Date:   Sun, 28 Jan 2018 13:01:59 -0800
From:   "Samudrala, Sridhar" <sridhar.samudrala@...el.com>
To:     Alexander Duyck <alexander.duyck@...il.com>
Cc:     Jakub Kicinski <kubakici@...pl>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Siwei Liu <loseweigh@...il.com>,
        Stephen Hemminger <stephen@...workplumber.org>,
        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 1/28/2018 12:18 PM, Alexander Duyck wrote:
> On Sun, Jan 28, 2018 at 11:18 AM, Samudrala, Sridhar
> <sridhar.samudrala@...el.com> wrote:
>> On 1/28/2018 9:35 AM, Alexander Duyck wrote:
>>> On Fri, Jan 26, 2018 at 9:58 PM, Jakub Kicinski <kubakici@...pl> wrote:
>>>> On Fri, 26 Jan 2018 21:33:01 -0800, Samudrala, Sridhar wrote:
>>>>>>> 3 netdev model breaks this configuration starting with the creation
>>>>>>> and naming of the 2 devices to udev needing to be aware of master and
>>>>>>> slave virtio-net devices.
>>>>>> I don't understand this comment.  There is one virtio-net device and
>>>>>> one "virtio-bond" netdev.  And user space has to be aware of the
>>>>>> special
>>>>>> automatic arrangement anyway, because it can't touch the VF.  It
>>>>>> doesn't make any difference whether it ignores the VF or PV and VF.
>>>>>> It simply can't touch the slaves, no matter how many there are.
>>>>> If the userspace is not expected to touch the slaves, then why do we
>>>>> need to
>>>>> take extra effort to expose a netdev that is just not really useful.
>>>> You said:
>>>> "[user space] needs to be aware of master and slave virtio-net devices."
>>>>
>>>> I'm saying:
>>>> It has to be aware of the special arrangement whether there is an
>>>> explicit bond netdev or not.
>>> To clarify here the kernel should be aware that there is a device that
>>> is an aggregate of 2 other devices. It isn't as if we need to insert
>>> the new device "above" the virtio.
>>>
>>> I have been doing a bit of messing around with a few ideas and it
>>> seems like it would be better if we could replace the virtio interface
>>> with the virtio-bond, renaming my virt-bond concept to this since it
>>> is now supposedly going to live in the virtio driver, interface, and
>>> then push the original virtio down one layer and call it a
>>> virtio-backup. If I am not mistaken we could assign the same dev
>>> pointer used by the virtio netdev to the virtio-bond, and if we
>>> register it first with the "eth%d" name then udev will assume that the
>>> virtio-bond device is the original virtio and all existing scripts
>>> should just work with that. We then would want to change the name of
>>> the virtio interface with the backup feature bit set, maybe call it
>>> something like bkup-00:00:00 where the 00:00:00 would be the last 3
>>> octets of the MAC address. It should solve the issue of inserting an
>>> interface "above" the virtio by making the virtio-bond become the
>>> virtio. The only limitation is that we will probably need to remove
>>> the back-up if the virtio device is removed, however that was already
>>> a limitation of this solution and others like the netvsc solution
>>> anyway.
>>
>> With 3 netdev model, if we make the the master virtio-net associated with
>> the
>> real virtio pci device,  i think  the userspace scripts would not break.
>> If we go this route, i am still not clear on the purpose of exposing the
>> bkup netdev.
>> Can we start with the 2 netdev model and move to 3 netdev model later if we
>> find out that there are limitiations with the 2 netdev model? I don't think
>> this will
>> break any user API as the userspace is not expected to use the bkup netdev.
> The 2 netdev model breaks a large number of expectations of user
> space. Among them is XDP since we cannot guarantee a symmetric setup
> between any entity and the virtio. How does it make sense that
> enabling XDP on virtio shows zero Rx packets, and in the meantime you
> are getting all of the packets coming in off of the VF?
Sure we cannot support XDP in this model and it needs to be disabled.
>
> In addition we would need to rewrite the VLAN and MAC address
> filtering ndo operations since we likely cannot add any VLANs since in
> most cases VFs are VLAN locked due to things like port VLAN and we
> cannot change the MAC address since the whole bonding concept is built
> around it.
>
> The last bit is the netpoll packet routing which the current code
> assumes is using the virtio only, but I don't know if that is a valid
> assumption since the VF is expected to be the default route for
> everything else when it is available.
>
> The point is by the time you are done you will have rewritten pretty
> much all the network device ops. With that being the case why add all
> the code to virtio itself instead of just coming up with a brand new
> set of ndo_ops that belong to this new device, and you could leave the
> existing virtio code in place and save yourself a bunch of time by
> just accessing it as an existing call as a separate netdev.

When the BACKUP feature is enabled, we can simply disable most of these 
ndo ops
that cannot be supported. Not sure we need an additional netdev and ndo_ops.

When we can support all these usecases along with live migration we can move
to the 3 netdev model and i think we will need a new feature bit so that the
hypervisor can allow VM to use both datapaths and configure PF accordingly.

Thanks
Sridhar

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