[Libvir] default hypervisor selection

[Daniel Veillard]

  While going though the ovirt installation steps I got annoyed with
the need to add "-c qemu:///system" each time we are trying to start a
virsh command if using KVM. The dfault setup is basically to assume
a Xen hypervisor if the URI is not specified, this is glued in the
do_open internal opening routine associated to the opening of connections:

--------------------------------------
static virConnectPtr
do_open (const char *name,
         virConnectAuthPtr auth,
         int flags)
{
    int i, res;
    virConnectPtr ret = NULL;
    xmlURIPtr uri;

    /* Convert NULL or "" to xen:/// for back compat */
    if (!name || name[0] == '\0')
        name = "xen:///";
--------------------------------------

  In one hand this is the default behaviour of the library, but
in my opinion it's not very smart. We should be able to be smarter than
that, for example:

   - if /proc/xen doesn't exist (on linux, or /dev/xen on Solaris) well
     we should not do that we are pretty sure we will get an error when
     trying to connect
   - if /proc/vz is present, well it's very likely that if the kernel
     has been compiled with OpenVZ support, it's likely to be used as the
     default virtualization
   - if there is a kvm module loaded well we should probably use
     qemu:///system if running as root or qemu:///session otherwise

I guess on Solaris an easy heuristic would allow to pick the right
hypervisor by default too.
At some point we may have multiple hypervisor support simultaneously
on a linux system thanks to pv_ops, but right now it doesn't make too
much sense to force a default Xen connection even when we know it won't
work. 
For a virsh specific solution there is the VIRSH_DEFAULT_CONNECT_URI
environment variable, but it's not really user friendly and not very
generic,

 What do people think ? I would be tempted to provide a patch to change
do_open() behaviour on linux in the case name is NULL or "", and 
then check what hypervisor might be present and running,

Daniel

-- 
Red Hat Virtualization group http://redhat.com/virtualization/
Daniel Veillard      | virtualization library  http://libvirt.org/
veillard at redhat.com  | libxml GNOME XML XSLT toolkit  http://xmlsoft.org/
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