Disclaimer: I am neither an SR-IOV nor a vhost-net expert, but I've CC'd people that are who can throw tomatoes at me for getting bits wrong :-) I wanted to start a discussion about supporting vhost-net in libvirt. vhost-net has not yet been merged into qemu but I expect it will be soon so it's a good time to start this discussion. There are two modes worth supporting for vhost-net in libvirt. The first mode is where vhost-net backs to a tun/tap device. This is behaves in very much the same way that -net tap behaves in qemu today. Basically, the difference is that the virtio backend is in the kernel instead of in qemu so there should be some performance improvement. Current, libvirt invokes qemu with -net tap,fd=X where X is an already open fd to a tun/tap device. I suspect that after we merge vhost-net, libvirt could support vhost-net in this mode by just doing -net vhost,fd=X. I think the only real question for libvirt is whether to provide a user visible switch to use vhost or to just always use vhost when it's available and it makes sense. Personally, I think the later makes sense.

The more interesting invocation of vhost-net though is one where the vhost-net device backs directly to a physical network card. In this mode, vhost should get considerably better performance than the current implementation. I don't know the syntax yet, but I think it's reasonable to assume that it will look something like -net tap,dev=eth0. The effect will be that eth0 is dedicated to the guest.

On most modern systems, there is a small number of network devices so this model is not all that useful except when dealing with SR-IOV adapters. In that case, each physical device can be exposed as many virtual devices (VFs). There are a few restrictions here though. The biggest is that currently, you can only change the number of VFs by reloading a kernel module so it's really a parameter that must be set at startup time.

I think there are a few ways libvirt could support vhost-net in this second mode. The simplest would be to introduce a new tag similar to <source network='br0'>. In fact, if you probed the device type for the network parameter, you could probably do something like <source network='eth0'> and have it Just Work. Another model would be to have libvirt see an SR-IOV adapter as a network pool whereas it handled all of the VF management. Considering how inflexible SR-IOV is today, I'm not sure whether this is the best model.

Daniel P. Berrange wrote: >>The more interesting invocation of vhost-net though is one where the >>vhost-net device backs directly to a physical network card. In this >>mode, vhost should get considerably better performance than the current >>implementation. I don't know the syntax yet, but I think it's >>reasonable to assume that it will look something like -net >>tap,dev=eth0. The effect will be that eth0 is dedicated to the guest. >> > >Ok, so in this model you have to create a dedicated ethXX device for >every guest, no sharing ? > Yup. You may be sharing a physical network device via SR-IOV, but from libvirt's perspective, we're dedicating a physical device to a guest virtual nic. >>I think there are a few ways libvirt could support vhost-net in this >>second mode. The simplest would be to introduce a new tag similar to >><source network='br0'>. In fact, if you probed the device type for the >>network parameter, you could probably do something like <source >>network='eth0'> and have it Just Work. >> >>Another model would be to have libvirt see an SR-IOV adapter as a >>network pool whereas it handled all of the VF management. Considering >>how inflexible SR-IOV is today, I'm not sure whether this is the best >>model. >> > >Agreed, given the hardware limitations I don't see that it is worth the >bother. > >This new mode is not really what we'd call 'bridging' in libvirt network >XML format, so I think we'll want to define a new type of network config >for it in libvirt. Perhaps > > <network type='physical'> > <source dev='eth0'/> > </network>

> >Or type='passthru' > That certainly simplifies the problem. I don't know whether SR-IOV requires additional setup though wrt programming the VF's mac address. It may make sense for libvirt to at least do that.

On Wed, Dec 16, 2009 at 07:48:08PM -0600, Anthony Liguori wrote: > The more interesting invocation of vhost-net though is one where the > vhost-net device backs directly to a physical network card. In this > mode, vhost should get considerably better performance than the current > implementation. I don't know the syntax yet, but I think it's > reasonable to assume that it will look something like -net > tap,dev=eth0. The effect will be that eth0 is dedicated to the guest. Ok, so in this model you have to create a dedicated ethXX device for every guest, no sharing ?

Yes, since the hardware doesn't allow for any usable configurability of the number of VFs, we'll guest assume that they have already been setup. Likely the kernel can just enable the max # of VFs at all times.

