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Re: [libvirt] Network device abstraction aka virtual switch - V3

On 06/12/2011 08:29 PM, Laine Stump wrote:
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 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

This case sounds to me like the first one with for example OpenVPN's tap interface also added to the bridge.

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

<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?)

I think the one in the domain XML should take precedence assuming the user wants to make some parameter different for one particular interface.

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


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?)

Just 'bridge'?
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'/>

So this becomes a pool now where libvirt keeps track of which ones of these interfaces is already in use.
Note that, as a convenience, the first of these elements will always
be a duplicate of the "dev" attribute in <forward> itself. (Is this
It feels like this would require special handling in the code. If there was no dev in the forward node then that would require one to look into the pool right away. So maybe the dev attribute in the forward node would just be ignored if there is a pool of interfaces.

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?)



Open Questions:

* 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?
As a user/administrator I may be interested to see it in both places, network and domain XML. At least that way I wouldn't have to dig too much...

I think this is necessary work and it feels like a lot of new complexity will need to be added...


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