[libvirt] Supporting vhost-net and macvtap in libvirt for QEMU

Vivek Kashyap kashyapv at us.ibm.com
Wed Jan 27 07:15:27 UTC 2010

On Wed, 27 Jan 2010, Arnd Bergmann wrote:

> Date: Wed, 27 Jan 2010 04:10:28 +0100
> From: Arnd Bergmann <arnd at arndb.de>
> To: Vivek Kashyap <kashyapv at us.ibm.com>
> Cc: Anthony Liguori <aliguori at linux.vnet.ibm.com>,
>     Chris Wright <chrisw at redhat.com>,
>     "libvir-list at redhat.com" <libvir-list at redhat.com>,
>     Michael S. Tsirkin <mst at redhat.com>, vivk at us.ibm.com
> Subject: Re: [libvirt] Re: Supporting vhost-net and macvtap in libvirt for
>     QEMU
> On Wednesday 20 January 2010, Vivek Kashyap wrote:
>>> As you can see, the policy is mostly independent from the qemu
>>> implementation and even from the kernel implementation. Naming the
>>> macvtap code in qemu '-net vepa' would completely mix up things
>>> for people that want to use vepa with an SR-IOV card, or macvtap
>>> in bridge mode!
>> Qemu can continue to name the interface '-net tap'. libvrt can invoke it
>> as '-net tap, fd=x', whether the fd is of type tap or macvtap.
> right.
>> With the above, in the domain xml, we specify:
>> <interface type='physical'/>
>> <name='somename'/>
>> <type='macvtap|tap'/>  // one of the two to be specified
>> <target mode='vepa|pepa|bridge'/> //specify the mode needed for the VM
>> </interface>
>> With the above, when instantiating a guest libvirt will determine the
>> type of interface. Example: for a 'vepa' on device eth0, libvirt will
>> create a macvtap interface while setting the mode to vepa.
> Sounds good. So you could passs macvtap with any of the target modes or
> tap with bridge mode to get the current behaviour.

Exactly. Also, the 'target mode' comes into play only if one wants to
override the default mode for the 'bridge'. For example, macvlan bridge
can allow both 'vepa' and 'veb' (ie. briding) modes. The VM can then
specify that its interface needs to be in target mode='veb' to overcome
the default of 'vepa'.

>> This fd is what is passed to qemu. Since macvtap/tap appear the same to
>> qemu we should not have to modify anything beyond libvirt.
>> thoughts?
> There is still the question of whether the macvtap devices should be
> kept persistant and created when the daemon is started or only when
> instantiating a particular VM. Normal taps are not persistent unless
> you mark them to be so, while macvtap is persistant by default.
> We could also add the TUNSETPERSIST ioctl to macvtap to give it
> autodestruct behavior, but I'd rather avoid that if possible in order
> to keep the lifetime rules simple.

Yes, we ran into this. We are creating the 'macvtap' interface when the
VM is created. The problem certainly is when the VM terminates. It seems
to me that we should, like 'tap', let macvtap auto-destruct. An option
like you suggest would be ok too.


> 	Arnd


Vivek Kashyap
Linux Technology Center, IBM

More information about the libvir-list mailing list