac address of the vServer that is connected to the tap interface.
>> It does not help to toggle the bridge link status,  the tap interface status or the interface in the vServer.
>> It seems that problem occurs if a tap interface that has previously been used, but set to nonpersistent
>> is set persistent again and then is by chance assigned to the same vServer (=same mac address on same
>> bridge) again. Unfortunately it seems not to be reproducible.
> 
> Not sure but this patch from Michael Tsirkin may help - it solves an
> issue with persistent tap devices:
> 
> http://patchwork.ozlabs.org/patch/198598/

Hi Stefan,

thanks for the pointer. I have seen this patch, but I have neglected it because it was dealing
with persistent taps. But maybe the taps in the kernel are not deleted directly. 
Can you remember what the syptomps of the above issue have been? Sorry for
being vague, but I currently have no clue whats going on.

Can someone who has more internal knowledge of the bridging/tap code say if qemu can
be responsible at all if the tap device is not receiving packets from the bridge.

If I have the following config. Lets say packets coming in via physical interface eth1.123,
and a bridge called br123.I further have a virtual machine with tap0. Both eth1.123
and tap0 are member of br123. 

If the issue occurs the vServer has no network connectivity inbound. If I sent a ping
from the vServer I see it on tap0 and leaving on eth1.123. I see further the arp reply coming
in via eth1.123, but the reply can't be seen on tap0.

Peter

> 
> Stefan

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