Subject: Re: [RFC] virtio-net: help live migrate SR-IOV devices

From: Stephen Hemminger <[email-masked]>
Date: Wed, 29 Nov 2017 20:10:09 -0800

On Wed, 29 Nov 2017 19:51:38 -0800
Jakub Kicinski <[email-masked]> wrote:

> 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 <[email-masked]>
> > 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 <[email-masked]>
> >      Signed-off-by: David S. Miller <[email-masked]>
> > 
> > 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... 

Although "policy should not be in the kernel" is a a great mantra,
it is not practical in the real world.

If you think it can be solved in userspace, then you haven't had to
deal with four different network initialization
systems, multiple orchestration systems and customers on ancient
Enterprise distributions.