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

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Date:	Wed, 15 May 2013 12:00:03 +0100
From:	Nicholas Thomas <nick@...emark.co.uk>
To:	Peter Lieven <pl@...net.de>
Cc:	"Michael S. Tsirkin" <mst@...hat.com>,
	Stefan Hajnoczi <stefanha@...il.com>, qemu-devel@...gnu.org,
	netdev@...r.kernel.org
Subject: Re: [Qemu-devel] tap devices not receiving packets from a bridge

Hi again,

On Tue, 2013-05-14 at 15:49 +0100, Nicholas Thomas wrote:
> /sys/devices/virtual/net/t100/tun_flags is 0x5002 - so it looks like
> IFF_ONE_QUEUE was indeed unset by qemu (which is lacking the patch). It
> surprises me, but that's probably my fault, rather than qemu's.

I've rebuilt 1.4.1 with the IFF_ONE_QUEUE patch and tun_flags is now
0x7002; unfortunately, I'm still seeing this bug, twice in five trials.
Symptoms in `ifconfig t100` now differ; overruns stays at 0, and
"dropped" increases monotonically as I send packets. Those packets do
appear if I tcpdump t100 on the host, but not if I tcpdump t100 on the
guest.

I've turned off gro in the guest, which makes no difference, and tried
changing the queue sizes (post-hoc) in both guest and host, in the hope
of causing them to be emptied out, clearing the condition; again to no
effect.

The VMs in question are bridged to a large (and busy) VLAN with no
ingress filtering to speak of; I guess what's happening is that the
transmit queue is filled up by that traffic while the guest is in ipxe,
and it never gets out of that state when it happens... so maybe there is
still an underlying problem?

/Nick

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