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Re: [libvirt] RFC: setting mac address on network devices being assigned to a guest via PCI passthrough (<hostdev>)



On 01/20/2012 10:50 PM, Laine Stump wrote:
To refresh everyone's memory, the origin of the problem I'm trying to
solve here is that the VFs of an SRIOV-capable ethernet card are given
new random MAC addresses each time the card is initialized. If those VFs
are then passed-through to a guest using the existing <hostdev> config,
the guest will see a new MAC address each time the host is restarted,
and will thus believe that a new ethernet card has been installed. This
can result in anything from a dialog claiming that the guest has
connected to a new network (MS products) to a new network device name
showing up (Linux - "hmm, eth0 was unplugged, but here's this new
device. Let's call it "eth1"!)

Several months ago I sent out some mail proposing a scheme for
automatically allocating network devices from a pool to be assigned to
guests via PCI passthrough:

https://www.redhat.com/archives/libvir-list/2011-August/msg00937.html


My idea was to have a new <network> forward mode combined with guest
<interface> definitions that would end up auto-generating a transient
<hostdev> entry in the guest's config (and setting the VF's mac address
in the process). Dan Berrange pointed out in that thread that we really
do need to have a persistent <hostdev> entry for these devices in the
domain xml, if for no other reason than to guarantee that the same
guest-side PCI address is always used (thus preventing surprises in the
guest, such as re-activation demands from Microsoft OSes). (There were
other reasons, but that one was the real "hard stop" for me.)

I've come back to this problem, and have decided that, while having the
actual host device auto-allocated at runtime would be nice, first
implementing a less ambitious solution that uses a hand-picked device
would not preclude adding the more complicated/useful functionality
later. So, here's a new simpler proposal.


Step 1
------

In the end, the solution will be that the VF's auto-generated random MAC
address should be replaced with a fixed MAC address supplied by libvirt
prior to assigning the VF to the guest. As a first step to satisfy this
basic requirement, I'm figuring to just extend the <hostdev> xml in this
way:

|<hostdev mode='subsystem' type='pci' managed='yes'>
|<source>
|<address bus='0x06' slot='0x02' function='0x0'/>
|</source>
|<mac address='11:22:33:44:55:66"/>
|</hostdev>

In view of the discussion on SCSI passthrough, it seems to me that this should be attached to an <interface> element:

  <devices>
    <interface type='hostdev'>
      <source>
        <address type='pci' bus='0x06' slot='0x02' function='0x0'/>
      </source>
      <mac address='00:16:3e:5d:c7:9e'/>
      <address type='pci' .../>
    </interface>
  </devices>

3) I've seen requests from 2 places to do host-side virtual port
association (i.e. vepa / 802.1Qb[gh]). Would it be feasible to do that
association with the device after setting MAC address and before
assigning it to the guest? (and likewise for the inverse) Or would the
act of PCI assignment screw that up somehow? (one of the messages in the
earlier thread says something about the device initialization by the
guest un-doing necessary setup) (if it would work, a <virtualport> could
just be added along with <mac address>).

I know almost nothing about this, but it does sound like another hint that augmenting <interface> is a better plan.

Step 2
------

Once the basic functionality is in place, a further step would be one
just to simplify the admins job - we could do this by replacing this
config:

| <source>
| <address bus='x' slot='y' function='z'/>
| </source>

with:

| <source>
| <address netdev='eth22'/>
| </source>

  <devices>
    <interface type='hostdev'>
      <source dev='eth22'/>
      <address type='pci' .../>
    </interface>
  </devices>

To further simplify configuration, it would be very nice if the choice
of network device could be done automatically. Since libvirt's networks
already have the concept of a pool of devices (and also of portgroups
which can be used to set <virtualport> parameters), it kind of makes to
sense to use that. In this case, a network would be defined something
like this:

| <network>
| <name>passthrough-net</name>
| <forward dev='eth20' mode='hostdev'> <!-- or "hardware" or "device" -->
| <interface dev='eth20'/>
| <interface dev='eth21'/>
| <interface dev='eth22'/>
| ..
| </forward>
| </network>

(it could also contain a virtualport definition and/or portgroups
containing virtualport definitions. Obviously, we would have to prohibit
<bandwidth> elements (and several other things) in the definitions>)

Then, in lieu of a pci address or network device name (as "netdev"), the
<hostdev>'s <source> would have a reference to the network:

|<hostdev mode='subsystem' type='pci' managed='yes'>
|<source>
|<address network='passthrough-net'/>
|</source>
|<mac address='11:22:33:44:55:66"/>
|</hostdev>

  <devices>
    <interface type='hostdev'>
      <source network='passthrough-net'/>
      <mac address='11:22:33:44:55:66"/>
      <address type='pci' .../>
    </interface>
  </devices>

(or, again, maybe use the separate <network> element: "<network
name='passthrough-net'/>) At attach time, the pool of devices in
passthrough-net would be searched for a free device, and if found, that
device would have its MAC address changed and be assigned to the guest.
In this case, the live XML would be updated with the pci address
information, but when the guest was destroyed, the device would be
handed back to the pool, and the pci address info once again removed
from the config.

This sounds really nice, especially together with the auto-add VF functionality that was committed recently.

Paolo


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