[libvirt] question for <interface> in ethernet type and multiple queues in KVM (start VM failed)

[Daniel P. Berrange]

On Wed, Mar 26, 2014 at 08:59:04AM +0000, Wangrui (K) wrote:
> Hi,
>   
>     I use ethernet vif for VM (libvirt 1.1.0  qemu 1.5.1). xml as such
>     
>         <interface type='ethernet'>
>             <script path='' />
>             <target dev='tap_mq'/>
>             <mac address='52:54:00:19:9e:4e'/>
>             <model type='virtio' />
>             <driver name='vhost' queues='2'/>
>         </interface>
> 
>     tap_mq is a tap device with multi_queue property which was created on host by ip command, such as follows
>     #ip tuntap add tap_mq mode tap multi_queue
> 
>     I want to use multiple queue feature for this vif so I set queues='2' in xml.
>     
>     When start VM , an error occurs 
>     "error: unsupported configuration: Multiqueue network is not supported for: ethernet".
>     This error is reported from function qemuBuildInterfaceCommandLine
>     
>     /* Currently nothing besides TAP devices supports multiqueue. */
>     if (net->driver.virtio.queues > 0 &&
>         !(actualType == VIR_DOMAIN_NET_TYPE_NETWORK ||
>           actualType == VIR_DOMAIN_NET_TYPE_BRIDGE)) {
>         virReportError(VIR_ERR_CONFIG_UNSUPPORTED,
>                        _("Multiqueue network is not supported for: %s"),
>                        virDomainNetTypeToString(actualType));
>         return -1;
>     }
> 
>     The comment specifies that only TAP device supports multiqueue. But
>     VIR_DOMAIN_NET_TYPE_ETHERNET is also TAP device I think.

No, libvirt can't assume that - it just knows it is something that /looks/
like a tap device but it might not be. This is just an example of one of
the many problems with type=ethernet and why we recommend people never
to use this & why its taints any domain using it.


Regards,
Daniel
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