> Another model would be to have libvirt see an SR-IOV adapter as a > network pool whereas it handled all of the VF management. Considering > how inflexible SR-IOV is today, I'm not sure whether this is the best model. Agreed, given the hardware limitations I don't see that it is worth the bother. This new mode is not really what we'd call 'bridging' in libvirt network XML format, so I think we'll want to define a new type of network config for it in libvirt. Perhaps <network type='physical'> <source dev='eth0'/> </network> Or type='passthru'

Current, libvirt invokes qemu with -net tap,fd=X where X is an already open fd to a tun/tap device. I suspect that after we merge vhost-net, libvirt could support vhost-net in this mode by just doing -net vhost,fd=X. I think the only real question for libvirt is whether to provide a user visible switch to use vhost or to just always use vhost when it's available and it makes sense. Personally, I think the later makes sense.

The more interesting invocation of vhost-net though is one where the vhost-net device backs directly to a physical network card. In this mode, vhost should get considerably better performance than the current implementation. I don't know the syntax yet, but I think it's reasonable to assume that it will look something like -net tap,dev=eth0. The effect will be that eth0 is dedicated to the guest.

modeled in libvirt? Michael, could you share your current thinking for -net syntax?

From networking POV, these two are similar cases: vepa

-- Regards, Anthony Liguori

There are two modes worth supporting for vhost-net in libvirt. The first mode is where vhost-net backs to a tun/tap device. This is behaves in very much the same way that -net tap behaves in qemu today. Basically, the difference is that the virtio backend is in the kernel instead of in qemu so there should be some performance improvement. Current, libvirt invokes qemu with -net tap,fd=X where X is an already open fd to a tun/tap device. I suspect that after we merge vhost-net, libvirt could support vhost-net in this mode by just doing -net vhost,fd=X. I think the only real question for libvirt is whether to provide a user visible switch to use vhost or to just always use vhost when it's available and it makes sense. Personally, I think the later makes sense.

The more interesting invocation of vhost-net though is one where the vhost-net device backs directly to a physical network card. In this mode, vhost should get considerably better performance than the current implementation. I don't know the syntax yet, but I think it's reasonable to assume that it will look something like -net tap,dev=eth0. The effect will be that eth0 is dedicated to the guest.

On most modern systems, there is a small number of network devices so this model is not all that useful except when dealing with SR-IOV adapters. In that case, each physical device can be exposed as many virtual devices (VFs). There are a few restrictions here though. The biggest is that currently, you can only change the number of VFs by reloading a kernel module so it's really a parameter that must be set at startup time. I think there are a few ways libvirt could support vhost-net in this second mode. The simplest would be to introduce a new tag similar to <source network='br0'>. In fact, if you probed the device type for the network parameter, you could probably do something like <source network='eth0'> and have it Just Work.

Another model would be to have libvirt see an SR-IOV adapter as a network pool whereas it handled all of the VF management. Considering how inflexible SR-IOV is today, I'm not sure whether this is the best model.

Chris Wright wrote: >* Anthony Liguori (aliguori(a)linux.vnet.ibm.com) wrote: >>There are two modes worth supporting for vhost-net in libvirt. The >>first mode is where vhost-net backs to a tun/tap device. This is >>behaves in very much the same way that -net tap behaves in qemu >>today. Basically, the difference is that the virtio backend is in >>the kernel instead of in qemu so there should be some performance >>improvement. >> >>Current, libvirt invokes qemu with -net tap,fd=X where X is an >>already open fd to a tun/tap device. I suspect that after we merge >>vhost-net, libvirt could support vhost-net in this mode by just >>doing -net vhost,fd=X. I think the only real question for libvirt >>is whether to provide a user visible switch to use vhost or to just >>always use vhost when it's available and it makes sense. >>Personally, I think the later makes sense. > >Doesn't sound useful. Low-level, sure worth being able to turn things >on and off for testing/debugging, but probably not something a user >should be burdened with in libvirt. > >But I dont' understand your -net vhost,fd=X, that would still be -net >tap=fd=X, no? IOW, vhost is an internal qemu impl. detail of the virtio >backend (or if you get your wish, $nic_backend). I don't want to get bogged down in a qemu-devel discussion on libvirt-devel :-)

