Message-ID: <CAKgT0Uda7Bmzf06RMmD7hCVLr8hHQxaY8Fk7w9Qy=7Vi6NODwQ@mail.gmail.com>
Date:   Wed, 21 Feb 2018 16:17:19 -0800
From:   Alexander Duyck <alexander.duyck@...il.com>
To:     Siwei Liu <loseweigh@...il.com>
Cc:     Jakub Kicinski <kubakici@...pl>,
        Sridhar Samudrala <sridhar.samudrala@...el.com>,
        "Michael S. Tsirkin" <mst@...hat.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>,
        "Duyck, Alexander H" <alexander.h.duyck@...el.com>,
        Jason Wang <jasowang@...hat.com>
Subject: Re: [RFC PATCH v3 0/3] Enable virtio_net to act as a backup for a
 passthru device

On Wed, Feb 21, 2018 at 3:50 PM, Siwei Liu <loseweigh@...il.com> wrote:
> I haven't checked emails for days and did not realize the new revision
> had already came out. And thank you for the effort, this revision
> really looks to be a step forward towards our use case and is close to
> what we wanted to do. A few questions in line.
>
> On Sat, Feb 17, 2018 at 9:12 AM, Alexander Duyck
> <alexander.duyck@...il.com> wrote:
>> On Fri, Feb 16, 2018 at 6:38 PM, Jakub Kicinski <kubakici@...pl> wrote:
>>> On Fri, 16 Feb 2018 10:11:19 -0800, Sridhar Samudrala wrote:
>>>> Ppatch 2 is in response to the community request for a 3 netdev
>>>> solution.  However, it creates some issues we'll get into in a moment.
>>>> It extends virtio_net to use alternate datapath when available and
>>>> registered. When BACKUP feature is enabled, virtio_net driver creates
>>>> an additional 'bypass' netdev that acts as a master device and controls
>>>> 2 slave devices.  The original virtio_net netdev is registered as
>>>> 'backup' netdev and a passthru/vf device with the same MAC gets
>>>> registered as 'active' netdev. Both 'bypass' and 'backup' netdevs are
>>>> associated with the same 'pci' device.  The user accesses the network
>>>> interface via 'bypass' netdev. The 'bypass' netdev chooses 'active' netdev
>>>> as default for transmits when it is available with link up and running.
>>>
>>> Thank you do doing this.
>>>
>>>> We noticed a couple of issues with this approach during testing.
>>>> - As both 'bypass' and 'backup' netdevs are associated with the same
>>>>   virtio pci device, udev tries to rename both of them with the same name
>>>>   and the 2nd rename will fail. This would be OK as long as the first netdev
>>>>   to be renamed is the 'bypass' netdev, but the order in which udev gets
>>>>   to rename the 2 netdevs is not reliable.
>>>
>>> Out of curiosity - why do you link the master netdev to the virtio
>>> struct device?
>>
>> The basic idea of all this is that we wanted this to work with an
>> existing VM image that was using virtio. As such we were trying to
>> make it so that the bypass interface takes the place of the original
>> virtio and get udev to rename the bypass to what the original
>> virtio_net was.
>
> Could it made it also possible to take over the config from VF instead
> of virtio on an existing VM image? And get udev rename the bypass
> netdev to what the original VF was. I don't say tightly binding the
> bypass master to only virtio or VF, but I think we should provide both
> options to support different upgrade paths. Possibly we could tweak
> the device tree layout to reuse the same PCI slot for the master
> bypass netdev, such that udev would not get confused when renaming the
> device. The VF needs to use a different function slot afterwards.
> Perhaps we might need to a special multiseat like QEMU device for that
> purpose?
>
> Our case we'll upgrade the config from VF to virtio-bypass directly.

So if I am understanding what you are saying you are wanting to flip
the backup interface from the virtio to a VF. The problem is that
becomes a bit of a vendor lock-in solution since it would rely on a
specific VF driver. I would agree with Jiri that we don't want to go
down that path. We don't want every VF out there firing up its own
separate bond. Ideally you want the hypervisor to be able to manage
all of this which is why it makes sense to have virtio manage this and
why this is associated with the virtio_net interface.

The other bits get into more complexity then we are ready to handle
for now. I think I might have talked about something similar that I
was referring to as a "virtio-bond" where you would have a PCI/PCIe
tree topology that makes this easier to sort out, and the "virtio-bond
would be used to handle coordination/configuration of a much more
complex interface.

>>
>>> FWIW two solutions that immediately come to mind is to export "backup"
>>> as phys_port_name of the backup virtio link and/or assign a name to the
>>> master like you are doing already.  I think team uses team%d and bond
>>> uses bond%d, soft naming of master devices seems quite natural in this
>>> case.
>>
>> I figured I had overlooked something like that.. Thanks for pointing
>> this out. Okay so I think the phys_port_name approach might resolve
>> the original issue. If I am reading things correctly what we end up
>> with is the master showing up as "ens1" for example and the backup
>> showing up as "ens1nbackup". Am I understanding that right?
>>
>> The problem with the team/bond%d approach is that it creates a new
>> netdevice and so it would require guest configuration changes.
>>
>>> IMHO phys_port_name == "backup" if BACKUP bit is set on slave virtio
>>> link is quite neat.
>>
>> I agree. For non-"backup" virio_net devices would it be okay for us to
>> just return -EOPNOTSUPP? I assume it would be and that way the legacy
>> behavior could be maintained although the function still exists.
>>
>>>> - When the 'active' netdev is unplugged OR not present on a destination
>>>>   system after live migration, the user will see 2 virtio_net netdevs.
>>>
>>> That's necessary and expected, all configuration applies to the master
>>> so master must exist.
>>
>> With the naming issue resolved this is the only item left outstanding.
>> This becomes a matter of form vs function.
>>
>> The main complaint about the "3 netdev" solution is a bit confusing to
>> have the 2 netdevs present if the VF isn't there. The idea is that
>> having the extra "master" netdev there if there isn't really a bond is
>> a bit ugly.
>
> Is it this uglier in terms of user experience rather than
> functionality? I don't want it dynamically changed between 2-netdev
> and 3-netdev depending on the presence of VF. That gets back to my
> original question and suggestion earlier: why not just hide the lower
> netdevs from udev renaming and such? Which important observability
> benefits users may get if exposing the lower netdevs?
>
> Thanks,
> -Siwei

The only real advantage to a 2 netdev solution is that it looks like
the netvsc solution, however it doesn't behave like it since there are
some features like XDP that may not function correctly if they are
left enabled in the virtio_net interface.

As far as functionality the advantage of not hiding the lower devices
is that they are free to be managed. The problem with pushing all of
the configuration into the upper device is that you are limited to the
intersection of the features of the lower devices. This can be
limiting for some setups as some VFs support things like more queues,
or better interrupt moderation options than others so trying to make
everything work with one config would be ugly.

- Alex

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