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[libvirt] Network device abstraction aka virtual switch - V3

This is a followup to https://www.redhat.com/archives/libvir-list/2011-April/msg00591.html
(and an even earlier draft) which I alluded to here:


Network device abstraction aka virtual switch - V3

The <interface> element of a guest's domain config in libvirt has a
<source> element that describes what resources on a host will be used
to connect the guest's network interface to the rest of the
world. This is very flexible, allowing several different types of
connection (virtual network, host bridge, direct macvtap connection to
physical interface, qemu usermode, user-defined via an external
script), but currently has the problem that unnecessary details of the
host resources are embedded into the guest's config; if the guest is
migrated to a different host, and that host has a different hardware
or network config (or possibly the same hardware, but that hardware is
currently in use by a different guest), the migration will fail.

I am proposing a change to libvirt's network XML that will allow us to
(optionally - old configs will remain valid) remove the host details
from the guest's domain XML (which can move around from host to host)
and place them in the network XML (which remains with a single host);
the domain XML will then use existing config elements to associate
each guest interface with a "network".

The motivating use case for this change is the "direct" connection
type (which uses macvtap for vepa and vnlink connections directly
between a guest and a physical interface, rather than through a
bridge), but it is applicable for all types of connection. (Another
hopeful side effect of this change will be to make libvirt's network
connection model easier to realize on non-Linux hypervisors (eg,
VMWare ESX) and for other network technologies, such as openvswitch,
VDE, and various VPN implementations).


(parts lifted from Dan Berrange's last mail on this subject)

Currently <network> supports 3 connectivity modes

- Non-routed network, separate subnet (no <forward> element present)
 - Routed network, separate subnet with NAT   (<forward mode='nat'/>)
 - Routed network, separate subnet            (<forward mode='route'/>)

Each of these is implemented in the existing network driver by
creating a bridge device using brctl, and connecting the guest network
interfaces via tap devices (a detail which, now that I've stated it,
you should promptly forget!). All traffic between that bridge and the
outside network is done via the host's IP routing stack (ie, there is
no physical device directly connected to the bridge)

In the future, these two additional routed modes might be useful:

 - Routed network, IP subnetting
 - Routed network, separate subnet with VPN

The core goal of this proposal, though, is to replace type=bridge and
type=direct from the domain interface XML with new types of <network>
definitions so that the domain can just give "type='network'" and have
all the necessary details filled in at runtime. This basically means
we're adding several bridging modes (the submodes of "direct" have
been flattened out here):

 - Bridged network, eth + bridge + tap
 - Bridged network, eth + macvtap + vepa
 - Bridged network, eth + macvtap + private
 - Bridged network, eth + macvtap + passthrough
 - Bridged network, eth + macvtap + bridge

Another "future expansion" could be to add:

 - Bridged network, with VPN

Likewise, support for other technologies, such as openvswitch and VDE
would each be another entry on this list.

(Dan also listed each of the above "+sriov" separately, but that ends
up being handled in an orthogonal manner (by just specifying a pool of
interfaces for a single network), so I'm only giving the abbreviated

I. Changes to domain <interface> element

In many cases, the <interface> element of the domain XML will be
identical to what is used now when connecting the interface to a
libvirt-style virtual network:

<interface type='network'>
<source network='red-network'/>
<mac address='xx:xx:xx:xx:xx:xx'/>

Depending on the definition of the network "red-network" on the host
the guest was started on / migrated to, this could be either a direct
(macvtap) connection using one of the various direct modes
(vepa/private/bridge/passthrough), a bridge (again, pointed to by the
definition of 'red-network'), or a virtual network (using the current
network definition syntax). This way the same guest could be migrated
not only between macvtap-enabled hosts, but from there to a host using
a bridge, or maybe a host in a remote location that used a virtual
network with a secure tunnel to connect back to the rest of the

 (Part of the migration process would of course check that the
destination host had a network of the proper name with adequate
available resources, and fail if it didn't; management software at a
level above libvirt would probably filter a list of candidate
migration destinations based on available networks and any various
details of those networks (eg. it could search for only networks using
vepa for the connection), and only attempt migration to one that had
the matching network available).

<virtualport> element of <interface>

Since many of the attributes/sub-elements of <virtualport> (used by
some modes of "direct" interface connections) are identical for all
interfaces connecting to any given switch, most of the information in
<virtualport> will be optional in the domain's interface definition -
it can be filled in from a similar <virtualport> element that will be
added to the <network> definition.

Some parameters in <virtualport> ("instanceid", for example) must be
unique for every interface, though, so those will still be specified
in the <interface> XML. The two <virtualport> elements will be OR'ed
at runtime to arrive at the actual set of parameters that are

(Open Question: What should be the policy when a parameter is
specified in both places? Should one take precedence? Or should it be
considered an error?)

portgroup attribute of <source>

The <source> element of an interface definition will be able to
optionally specify a "portgroup" attribute. If portgroup is *NOT*
given, the default (first) portgroup of the network will be used (if
any are defined). If portgroup *IS* specified, the source network must
have a portgroup by that name (or the domain startup/migration will
fail), and the attributes of that portgroup will be used for the
connection. Here is an example <interface> definition that has both a
reduced <virtualport> element, as well as a portgroup attribute:

<interface type='network'>
<source network='red-network' portgroup='engineering'/>
<virtualport type="802.1Qbg">
<parameters instanceid="09b11c53-8b5c-4eeb-8f00-d84eaa0aaa4f"/>
<mac address='de:ad:be:ef:ca:fe'/>

(The specifics of what can be in a portgroup are given below)

II. Changes to <network> definition

As Dan has pointed out, any additions to <network> must be designed so
that existing management applications (written to understand <network>
prior to these new additions) will at least recognize that the XML
they've been given is for something new that they don't fully
understand. At the same time, the new types of network definition
should attempt to re-use as much of the existing elements/attributes
as possible, both to make it easier to extend these applications, as
well as to make the status displays of un-updated applications make as
much sense as possible.

