[libvirt] Problem configuring selective dropping of root

[Stephan von Krawczynski]

Hello list,

I came across a fundamental flaw in the libvirt user configuration lately and
try to find a solution now. Here is the problem:
I run several qemu instances on arch linux all configured via libvirt. The
default config as user nobody:kvm was fine up to the day I tried to use a host
filesystem via 9p. If you want to gain all user rights on the guest inside
that fs you have to run qemu as root. So far so good. But if you have several
qemus running and only one needs to be root, what to do? You can try to give a
-runas by using <qemu:args>. But that does not work, qemu instantly crashes. I
think this is because to have _one_ root qemu, you have to configure libvirt
to use root user. This means all rights to fs and so on are set to root and
this is what lets qemu probably go crazy if dropping root by -runas.
The whole thing would be a lot easier and more transparent if the user in
libvirt wouldn't be a global config, but instead be part of the domain xml.
This way every qemu started could use a different user and have different
rights.
In my case all but one could be nobody:kvm, and one root:root.
This should not be to complicated based on whats already there, is it?

-- 
Regards,
Stephan