[libvirt] [RFC 0/7] Live Migration with Pass-through Devices proposal

Michael S. Tsirkin mst at redhat.com
Tue May 19 09:07:55 UTC 2015

On Wed, Apr 22, 2015 at 10:23:04AM +0100, Daniel P. Berrange wrote:
> On Fri, Apr 17, 2015 at 04:53:02PM +0800, Chen Fan wrote:
> > backgrond:
> > Live migration is one of the most important features of virtualization technology.
> > With regard to recent virtualization techniques, performance of network I/O is critical.
> > Current network I/O virtualization (e.g. Para-virtualized I/O, VMDq) has a significant
> > performance gap with native network I/O. Pass-through network devices have near
> > native performance, however, they have thus far prevented live migration. No existing
> > methods solve the problem of live migration with pass-through devices perfectly.
> > 
> > There was an idea to solve the problem in website:
> > https://www.kernel.org/doc/ols/2008/ols2008v2-pages-261-267.pdf
> > Please refer to above document for detailed information.
> > 
> > So I think this problem maybe could be solved by using the combination of existing
> > technology. and the following steps are we considering to implement:
> > 
> > -  before boot VM, we anticipate to specify two NICs for creating bonding device
> >    (one plugged and one virtual NIC) in XML. here we can specify the NIC's mac addresses
> >    in XML, which could facilitate qemu-guest-agent to find the network interfaces in guest.
> > 
> > -  when qemu-guest-agent startup in guest it would send a notification to libvirt,
> >    then libvirt will call the previous registered initialize callbacks. so through
> >    the callback functions, we can create the bonding device according to the XML
> >    configuration. and here we use netcf tool which can facilitate to create bonding device
> >    easily.
> I'm not really clear on why libvirt/guest agent needs to be involved in this.
> I think configuration of networking is really something that must be left to
> the guest OS admin to control. I don't think the guest agent should be trying
> to reconfigure guest networking itself, as that is inevitably going to conflict
> with configuration attempted by things in the guest like NetworkManager or
> systemd-networkd.

There should not be a conflict.
guest agent should just give NM the information, and have  NM do
the right thing.

> IOW, if you want to do this setup where the guest is given multiple NICs connected
> to the same host LAN, then I think we should just let the gues admin configure
> bonding in whatever manner they decide is best for their OS install.
> > -  during migration, unplug the passthroughed NIC. then do native migration.
> > 
> > -  on destination side, check whether need to hotplug new NIC according to specified XML.
> >    usually, we use migrate "--xml" command option to specify the destination host NIC mac
> >    address to hotplug a new NIC, because source side passthrough NIC mac address is different,
> >    then hotplug the deivce according to the destination XML configuration.
> Regards,
> Daniel

Users are actually asking for this functionality.

Configuring everything manually is possible but error
prone. We probably should leave manual configuration
as an option for the 10% of people who want to tweak
guest networking config, but this does not mean we shouldn't
have it all work out of the box for 90% of people that
just want networking to go fast with no tweaks.

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