[Xen-users] how to fix problems with ioemu, tap, windows and qemu.

[Ryan Worsley <R.C.Worsley@xxxxxxxxxxxx>]

Hey all,

I've just finished getting windows xp working properly in xen under 
RHELx 2.6.9-42.0.2.  The primary problem I had was that the domain would 
be created but wouldn't get any further than that, an xm list would show 
the domain but the running time would not increase and the status would 
not be listed.  This is a bad thing.  Looking through the logs isn't 
very revealing until you look at the qemu logs (in /var/log/xen).  The 
show the following error;
domid: 2
qemu: the number of cpus is 1
/etc/xen/qemu-ifup: could not launch network script
Could not initialize device 'tap'

Some looking around taught me that I didn't have the tap device 
installed, so I went and installed it.  You can load it as a kernel 
module by doing the usual make menuconfig and going into Device drivers 
--> networking --> Universal TUN/TAP adapter.  Once installed it can be 
loaded with the usual insmod or modprobe tun.  That should be it, 
however it isn't the problem doesn't go away quite that easy.  One may 
then notice that attempting to do
ifconfig tap0 0.0.0.0 up

which is what is done by /etc/xen/qemu-ifup will result in an error 
along the lines of no such device tap0.  It turns out there's a UML tool 
called tunctl that allows you to create a tap device with something like;
tunctl -d tap0

You can then create interfaces and everything is lovely :D  If you, like 
me, don't have this tunctl tool, then you should google for tunctl.c and 
its makefile.  You can then create it and put it somewhere pretty like /bin.
However this still isn't the end of the story, I found that to get 
things to work properly I had to modify the qemu-ifup script to do the 
following;
/bin/tunctl -d $1
ifconfig $1 0.0.0.0 up
brctl addif $2 $1

This simply attempts to create the tap interface tap0 (normally) and 
then attempts to configure this.  Which is all groovy.
So this is all as cool as a big tree.

Thanks and good luck to whoever might read this.  Any comments welcome.

Ryan.


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