netdev - Re: [RFC] virtio-net: help live migrate SR-IOV devices

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Date:   Sun, 3 Dec 2017 11:14:37 +0200
From:   achiad shochat <achiad.mellanox@...il.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     Shannon Nelson <shannon.nelson@...cle.com>,
        Jason Wang <jasowang@...hat.com>,
        Jesse Brandeburg <jesse.brandeburg@...el.com>,
        virtualization@...ts.linux-foundation.org,
        Jakub Kicinski <jakub.kicinski@...ronome.com>,
        Sridhar Samudrala <sridhar.samudrala@...el.com>,
        Achiad <achiad@...lanox.com>,
        Peter Waskiewicz Jr <peter.waskiewicz.jr@...el.com>,
        "Singhai, Anjali" <anjali.singhai@...el.com>,
        Andy Gospodarek <gospo@...adcom.com>,
        Or Gerlitz <gerlitz.or@...il.com>,
        Hannes Frederic Sowa <hannes@...hat.com>,
        netdev <netdev@...r.kernel.org>
Subject: Re: [RFC] virtio-net: help live migrate SR-IOV devices

On 3 December 2017 at 07:05, Michael S. Tsirkin <mst@...hat.com> wrote:
> On Fri, Dec 01, 2017 at 12:08:59PM -0800, Shannon Nelson wrote:
>> On 11/30/2017 6:11 AM, Michael S. Tsirkin wrote:
>> > On Thu, Nov 30, 2017 at 10:08:45AM +0200, achiad shochat wrote:
>> > > Re. problem #2:
>> > > Indeed the best way to address it seems to be to enslave the VF driver
>> > > netdev under a persistent anchor netdev.
>> > > And it's indeed desired to allow (but not enforce) PV netdev and VF
>> > > netdev to work in conjunction.
>> > > And it's indeed desired that this enslavement logic work out-of-the box.
>> > > But in case of PV+VF some configurable policies must be in place (and
>> > > they'd better be generic rather than differ per PV technology).
>> > > For example - based on which characteristics should the PV+VF coupling
>> > > be done? netvsc uses MAC address, but that might not always be the
>> > > desire.
>> >
>> > It's a policy but not guest userspace policy.
>> >
>> > The hypervisor certainly knows.
>> >
>> > Are you concerned that someone might want to create two devices with the
>> > same MAC for an unrelated reason?  If so, hypervisor could easily set a
>> > flag in the virtio device to say "this is a backup, use MAC to find
>> > another device".
>>
>> This is something I was going to suggest: a flag or other configuration on
>> the virtio device to help control how this new feature is used.  I can
>> imagine this might be useful to control from either the hypervisor side or
>> the VM side.
>>
>> The hypervisor might want to (1) disable it (force it off), (2) enable it
>> for VM choice, or (3) force it on for the VM.  In case (2), the VM might be
>> able to chose whether it wants to make use of the feature, or stick with the
>> bonding solution.
>>
>> Either way, the kernel is making a feature available, and the user (VM or
>> hypervisor) is able to control it by selecting the feature based on the
>> policy desired.
>>
>> sln
>
> I'm not sure what's the feature that is available here.
>
> I saw this as a flag that says "this device shares backend with another
> network device which can be found using MAC, and that backend should be
> preferred".  kernel then forces configuration which uses that other
> backend - as long as it exists.
>
> However, please Cc virtio-dev mailing list if we are doing this since
> this is a spec extension.
>
> --
> MST


Can someone please explain why assume a virtio device is there at all??
I specified a case where there isn't any.

I second Jacob - having a netdev of one device driver enslave a netdev
of another device driver is an awkward a-symmetric model.
Regardless of whether they share the same backend device.
Only I am not sure the Linux Bond is the right choice.
e.g one may well want to use the virtio device also when the
pass-through device is available, e.g for multicasts, east-west
traffic, etc.
I'm not sure the Linux Bond fits that functionality.
And, as I hear in this thread, it is hard to make it work out of the box.
So I think the right thing would be to write a new dedicated module
for this purpose.

Re policy -
Indeed the HV can request a policy from the guest but that's not a
claim for the virtio device enslaving the pass-through device.
Any policy can be queried by the upper enslaving device.

Bottom line - I do not see a single reason to have the virtio netdev
(nor netvsc or any other PV netdev) enslave another netdev by itself.
If we'd do it right with netvsc from the beginning we wouldn't need
this discussion at all...