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Subject: Re: [PATCH net-next 0/2] Enable virtio to act as a master for a passthru device

On 1/3/2018 10:28 AM, Alexander Duyck wrote:
On Wed, Jan 3, 2018 at 10:14 AM, Samudrala, Sridhar
<sridhar.samudrala@intel.com> wrote:

On 1/3/2018 8:59 AM, Alexander Duyck wrote:
On Tue, Jan 2, 2018 at 6:16 PM, Jakub Kicinski <kubakici@wp.pl> wrote:
On Tue,  2 Jan 2018 16:35:36 -0800, Sridhar Samudrala wrote:
This patch series enables virtio to switch over to a VF datapath when a
netdev is present with the same MAC address. It allows live migration of
a VM
with a direct attached VF without the need to setup a bond/team between
VF and virtio net device in the guest.

The hypervisor needs to unplug the VF device from the guest on the
host and reset the MAC filter of the VF to initiate failover of datapath
virtio before starting the migration. After the migration is completed,
destination hypervisor sets the MAC filter on the VF and plugs it back
the guest to switch over to VF datapath.

It is based on netvsc implementation and it may be possible to make this
generic and move it to a common location that can be shared by netvsc
and virtio.

This patch series is based on the discussion initiated by Jesse on this
How does the notion of a device which is both a bond and a leg of a
bond fit with Alex's recent discussions about feature propagation?
Which propagation rules will apply to VirtIO master?  Meaning of the
flags on a software upper device may be different.  Why muddy the
architecture like this and not introduce a synthetic bond device?
It doesn't really fit with the notion I had. I think there may have
been a bit of a disconnect as I have been out for the last week or so
for the holidays.

My thought on this was that the feature bit should be spawning a new
para-virtual bond device and that bond should have the virto and the
VF as slaves. Also I thought there was some discussion about trying to
reuse as much of the netvsc code as possible for this so that we could
avoid duplication of effort and have the two drivers use the same
approach. It seems like it should be pretty straight forward since you
would have the feature bit in the case of virto, and netvsc just does
this sort of thing by default if I am not mistaken.
This patch is mostly based on netvsc implementation. The only change is
avoiding the
explicit dev_open() call of the VF netdev after a delay. I am assuming that
the guest userspace
will bring up the VF netdev and the hypervisor will update the MAC filters
to switch to
the right data path.
We could commonize the code and make it shared between netvsc and virtio. Do
we want
to do this right away or later? If so, what would be a good location for
these shared functions?
Is it net/core/dev.c?
No, I would think about starting a new driver file in "/drivers/net/".
The idea is this driver would be utilized to create a bond
automatically and set the appropriate registration hooks. If nothing
else you could probably just call it something generic like virt-bond
or vbond or whatever.

We are trying to avoid creating another driver or a device.  Can we look into
consolidation of the 2 implementations(virtio & netvsc) as a later patch?

Also, if we want to go with a solution that creates a bond device, do we
want virtio_net/netvsc
drivers to create a upper device?  Such a solution is already possible via
config scripts that can
create a bond with virtio and a VF net device as slaves.  netvsc and this
patch series is trying to
make it as simple as possible for the VM to use directly attached devices
and support live migration
by switching to virtio datapath as a backup during the migration process
when the VF device
is unplugged.
We all understand that. But you are making the solution very virtio
specific. We want to see this be usable for other interfaces such as
netsc and whatever other virtual interfaces are floating around out

Also I haven't seen us address what happens as far as how we will
handle this on the host. My thought was we should have a paired
interface. Something like veth, but made up of a bond on each end. So
in the host we should have one bond that has a tap/vhost interface and
a VF port representor, and on the other we would be looking at the
virtio interface and the VF. Attaching the tap/vhost to the bond could
be a way of triggering the feature bit to be set in the virtio. That
way communication between the guest and the host won't get too
confusing as you will see all traffic from the bonded MAC address
always show up on the host side bond instead of potentially showing up
on two unrelated interfaces. It would also make for a good way to
resolve the east/west traffic problem on hosts since you could just
send the broadcast/multicast traffic via the tap/vhost/virtio channel
instead of having to send it back through the port representor and eat
up all that PCIe bus traffic.
From the host point of view, here is a simple script that needs to be run to do the
live migration. We don't need any bond configuration on the host.

virsh detach-interface $DOMAIN hostdev --mac $MAC
ip link set $PF vf $VF_NUM mac $ZERO_MAC

virsh migrate --live $DOMAIN qemu+ssh://$REMOTE_HOST/system

ssh $REMOTE_HOST ip link set $PF vf $VF_NUM mac $MAC
ssh $REMOTE_HOST virsh attach-interface $DOMAIN hostdev $REMOTE_HOSTDEV --mac $MAC

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