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

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Message-ID: <1368703672.15129.1501.camel@eboracum.office.bytemark.co.uk>
Date:	Thu, 16 May 2013 12:27:52 +0100
From:	Nicholas Thomas <nick@...emark.co.uk>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	Peter Lieven <pl@...net.de>, 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

On Thu, 2013-05-16 at 11:40 +0300, Michael S. Tsirkin wrote:
> On Thu, May 16, 2013 at 09:20:55AM +0100, Nicholas Thomas wrote:
> > Hi,
> > 
> > On Thu, 2013-05-16 at 09:27 +0300, Michael S. Tsirkin wrote:
> > > On Thu, May 16, 2013 at 09:24:05AM +0300, Michael S. Tsirkin wrote:
> > > > Is this with or without vhost-net in host?
> > > 
> > > never mind, I see it's without.
> > > Try to enable vhost-net (you'll have to switch to -netdev syntax
> > > for that to work) and see if this help.
> > > If it does it's likely a qemu bug if not probably a guest bug.
> > 
> > Switching to -netdev is non-trivial for me, unfortunately.
> 
> Interesting. Why is that?

Our setup is bond0 <-> vlanX <-> bridgeX <-> [ tap devices ] and we do
all that outside of qemu at the moment, specifying -net tap,ifname=... -
we also run some processes on the TAP interface and insert a bunch of
ebtables rules between creating it and starting qemu. Duplicating that
with -net bridge seemed close to impossible, and -netdev tap was
throwing EBUSY from /dev/net/tun. I guess our external magic should be
using ,fd= instead.

> > Anyway, it's
> > definitely a qemu bug - it happens on kernels 3.2 and 3.9 with 1.4.1,
> > but doesn't happen with qemu 0.15.0 or 1.5.0rc1.
> > 
> > I'll have a dig through git to see if I can identify the patch that
> > resolves it. It feels-like qemu sometimes stops reading from the tap
> > file descriptor between ipxe exiting and the linux kernel bringing up
> > the network interface, and never recovers from that.
> > 
> > /Nick
> 
> You can try to bisect, yes.

Work have decided to accept 1.5.0 when it arrives instead, so I'm afraid
I won't be working on this after all. 

/Nick



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