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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@intel.com> wrote:
On 1/28/2018 9:35 AM, Alexander Duyck wrote:
On Fri, Jan 26, 2018 at 9:58 PM, Jakub Kicinski <kubakici@wp.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
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

With 3 netdev model, if we make the the master virtio-net associated with
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.


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