[libvirt] SR-IOV with libvirt

[Florin Stingaciu]

Hello,

For the last little while I've been trying to configure a VM with a
passthrough interface using SR-IOV. I've managed to configure SR-IOV on the
hypervisor correctly. I'm working with a Mellanox ConnectX3 card with two
infiniband ports and hypervisors running Centos 7. I can see the PCI
devices (ie. virtual functions) using the `virsh nodedev-list` command.

The majority of tutorials online (including the tutorials on the libvirt
site), suggest one configures an SR-IOV passthrough interface in the
following manner in the VM definition file:

    <interface type='hostdev' managed='yes'>
      <mac address='52:54:00:d7:cd:aa'/>
      <source>
        <address type='pci' domain='0x0000' bus='0x01' slot='0x00'
function='0x2'/>
      </source>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x03'
function='0x0'/>
    </interface>

For example the above configuration should configure the second VF as a
passthrough interface for this VM. When I try to launch the VM, I get the
following error:

    error: Failed to start domain test
    error: internal error: missing IFLA_VF_INFO in netlink response

I've done research on this error quite extensively. This used to be a very
popular error a while back for scenarios where the total number of VFs is >
64. This is not the case here as I only have 8.

I've was never able to overcome this error but I did manage to get this
working another way. The VM will boot just fine and detect a new interface
if you define one of the VFs as a PCI in the following manner within the VM
definition file:

    <hostdev mode='subsystem' type='pci' managed='yes'>
      <source>
        <address domain='0x0000' bus='0x01' slot='0x00' function='0x2'/>
      </source>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x06'
function='0x0'/>
    </hostdev>

As you can see the from the source bus and function, it's the same VF I was
trying with the definition above. I'm wondering if there is any particular
reason for this or how could I get around this error. Normally, I would
just use the second configuration, however I'm about to begin work within
an openstack environment where the sriovnicswitch ml2 plugin will be used
for networking. This plugin, when configuring passthrough interfaces uses
the first definition that does not work and spits out that error.

Considering even the official libvirt tutorials suggest the first way, I
feel like I must be missing something, or something broke and noone noticed
in one of the latest updates. Any help would be greatly appreciated.

Thanks,
-F
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