[ovirt-users] feedback and questions about SR-IOV feature

[Martin Polednik]

On 06/12/16 12:14 +0100, Nathanaël Blanchet wrote:
>Hello,
>
>My new 10G NICS support now SR-IOV, and I've played with this new 
>feature as passthrough device, so as to reduce my host CPU 
>consumption.
>
>At the origin, I set up a bond on both 10G PF nics.
>
>After many configurations, the only way I manage to use a VF into a 
>VM, is to get out of the bond one nic.
>
>So does it mean that it is impossible to run a VM with VF with PF 
>attached to a bond?

As far as I know, it's not possible to do that. The reason is that the
bond normally creates new (logical) interface, what you are doing is
assigning "part" of the bond directly to a VM and the driver within VM
isn't aware of the bond.

>Moreover, something strange happens : during the boot of the VM, the 
>passthrough device gets an dhcp IP on the native vlan of the bond, and 
>once finally up, the real vlan used by this device is on the different 
>predifined vlan. It implies to me to reconfigure the network to ping 
>something on the wanted vlan. Really crazy.

This could be explained by previous statement: bonding PFs at
hypervisor level and then assigning VFs to a VM can most likely cause
undefined behavior.

>Other question is : In which case can it be useful to be able to bond 
>2 VF? UI let us to do so, but it is impossible to add any bridge on 
>that virtual bond.

At hypervisor level? I believe it doesn't make sense.

If you require bond between 2 PFs, you can assign 2 VFs each from
different PF to a VM and bond them within the guest.

>Comparing to a large number of restrictions (migration and others), my 
>opinion is that this feature seems to be very difficult to use in 
>production...

The use case for SR-IOV is maximum performance at the cost of
convenience while still (somewhat) allowing you to scale.

>-- 
>Nathanaël Blanchet
>
>Supervision réseau
>Pôle Infrastrutures Informatiques
>227 avenue Professeur-Jean-Louis-Viala
>34193 MONTPELLIER CEDEX 5 	
>Tél. 33 (0)4 67 54 84 55
>Fax  33 (0)4 67 54 84 14
>blanchet at abes.fr
>

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