Date:   Wed, 29 Nov 2017 19:51:38 -0800
From:   Jakub Kicinski <jakub.kicinski@...ronome.com>
To:     Jason Wang <jasowang@...hat.com>
Cc:     Jesse Brandeburg <jesse.brandeburg@...el.com>,
        virtualization@...ts.linux-foundation.org, mst@...hat.com,
        Sridhar Samudrala <sridhar.samudrala@...el.com>,
        Achiad <achiad@...lanox.com>,
        Peter Waskiewicz Jr <peter.waskiewicz.jr@...el.com>,
        "Singhai, Anjali" <anjali.singhai@...el.com>,
        Andy Gospodarek <gospo@...adcom.com>,
        Or Gerlitz <gerlitz.or@...il.com>,
        netdev <netdev@...r.kernel.org>,
        Hannes Frederic Sowa <hannes@...essinduktion.org>
Subject: Re: [RFC] virtio-net: help live migrate SR-IOV devices

On Thu, 30 Nov 2017 11:29:56 +0800, Jason Wang wrote:
> On 2017年11月29日 03:27, Jesse Brandeburg wrote:
> > Hi, I'd like to get some feedback on a proposal to enhance virtio-net
> > to ease configuration of a VM and that would enable live migration of
> > passthrough network SR-IOV devices.
> >
> > Today we have SR-IOV network devices (VFs) that can be passed into a VM
> > in order to enable high performance networking direct within the VM.
> > The problem I am trying to address is that this configuration is
> > generally difficult to live-migrate.  There is documentation [1]
> > indicating that some OS/Hypervisor vendors will support live migration
> > of a system with a direct assigned networking device.  The problem I
> > see with these implementations is that the network configuration
> > requirements that are passed on to the owner of the VM are quite
> > complicated.  You have to set up bonding, you have to configure it to
> > enslave two interfaces, those interfaces (one is virtio-net, the other
> > is SR-IOV device/driver like ixgbevf) must support MAC address changes
> > requested in the VM, and on and on...
> >
> > So, on to the proposal:
> > Modify virtio-net driver to be a single VM network device that
> > enslaves an SR-IOV network device (inside the VM) with the same MAC
> > address. This would cause the virtio-net driver to appear and work like
> > a simplified bonding/team driver.  The live migration problem would be
> > solved just like today's bonding solution, but the VM user's networking
> > config would be greatly simplified.
> >
> > At it's simplest, it would appear something like this in the VM.
> >
> > ==========
> > = vnet0  =
> >           =============
> > (virtio- =       |
> >   net)    =       |
> >           =  ==========
> >           =  = ixgbef =
> > ==========  ==========
> >
> > (forgive the ASCII art)
> >
> > The fast path traffic would prefer the ixgbevf or other SR-IOV device
> > path, and fall back to virtio's transmit/receive when migrating.
> >
> > Compared to today's options this proposal would
> > 1) make virtio-net more sticky, allow fast path traffic at SR-IOV
> >     speeds
> > 2) simplify end user configuration in the VM (most if not all of the
> >     set up to enable migration would be done in the hypervisor)
> > 3) allow live migration via a simple link down and maybe a PCI
> >     hot-unplug of the SR-IOV device, with failover to the virtio-net
> >     driver core
> > 4) allow vendor agnostic hardware acceleration, and live migration
> >     between vendors if the VM os has driver support for all the required
> >     SR-IOV devices.
> >
> > Runtime operation proposed:
> > - <in either order> virtio-net driver loads, SR-IOV driver loads
> > - virtio-net finds other NICs that match it's MAC address by
> >    both examining existing interfaces, and sets up a new device notifier
> > - virtio-net enslaves the first NIC with the same MAC address
> > - virtio-net brings up the slave, and makes it the "preferred" path
> > - virtio-net follows the behavior of an active backup bond/team
> > - virtio-net acts as the interface to the VM
> > - live migration initiates
> > - link goes down on SR-IOV, or SR-IOV device is removed
> > - failover to virtio-net as primary path
> > - migration continues to new host
> > - new host is started with virio-net as primary
> > - if no SR-IOV, virtio-net stays primary
> > - hypervisor can hot-add SR-IOV NIC, with same MAC addr as virtio
> > - virtio-net notices new NIC and starts over at enslave step above
> >
> > Future ideas (brainstorming):
> > - Optimize Fast east-west by having special rules to direct east-west
> >    traffic through virtio-net traffic path
> >
> > Thanks for reading!
> > Jesse  
> 
> Cc netdev.
> 
> Interesting, and this method is actually used by netvsc now:
> 
> commit 0c195567a8f6e82ea5535cd9f1d54a1626dd233e
> Author: stephen hemminger <stephen@...workplumber.org>
> Date:   Tue Aug 1 19:58:53 2017 -0700
> 
>      netvsc: transparent VF management
> 
>      This patch implements transparent fail over from synthetic NIC to
>      SR-IOV virtual function NIC in Hyper-V environment. It is a better
>      alternative to using bonding as is done now. Instead, the receive and
>      transmit fail over is done internally inside the driver.
> 
>      Using bonding driver has lots of issues because it depends on the
>      script being run early enough in the boot process and with sufficient
>      information to make the association. This patch moves all that
>      functionality into the kernel.
> 
>      Signed-off-by: Stephen Hemminger <sthemmin@...rosoft.com>
>      Signed-off-by: David S. Miller <davem@...emloft.net>
> 
> If my understanding is correct there's no need to for any extension of 
> virtio spec. If this is true, maybe you can start to prepare the patch?

IMHO this is as close to policy in the kernel as one can get.  User
land has all the information it needs to instantiate that bond/team
automatically.  In fact I'm trying to discuss this with NetworkManager
folks and Red Hat right now:

https://mail.gnome.org/archives/networkmanager-list/2017-November/msg00038.html

Can we flip the argument and ask why is the kernel supposed to be
responsible for this?  It's not like we run DHCP out of the kernel
on new interfaces... 

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