But from a libvirt perspective, I assume that it wants to open up /dev/vhost in order to not have to grant the qemu instance privileges which means that it needs to hand qemu the file descriptor to it. Given a file descriptor, I don't think qemu can easily tell whether it's a tun/tap fd or whether it's a vhost fd. Since they have different interfaces, we need libvirt to tell us which one it is. Whether that's -net tap,vhost or -net vhost, we can figure that part out on qemu-devel :-)

>>The more interesting invocation of vhost-net though is one where the >>vhost-net device backs directly to a physical network card. In this >>mode, vhost should get considerably better performance than the >>current implementation. I don't know the syntax yet, but I think >>it's reasonable to assume that it will look something like -net >>tap,dev=eth0. The effect will be that eth0 is dedicated to the >>guest. > >tap? we'd want either macvtap or raw socket here. I screwed up. I meant to say, -net vhost,dev=eth0. But maybe it doesn't matter if libvirt is the one that initializes the vhost device, setups up the raw socket (or macvtap), and hands us a file descriptor.

In general, I think it's best to avoid as much network configuration in qemu as humanly possible so I'd rather see libvirt configure the vhost device ahead of time and pass us an fd that we can start using.

>>On most modern systems, there is a small number of network devices >>so this model is not all that useful except when dealing with SR-IOV >>adapters. In that case, each physical device can be exposed as many >>virtual devices (VFs). There are a few restrictions here though. >>The biggest is that currently, you can only change the number of VFs >>by reloading a kernel module so it's really a parameter that must be >>set at startup time. >> >>I think there are a few ways libvirt could support vhost-net in this >>second mode. The simplest would be to introduce a new tag similar >>to <source network='br0'>. In fact, if you probed the device type >>for the network parameter, you could probably do something like >><source network='eth0'> and have it Just Work. > >We'll need to keep track of more than just the other en >We need to 0 Is something missing here?

>>Another model would be to have libvirt see an SR-IOV adapter as a >>network pool whereas it handled all of the VF management. >>Considering how inflexible SR-IOV is today, I'm not sure whether >>this is the best model. > >We already need to know the VF<->PF relationship. For example, don't >want to assign a VF to a guest, then a PF to another guest for basic >sanity reasons. As we get better ability to manage the embedded switch >in an SR-IOV NIC we will need to manage them as well. So we do need >to have some concept of managing an SR-IOV adapter. But we still need to support the notion of backing a VNIC to a NIC, no? If this just happens to also work with a naive usage of SR-IOV, is that so bad? :-)

Long term, yes, I think you want to manage SR-IOV adapters as if they're a network pool. But since they're sufficiently inflexible right now, I'm not sure it's all that useful today. >So I think we want to maintain a concept of the qemu backend (virtio, >e1000, etc), tbhe fd that connects the qemu backend to the host (tap, >socket, macvtap, etc), and the bridge. The bridge bit gets a little >complicated. We have the following bridge cases: > >- sw bridge > - normal existing setup, w/ Linux bridging code > - macvlan >- hw bridge > - on SR-IOV card > - configured to simply fwd to external hw bridge (like VEPA mode) > - configured as a bridge w/ policies (QoS, ACL, port mirroring, > etc. and allows inter-guest traffic and looks a bit like above > sw switch) > - external > - need to possibly inform switch of incoming vport I've got mixed feelings here. With respect to sw vs. hw bridge, I really think that that's an implementation detail that should not be exposed to a user. A user doesn't typically want to think about whether they're using a hardware switch vs. software switch. Instead, they approach it from, I want to have this network topology, and these features enabled.

I think the notion of network pools as being somewhat opaque really works well for this. Ideally you would create a network pool based on the requirements you had, and the management tool would figure out what the best set of implementations to use was. VEPA is really a unique use-case in my mind. It's when someone wants to use an external switch for their network management.

When invoking qemu directly, for the first go about, I'd expect -net vhost,dev=eth0 for a raw device and -net vhost,mode=tap,tap-arguments. Long term, there are so many possible ways to layer things, that I'd really like to see: -net vepa,dev=eth0 Which ends up invoking /usr/libexec/qemu-net-helper-vepa --arg-dev=eth0 --socketpair=X --try-vhost. qemu-net-helper-vepa would do all of the fancy stuff of creating a macvtap device, trying to hook that up with vhost, sending us an fd over the socketpair telling us which interface it's using and what features were enabled.