Dan's suggestion (which I obviously endorse :-) is that the new types
of network should be specified by extending the choices for <forward

He also suggested adding a new "layer='network|link'" attribute to
<forward>. I'm not convinced that item is necessary (it seems
redundant), but am including it here for sake of discussion.

The current modes are:

<forward layer='network' mode='route|nat'/>

(in addition to not listing any mode, which equates to "isolated")

Here are suggested new modes:

<forward layer='link'

A description of each:

bridge-brctl - equivalent to "<interface type='bridge'>" in the
               interface definition. The bridge device to use would be
               given in the existing <forward dev='xxx'>. (Dan also
               suggests putting this in <network>'s <bridge
               name='xxx'/> - opinions?)
               (Question: better name for this?)

vepa         - same as "<interface type='direct'>..." with <source

private      - <interface type='direct'> ... <source mode='private'/>

passthrough  - <interface type='direct'> ... <source mode='passthrough'/>

bridge-macvtap - <interface type='direct'> ... <source mode='bridge'/>
               (Question: better name for this?)

Interface Pools

In many cases, a single host network may have multiple physical
network devices associated with it (especially in the case of an
SRIOV-capable ethernet card, which will have several "virtual
functions" associated with a single physical ethernet connection). The
host will at least want to balance the load of multiple guests between
these multiple devices, and may even require (in the case of
passthrough mode, for example) that only a single guest interface be
attached to each host device.

The current specification for <forward> only allows for a single "dev"
attribute, though. In order to support multiple device names, we will
extend <forward> to allow 0 or more <interface> sub-elements:

<forward mode='vepa' dev='eth10'/>
<interface dev='eth10'/>
<interface dev='eth11'/>
<interface dev='eth12'/>
<interface dev='eth13'/>

Note that, as a convenience, the first of these elements will always
be a duplicate of the "dev" attribute in <forward> itself. (Is this

In the case of mode='passthrough', only one guest interface can be
connected to a device at a time. libvirt will keep track of which
devices are in use, and attempt to assign a free device; failure to
assign a device will result in a failure of the domain to
start/migrate. For the other direct modes, libvirt will simply keep
track of the number of guest interfaces currently using each device,
and attempt to keep them balanced.

(Open question: where will we keep track of this allocation/assignment?)


A <portgroup> (sub-element of <network>) is just a way of easily
putting connections to the network into different classes, with each
class having a different level/type of service. Each <network> can
have multiple <portgroup> elements, and each <portgroup> has a name,
as well as various attributes associated with it. The first thing we
will use portgroups for is as an alternate place to specify
<virtualport> parameters:

<portgroup name='engineering'>
<virtualport type="802.1Qbg">
<parameters managerid="11" typeid="1193047" typeidversion="2"/>

Anything that is valid in an interface's <virtualport> is also valid here.

The next thing to specify in a portgroup will be bandwidth limiting /
QoS configuration. Since I don't know exactly what's needed for that,
I won't specify it here.

If anything is specified both directly under <network> and in a
<portgroup>, the value in portgroup will take precedence. (Again -
what will the precedence of items specified in the <interface> be?)


Examples of 'red-network' for different types of connections (all of
these would work with minor variations of the interface XML given
above, e.g. the 'vepa' version would require <virtualport> in the
interface that specified an instanceid, and if the <interface>
specified a portgroup, it would need to also be in the <network>
definition (even if it was empty aside from name).

<!-- Existing usage - a libvirt virtual network -->
<bridge name='virbr0'/>
<forward layer='network' mode='route'/>

<!-- The simplest - an existing host bridge -->
<forward mode='bridge-brctl' dev='br0'/>

<!-- A macvtap connection to a vepa bridge -->
<forward layer='link' mode='vepa' dev='eth10'/>
<virtualport type='802.1Qbg'>
<parameters managerid='11' typeid='1193047' typeidversion='2'/>
<!-- NB: if <interface> doesn't specify portgroup, -->
<!-- 'accounting' is assumed -->
<portgroup name='accounting'>
<parameters typeid='22'/>
<portgroup name='engineering'>
<parameters typeid='33'/>

<!-- A macvtap passthrough connection (one guest interface per dev) -->
<forward layer='link' mode='passthrough' dev='eth10'/>
<interface dev='eth10'/>
<interface dev='eth11'/>
<interface dev='eth12'/>
<interface dev='eth13'/>
<interface dev='eth14'/>
<interface dev='eth15'/>
<interface dev='eth16'/>
<interface dev='eth17'/>


Open Questions:

* Is there a good reason to include the "layer='network|link'"
  attribute in forward? (maybe just because it's useful info for a
  management application that doesn't know the details of the modes?)
  Or is it redundant?

* What should be the policy when a virtualport parameter is specified
  in both the <interface> and the <network>/<portgroup>? Should one take
  precedence? Or should it be considered an error?

* Is it okay for the domain's own definition to specify what portgroup
  it will be in? Or are there cases where we want to allow someone to
  modify their domain XML, but force them into a particular portgroup
  beyond their control?

* Is it really necessary/desirable for the first ethernet device in a
  pool to be duplicated in the <forward dev='xxx'...> attribute? Or
  can that attribute be omitted when there is a pool of devices?

* Where will we keep track of the count of guest interfaces connected
  to each host interface device, and where will we keep track of which
  device is being used by a particular guest interface? In the
  network/domain XML?

* Does anyone have better names for "brctl-bridge" and

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