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

[Laine Stump]

On 05/19/2015 05:07 AM, Michael S. Tsirkin wrote:
> 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.

That assumes the guest will have NM running. Unless you want to severely
limit the scope of usefulness, you also need to handle systems that have
NM disabled, and among those the different styles of system network
config. It gets messy very fast.

>
> Users are actually asking for this functionality.
>
> Configuring everything manually is possible but error
> prone.

Yes, but attempting to do it automatically is also error prone (due to
the myriad of different guest network config systems, even just within
the seemingly narrow category of "Linux guests"). Pick your poison :-)