[libvirt] RFC: exposing a config setting to force vhost-net support on/off

[Daniel P. Berrange]

On Tue, Jan 04, 2011 at 02:37:15PM -0500, Laine Stump wrote:
> There's a request to allow libvirt to explicitly turn on/off the new
> vhost-net feature of virtio network cards. I see a few ways to do
> it, and am looking for opinions on which is best.
> 
> (For the uninitiated, vhost-net is a new kernel-based virtio
> implementation that saves the overhead of having all network traffic
> be handled by the userlevel qemu process.)
> 
> The original implementation of vhost-net support (what's been in
> libvirt for a few releases now) doesn't expose any knobs to the user
> - if the vhost-net kernel module is loaded, libvirt uses it for all
> virtio network devices of all newly created guests, and if the
> kernel module isn't loaded, it reverts to using the old user-level
> virtio.
> 
> It's simple enough to put a bit of extra logic at the point where we
> make that decision. I see 3 possibilities:
> 
> 1) default - use vhost-net if it's loaded, don't if it isn't
> (current behavior)
> 
> 2) require - use vhost-net if it's loaded, and refuse to start the
> guest if it isn't (for those who want
>    to be 100% sure they're using it)
> 
> 3) disable - don't use vhost-net, whether or not it's loaded (to
> disable it, eg in case a compatibility problem
>    is found between vhost-net and some particular guest)
> 
> The question is how to describe that in the XML. Here's what a
> virtio network interface might look like currently:
> 
>   <devices>
>     <interface type='network'>
>       <source network='default'/>
>       <model type='virtio'/>
>     </interface>
>   </devices>
> 
> 
> 1) One possibility (the simplest) would be to add an optional
> attribute to <model>:
> 
>   <devices>
>     <interface type='network'>
>       <source network='default'/>
>       <model type='virtio' vhost='default|require|disable'/>  (or 'default|on|off' ?)
>     </interface>
>   </devices>
> 
> or maybe:
> 
>   <devices>
>     <interface type='network'>
>       <source network='default'/>
>       <model type='virtio' mode='default|kernel|user'/>
>     </interface>
>   </devices>
> 
> 
> 2) Another possibility would be to define a new sub-element of<interface>, called "<driver>", similar
>    to what's done in the storage device XML. In the future, other backend driver-related items could be placed there:
> 
>   <devices>
>     <interface type='network'>
>       <source network='default'/>
>       <model type='virtio'/>
>       <driver vhost='default|require|disable'/>   (or "mode='default|kernel|user'")
>     </interface>
>   </devices>

We should try to keep terminology matching the disk <driver>
so I think 

  <driver name='qemu|vhost'/>

with omission of <driver> resulting in us automatically
adding either 'qemu' or 'vhost' to the XML. We don't
want to have an explicit 'default' value in the XML,
because users should be able to see the guest is running
with.

Regards,
Daniel