Message-ID: <CAKgT0UdV_Vppykx7TVeRoOSmc+6E1eBPYoF0+8eP3qXfWgH0gw@mail.gmail.com>
Date:   Mon, 29 Jan 2018 12:09:16 -0800
From:   Alexander Duyck <alexander.duyck@...il.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     Stephen Hemminger <stephen@...workplumber.org>,
        Jakub Kicinski <kubakici@...pl>,
        "Samudrala, Sridhar" <sridhar.samudrala@...el.com>,
        Siwei Liu <loseweigh@...il.com>,
        David Miller <davem@...emloft.net>,
        Netdev <netdev@...r.kernel.org>,
        virtualization@...ts.linux-foundation.org,
        virtio-dev@...ts.oasis-open.org,
        "Brandeburg, Jesse" <jesse.brandeburg@...el.com>,
        Alexander Duyck <alexander.h.duyck@...el.com>
Subject: Re: [virtio-dev] Re: [RFC PATCH net-next v2 2/2] virtio_net: Extend
 virtio to use VF datapath when available

On Mon, Jan 29, 2018 at 10:24 AM, Michael S. Tsirkin <mst@...hat.com> wrote:
> On Sun, Jan 28, 2018 at 08:26:53PM -0800, Alexander Duyck wrote:
>> >> > For live migration with advanced usecases that Siwei is suggesting, i
>> >> > think we need a new driver with a new device type that can track the
>> >> > VF specific feature settings even when the VF driver is unloaded.
>> >
>> > I see no added value of the 3 netdev model, there is no need for a bond
>> > device.
>>
>> I agree a full-blown bond isn't what is needed. However, just forking
>> traffic out from virtio to a VF doesn't really solve things either.
>>
>> One of the issues as I see it is the fact that the qdisc model with
>> the merged device gets to be pretty ugly. There is the fact that every
>> packet that goes to the VF has to pass through the qdisc code twice.
>> There is the dual nature of the 2 netdev solution that also introduces
>> the potential for head-of-line blocking since the virtio could put
>> back pressure on the upper qdisc layer which could stall the VF
>> traffic when switching over. I hope we could avoid issues like that by
>> maintaining qdiscs per device queue instead of on an upper device that
>> is half software interface and half not. Ideally the virtio-bond
>> device could operate without a qdisc and without needing any
>> additional locks so there shouldn't be head of line blocking occurring
>> between the two interfaces and overhead could be kept minimal.
>>
>> Also in the case of virtio there is support for in-driver XDP. As
>> Sridhar stated, when using the 2 netdev model "we cannot support XDP
>> in this model and it needs to be disabled". That sounds like a step
>> backwards instead of forwards. I would much rather leave the XDP
>> enabled at the lower dev level, and then if we want we can use the
>> generic XDP at the virtio-bond level to capture traffic on both
>> interfaces at the same time.
>
> I agree dropping XDP makes everything very iffy.
>
>> In the case of netvsc you have control of both sides of a given link
>> so you can match up the RSS tables, queue configuration and everything
>> is somewhat symmetric since you are running the PF and all the HyperV
>> subchannels. Most of the complexity is pushed down into the host and
>> your subchannel management is synchronized there if I am not mistaken.
>> We don't have this in the case of this virtio-bond setup. Instead a
>> single bit is set indicating "backup" without indicating what that
>> means to topology other than the fact that this virtio interface is
>> the backup for some other interface. We are essentially blind other
>> than having the link status for the VF and virtio and knowing that the
>> virtio is intended to be the backup.
>
> Would you be interested in posting at least a proof of concept
> patch for this approach? It's OK if there are some TODO stubs.
> It would be much easier to compare code to code rather than
> a high level description to code.

That is the general idea. I was hoping to go the bonding route last
week but I got too snarled up trying to untangle the features we
didn't need. I have some code I am working on but don't have an ETA
for when it will be done.

At this point I am hoping we can build something based on Sridhar's
original patch that can addresses the items I brought up and shifts to
more of a 3 netdev model. If I am not mistaken we might be able to do
it as a separate driver that has a pair of calls that allow for
adding/removing a virt-bond that is provided with a MAC address and a
device pointer so that we can do the bits necessary to get ourselves
swapped with the original virtio device and identify the virtio as the
backup channel.

Thanks.

- Alex

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