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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 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 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

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.

Also, from a user experience point of view, loading a virtio-net with
BACKUP feature enabled will now show 2 virtio-net netdevs.
One virtio-net and one virtio-bond, which represents what's happening.
This again assumes that we want to represent a bond setup. Can't we
treat this
as virtio-net providing an alternate low-latency datapath by taking over
VF datapath?
Bond is just a familiar name, we can call it something else if you
prefer.  The point is there are two data paths which can have
independent low-level settings and a higher level entity with
global settings which represents any path to the outside world.

Hiding low-level netdevs from a lay user requires a generic solution.
The last thing I think we should be doing is hiding the low level
netdevs. It doesn't solve anythying. We are already trusting the owner
of the VM enough to let them have root access of the VM. That means
they can load/unload any driver they want. They don't have to use the
kernel provided virtio driver, they could load their own. They could
even do something like run DPDK on top of it, or they could run DPDK
on top of the VF. In either case there is no way the bond would ever
be created and all hiding devices does is make it easier to fix
problems when something gets broken. Unless I am mistaken, and
"security through obscurity" has somehow become a valid security

As I mentioned to Sridhar on an off-list thread I think the goal
should be to make it so that the user wants to use the virtio-bond,
not make it so that they have no choice but to use it. The idea is we
should be making things easier for the administrator of the VM, not

When the hypervisor has enabled BACKUP bit along with a VF with the same
MAC address, the VM can use either VF only OR extended virtio with attached  VF. Although there are 2 datapaths to the device, the hypervisor will enable only one at any time.  The virtio via PF datapath is only enabled during live migration when the VF is unplugged. If not, VF broadcasts/multicasts will get looped back to the VM via the PF datapath. In the RFC path i was assuming that the VM can use both datapaths and i had broadcasts/multicasts going over virtio datapath, but i don' think it is a good idea.  It requires the device to disable broadcast
replication on the VFs.


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