Re: [Qemu-devel] [RFC 0/7] Live Migration with Pass-through Devices proposal

[Daniel P. Berrange]

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.

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
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