That lets people infinitely extend qemu's networking support while allow us to focus on just implementing backends for the interfaces we're exposed to. AFAICT, that's just /dev/vhost, /dev/net/tun, and a normal socket. The later two can be reduced to a single read/write interface honestly.

No, net/ would essentially become a series of helper programs. What's nice about this approach is that libvirt could potentially use helpers too which would allow people to run qemu directly based on the output of ps -ef. Would certainly make debugging easier.

> Nope, not at all ;-) > > We do need to know if a VF is available or not (and if a PF has any of > its VFs used). "We need to know" or "it would be nice to know"? You can make the same argument about a physical network interface.

> > 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.

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.

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?

The reason I brought it up here is in case libvirt would be doing both. /dev/vhost takes an fd for a tap device or raw socket. So libvirt would need to open both, and then becomes a question of whether libvirt only passes the single vhost fd (after setting it up completely) or passes both the vhost fd and connecting fd for qemu to put the two together. I didn't recall migration (if qemu would need tap fd again).

>> So I think we want to maintain a concept of the qemu backend (virtio, >> e1000, etc), tbhe fd that connects the qemu backend to the host (tap, >> socket, macvtap, etc), and the bridge. The bridge bit gets a little >> complicated. We have the following bridge cases: >> >> - sw bridge >> - normal existing setup, w/ Linux bridging code >> - macvlan >> - hw bridge >> - on SR-IOV card >> - configured to simply fwd to external hw bridge (like VEPA mode) >> - configured as a bridge w/ policies (QoS, ACL, port mirroring, >> etc. and allows inter-guest traffic and looks a bit like above >> sw switch) >> - external >> - need to possibly inform switch of incoming vport > > I've got mixed feelings here. With respect to sw vs. hw bridge, I > really think that that's an implementation detail that should not be > exposed to a user. A user doesn't typically want to think about whether > they're using a hardware switch vs. software switch. Instead, they > approach it from, I want to have this network topology, and these > features enabled. Agree there is alot of low level detail there, and I think it will be very hard for users, or apps to gain enough knowledge to make intelligent decisions about which they should use. So I don't think we want to expose all that detail. For a libvirt representation we need to consider it more in terms of what capabilities does each options provide, rather than what implementation each option uses

-- |: Red Hat, Engineering, London -o- http://people.redhat.com/berrange/ :| |: http://libvirt.org -o- http://virt-manager.org -o- http://ovirt.org :| |: http://autobuild.org -o- http://search.cpan.org/~danberr/ :| |: GnuPG: 7D3B9505 -o- F3C9 553F A1DA 4AC2 5648 23C1 B3DF F742 7D3B 9505 :| -- Libvir-list mailing list Libvir-list(a)redhat.com https://www.redhat.com/mailman/listinfo/libvir-list

