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Re: [libvirt] RFC: managing "pci passthrough" usage of sriov VFs via a new network forward type

On Aug 23, 2011, at 12:50 PM, Daniel P. Berrange wrote:


This makes using SRIOV VFs via PCI passthrough very unpalatable. The
problem can be solved by setting the MAC address of the ethernet
device prior to assigning it to the guest, but of course the
<hostdev> element used to assign PCI devices to guests has no place
to specify a MAC address (and I'm not sure it would be appropriate
to add something that function-specific to <hostdev>).

In discussions at the KVM forum, other related problems were
noted too. Specifically when using an SRIOV VF with VEPA/VNLink
we need to be able to set the port profile on the VF before
assigning it to the guest, to lock down what the guest can
do. We also likely need to a specify a VLAN tag on the NIC.
The VLAN tag is actally something we need to be able todo
for normal non-PCI passthrough usage of SRIOV networks too.

I guess there is a issue with PCI-passtrough here, If the VEPA link
is set up prior to VM start then that information is lost when the VM
OS resets the device during initialization.
Only on NICs with an integrated bridge can this setup be persistent
because the bridge can handle the VLAN tagging and port setup.
I see a major drawback with storing MAC adresses in <hostdev>
elements: It would require great care to make sure that MAC adresses
are unique across a big datacenter.

                                                       Dave Allan
and I have discussed a different possible method of eliminating this
problem (using a new forward type for libvirt networks) that I've
outlined below. Please let me know what you think - is this
reasonable in general? If so, what about the details? If not, any
counter-proposals to solve the problem?

The issue I see is that if an application wants to know what
PCI devices have been assigned to a guest, they can no longer
just look at <hostdev> elements. They also need to look at
<interface> elements. If we follow this proposed model in other
areas, we could end up with PCI devices appearing as <disks>
<controllers> and who knows what else. I think this is not
very desirable for applications, and it is also not good for
our internal code that manages PCI devices. ie the security
drivers now have to look at many different places to find
what PCI devices need labelling.

The same is true for network setups, the available options
are becomming more and more confusing.



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