[virt-tools-list] libvirt and virt-manager default Xen blktap driver name for file images, tap vs tap2?

[Cole Robinson]

On 06/09/2013 01:07 PM, Pasi Kärkkäinen wrote:
> Hello,
> 
> Does anyone know where/how to set the default libvirt Xen blktap "driver name" for disk image files?
> 
> If I have "blktap" module loaded in dom0 kernel, and I do this:
> 
> virt-install -d -n blktaptest01 -r 2048 --vcpus=2 --disk /var/lib/libvirt/images/blktaptest01.img,size=8 --vnc -p -l "http://ftp.funet.fi/pub/mirrors/centos.org/6.4/os/x86_64"
> 
> which results in this generated libvirt xml config for the VM:
> 
>     <disk type='file' device='disk'>
>       <driver name='tap' type='aio'/>
>       <source file='/var/lib/libvirt/images/blktaptest01.img'/>
>       <target dev='xvda' bus='xen'/>
>     </disk>
> 
> .. which doesn't seem to work for me with Xen 4.2.2 using xm/xend, and libvirt 0.10.x.
> 
> The problem is there's no tapdisk process started in dom0, and the block device cannot be 
> initialized or accessed inside the VM. I have the same problem with virt-manager, 
> using the "new VM" wizard with disk image files.
> 
> The correct "driver name" for libvirt is "tap2", which works for me, 
> so I need to manually edit the libvirt xml config to make the VM boot/work:
> 
>     <disk type='file' device='disk'>
>       <driver name='tap2' type='aio'/>
>       <source file='/var/lib/libvirt/images/blktaptest01.img'/>
>       <target dev='xvda' bus='xen'/>
>     </disk>
> 
> This matches xm/xend behaviour with text config files for VMs. 
> I need to use tap2 there aswell on the disk = [] line.
> 
> Where should I choose/set/patch the default libvirt driver name? Is that in libvirt sources,
> or in virt-install/virt-manager ? I tried reading the libvirt "xen" driver source, 
> but I couldn't find any "default" setting there.. 
> 

Hmm. This is old old behavior in virtinst. If the blktap module is loaded on
the local host we use the 'tap' driver.

According to the libvirt log, tap2 has been available since xen 4.0, so
virtinst should probably check if xen >= 4.0 and libvirt >= 0.9.7, which is
the first libvirt version where it sounds like tap2 is usable. This would be a
check in virtinst/support.py

You should also be able to manually work around it by doing --disk
...,driver_name=tap2,driver_type=aio

- Cole