On 01/21/2010 03:13 PM, Vivek Kashyap wrote: >..... >> >>>>So I think we want to maintain a concept of the qemu backend (virtio, >>>>e1000, etc), tbhe fd that connects the qemu backend to the host (tap, >>>>socket, macvtap, etc), and the bridge. The bridge bit gets a little >>>>complicated. We have the following bridge cases: >>>> >>>>- sw bridge >>>> - normal existing setup, w/ Linux bridging code >>>> - macvlan >>>>- hw bridge >>>> - on SR-IOV card >>>> - configured to simply fwd to external hw bridge (like VEPA mode) >>>> - configured as a bridge w/ policies (QoS, ACL, port mirroring, >>>> etc. and allows inter-guest traffic and looks a bit like above >>>> sw switch) >>>> - external >>>> - need to possibly inform switch of incoming vport >>> >>>I've got mixed feelings here. With respect to sw vs. hw bridge, I >>>really think that that's an implementation detail that should not be >>>exposed to a user. A user doesn't typically want to think about >>>whether >>>they're using a hardware switch vs. software switch. Instead, they >>>approach it from, I want to have this network topology, and these >>>features enabled. >> >>Agree there is alot of low level detail there, and I think it will be >>very hard for users, or apps to gain enough knowledge to make >>intelligent >>decisions about which they should use. So I don't think we want to >>expose >>all that detail. For a libvirt representation we need to consider it >>more >>in terms of what capabilities does each options provide, rather than >>what >>implementation each option uses >> > >Attached is some background information on VEPA bridging being >discussed in >this thread and then a proposal for defining it in libvirt xml. > >The 'Edge Virtual Bridging'(eVB) working group has proposed a >mechanism to >offload the bridging function from the server to a physical switch on >the network. This is referred to as VEPA (Virtual Ethernet Port >Aggregator). This is described here: > >http://www.ieee802.org/1/files/public/docs2009/new-evb-congdon-vepa-modular-0709-v01.pdf > > >The VEPA mode implies that the virtual machines on a host communicate >to each >other via the physical switch on the network instead of the bridge in >the Linux host. The filtering, quality of service enforcement, stats >etc. are all done in the external switch. > >The newer NICs with embedded switches (such as SR-IOV cards) will >also provide VEPA mode. This implies that the communication between two >virtual functions on the same physical NIC will also require a packet to >travel to the first hop switch on the network and then be reflected back. > >The 'macvlan' driver in Linux supports virtual interfaces that can be >attached to virtual machine interfaces. This patch provides tap backend >to macvlan: http://marc.info/?l=linux-kernel&m=125986323631311&w=2. If >such an interface is used the packets will be forwarded directly onto >the network bypassing the host bridge. This is exactly what is >required for VEPA mode. > >However, the 'macvlan' driver can support both VEPA and 'bridging' >mode. The bridging in this case is among its virtual interfaces only. >There is also a private mode in which the packets are transmitted to >the network >but are not forwarded among the VMs. > >Similarly, the sr-iov's embedded switch in the future will be settable >as 'VEPA', or 'private' or 'bridging' mode. > >In the eVB working group the 'private' mode is referred to as PEPA, and >the 'bridging' as VEB (Virtual ethernet bridge). I'll use the same >terms. > >The 'VEB' mode of macvlan or sr-iov is no different than the bridge >in Linux. The behaviour of the networking/switching on the network is >unaffected. > >Changes in the first-hop adjacent Switch on the network: >--------------------------------------------------------- >When the 'VEPA' (or PEPA) mode is used the packet switching is >occuring on the first hop switch. Therefore for VM to VM traffic, the >first hop switch must support reflecting the packets back on the port >on which they were received. This is referred to as the 'hairpin' or >'reflective relay' >mode. > >The IEEE 802.1 body is standardizing on the protocol with the switch >vendors, and various other server vendors working on the standard. This >is derived from the above mentioned eVB ('edge virtual bridging') >working group. > >To enable easy testing the Linux bridge can be put into the 'reflective >relay' (or hairpin) mode. The patches are included in 2.6.32. The mode >can be set using sysfs or brctl commands (in latest bridge utils bits). > >In the future the switch vendors (in eVB group) expect to support both >VEPA and VEB on the same switch port. That is the Linux host can have >some VM's using VEPA mode and some in VEB mode on the same outgoing >uplink. This protocol is to be fully defined and will require more >changes in the bridging function. The ethernet frame will carry tags to >identify the packet streams (for VEPA or VEB ports). See chart 4 in the >above linked IEEE document. > >However, from a libvirt defintion point of view it implies that a >'bridge' can be in multiple modes(VEPA or VEB or PEPA). An alternative >is to define separate bridges handling VEB/VEPA or PEPA modes for the >same 'sr-iov' or 'macvlan' backend. > >Determining the switch capability: >--------------------------------- >The Linux host can determine (and set) whether the remote bridge >supports 'hairpin' mode and also set this capability through a low level >protocol (DCBx) being extended in the above eVB working group. >Some drivers (for NICs/CNAs) are likely to do this detrmination >themselves and make the information available to the hypervisor/Linux >host. > >Summary: >-------- > >Based on above a virtual machine might be defined to work with the >Linux/hypervisor bridge, with the 'macvlan' in bridge, vepa/pepa modes, >or with sr-iov virtual function with switching in bridge, or vepa/pepa >modes. > > >Proposal: >-------- > >To support the above combinations we need to be able to define the bridge >to be used, the 'uplink' it is associated with, and the interface type >that the VM will use to connect to the bridge. > >Currently in libvirt we define a bridge and can associate an ethernet >with it (which is the uplink to the network). In the 'macvlan' and the >'sr-iov' cases there is no creation of the bridge itself. In 'sr-iov' it >is embedded in the 'nic', and in the case of macvlan the function is >enabled when the virtual interface is created. > >Describing the bridge and modes: >-------------------------------- >So, we can define the bridge function using a new type or maybe extend >the bridge.xml itself. > ><interface type='bridge' name='br0'> ><bridge> ><type='hypervisor|embedded|ethernet'/> //hypervisor is default ><mode='all|VEPA|PEPA|VEB'/> // 'all' is default if supported. ><interface type='ethernet' name='eth0'/> ></bridge> ></interface> Does this really map to how VEPA works? For a physical bridge, you create a br0 network interface that also has eth0 as a component. With VEPA and macv{lan,tap}, you do not create a single "br0" interface. Instead, for the given physical port, you create interfaces for each tap device and hand them over. IOW, while something like: <interface type='bridge' name='br0'> <bridge> <interface type='ethernet' name='eth0'/> <interface type='ethernet' name='eth1'/> </bridge> </interface> Makes sense, the following wouldn't: <interface type='bridge' name='br0'> <bridge mode='VEPA'> <interface type='ethernet' name='eth0'/> <interface type='ethernet' name='eth1'/> </bridge> </interface> I think the only use of the interface tag that would make sense is: <interface type='ethernet' name='eth0'> <vepa/> </interface> You can imagine doing something similar with SR-IOV: <interface type='ethernet' name='eth0> <sr-iov/> </interface>

