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

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

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.

If you mean adding it only to the dumpxml output (when the domain is running, and --inactive isn't specified), I suppose that would be okay, as long as there's an easy way for the low level functions to understand that's the case.

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.

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).

But what of the case where the device isn't virtio? Would you then specify a <driver> with no name attribute? (eg "<driver sndbuf='0'/>)

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