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

[Daniel P. Berrange]

On Wed, Jan 05, 2011 at 09:57:42AM -0500, Laine Stump wrote:
> On 01/05/2011 05:19 AM, Daniel P. Berrange wrote:
> >
> >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.
> 
> Do you mean to add it to the XML that's saved in the config? If so,
> that would mean that it would only be possible to configure it as
> "use whatever is best for the current situation" for the first
> startup of the domain. Once that happened, it would be stuck on
> whichever was used the first time (qemu or vhost), so if the domain
> was  first started when vhost-net was loaded, then later restarted
> when vhost-net wasn't loaded (or maybe migrated to another host that
> didn't have vhost support), it would fail to start.

Yes, I *did* actually mean to set it in the permanent XML
config, so once a choice is made, that choice is preserved
thereafter. This gives us better reliability in the future
if a further possible 'default' options are introduced and
we want to avoid existing guests accidentally getting the
new option for some reason.

> Aside from that, I'd been thinking that the "backend" driver in this
> case is virtio, not qemu or vhost; qemu(userland) vs vhost seems
> like just a setting within that driver. So it doesn't seem
> appropriate to me to have the name decide whether to use userland or
> vhost.

Hmm, I thought vhost was a property of any tap device based
backend, rather than virtio ?

> 
> One other twist - there's already another request for something else
> to be set for each network device: sndbuf.
> 
> <https://bugzilla.redhat.com/show_bug.cgi?id=665293>
> https://bugzilla.redhat.com/show_bug.cgi?id=665293
> <https://bugzilla.redhat.com/show_bug.cgi?id=665293>
> 
> The sndbuf setting is applicable to any network device that connects
> to the real world using a tap device (ie, not just virtio). If we
> want to add that setting via the same scheme, we would need
> something like:
> 
> <driver name='qemu|vhost' sndbuf='0'/>
> 
> (0 can't be the default, because 0 is actually one of the settings
> that they want to explicitly specify (if sndbuf isn't given on the
> commandline, qemu defaults to 1048576).

sndbuf is much more like a true "tunable" than vhost is, so
I think it makes sense to have a generic representation for
NIC tunables.

Daniel