and in the guest: <interface type='direct'> <source physical='eth0'> ... </interface>

> and in the guest: > > <interface type='direct'> > <source physical='eth0'> > ... > </interface> I like the simplicity of just having this in the guest XML and a way to just indicate macvlan vs macvtap somehow.

Daniel -- |: Red Hat, Engineering, London -o-

|: http://libvirt.org -o- http://virt-manager.org -o-

|: http://autobuild.org -o-

|: GnuPG: 7D3B9505 -o- F3C9 553F A1DA 4AC2 5648 23C1 B3DF F742 7D3B

-- libvir-list mailing list libvir-list(a)redhat.com https://www.redhat.com/mailman/listinfo/libvir-list

On Mon, 25 Jan 2010, Anthony Liguori wrote: >>Describing the bridge and modes: >>-------------------------------- >>So, we can define the bridge function using a new type or maybe extend >>the bridge.xml itself. >> >><interface type='bridge' name='br0'> >><bridge> >><type='hypervisor|embedded|ethernet'/> //hypervisor is default >><mode='all|VEPA|PEPA|VEB'/> // 'all' is default if supported. >><interface type='ethernet' name='eth0'/> >></bridge> >></interface> > >Does this really map to how VEPA works? > >For a physical bridge, you create a br0 network interface that also has >eth0 as a component. Right. So a bridge has at least one 'uplink'. In this case the bridge is an abstract concept. It still has an 'uplink' which is the device (eth0 in this instance). > >With VEPA and macv{lan,tap}, you do not create a single "br0" interface. >Instead, for the given physical port, you create interfaces for each tap >device and hand them over. IOW, while something like: > ><interface type='bridge' name='br0'> ><bridge> ><interface type='ethernet' name='eth0'/> ><interface type='ethernet' name='eth1'/> ></bridge> ></interface> The above is not in the domain xml but was proposed in the bridge xml. The advantage of using the bridge concept is that it appears the same for macvlan and the virtual Linux host bridge. The 'macvlan' interface itself can support 'bridge' mode in addition to the 'vepa' mode. Therefore, one is creating the bridge, attaching it to the physical device. This device is the one which provides the 'uplink' i.e. is either the sr-iov card or is the device associated with the macvlan driver. The domain xml can now point to the above bridge. For the interfaces it creates it can associate target names.