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>
When libvirt sees <mac address...> in the hostdev at device attach time,
it would first verify that the device is a network device (if not, it
would log an error and fail the operation). If it is a network device,
the pci address would be converted into a network device name, and that
device would have its MAC address set to the configured value, and then
the attach would proceed.
My main questions here are:
1) Is this the right place for the new element? Or should it go into
<source>?
2) Should we bother trying to save the original MAC address to restore
when the device is released? (I guess that might be important if, for
example, the guest config was changed to use a different device but same
MAC address - you could end up with two devices having the same MAC
address).
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>).
Beyond those 3 questons, this all seems rather uncontroversial, so I'll
start coding something up right away (and modify as necessary after
discussion).
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>
(or possibly it could be a separate element within <source>, e.g.
"<network dev='eth22'/>") As long as the domain isn't running, the
config would remain like this. The first time the device was attached,
the name in netdev would be resolved to a pci address (or failed if the
given netdev wasn't a PCI device); and the config auto-filled as follows:
| <source>
| <address bus='x' slot='y' function='z' netdev='eth22'/>
| </source>
On subsequent attaches (i.e. when both netdev and a pci address are
present) the netdev would again be resolved and compared to the pci
address to make sure they still agree; if not, the operation would fail.
This would satisfy management applications' desire to see the pci
address info of all devices assigned to guests, while retaining the
original config info (and also lead quite nicely into step 3...).
Step 3
------
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>
(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.
Step 2 & 3 probably won't be implemented right away, but I thought I
should toss the ideas out there in case they lead to something else that
would require a change in step 1.
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