First some background information on the server:

Host OS: Debian Linux Squeeze + qemu-kvm version 1.0+dfsg-8~bpo60+1

Guest OS: Debian Linux Squeeze

I use qemu-kvm via libvirt. I have set up 9p VirtFS with the following in Guest's XML config:

<filesystem type='mount' accessmode='passthrough'>
  <source dir='/srv/www'/>
  <target dir='wwwdata'/>
  <address type='pci' domain='0x0000' bus='0x00' slot='0x06' function='0x0'/>
</filesystem>

That is, I want to share /srv/www to the guest OS using mount tag wwwdata.

When I try to mount the VirtFS share from the guest, I get an error message:

root@server:~# mount -t 9p -o trans=virtio,version=9p2000.L2 wwwdata /srv/www/
mount: wwwdata: can't read superblock

I also tried virtfs target dir/mount_tag www at first. I got the same error message. However, I was able to mount the VirtFS share using mount tag www1111, or www1 or similar.

Some more notes on this one. dmesg doesn't show anything useful either in guest or the host. The only sign is this entry in the guest dmesg:

[ 36.054936] Installing v9fs 9p2000 file system support

Does anyone know how to get this working correctly? Google gives no useful information on this issue; I've tried several searches.