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

[Nathanaël Blanchet]

Le 06/12/2016 à 13:19, Martin Polednik a écrit :
> 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.
This is what I supposed, UI should prevent us to create VFfrom when nic 
is attached to a bond. Pencil icon should'nt appear in this case.
>
>> 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.
The issue is the same when the PF is not attached to a bond, so in an 
expected working situation.
>
>> 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.
I wonder this because UI allows to do it. The same as above, user 
shouldn't be allowed to bond two VFs, and not allowed to add virtual 
network to a VF
>
> 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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>

-- 
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