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

[Michael S. Tsirkin]

On Tue, May 19, 2015 at 10:15:17AM -0400, Laine Stump wrote:
> 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.

Systems with system network config can just do the configuration
manually, they won't be worse off than they are now.

> >
> > 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 :-)

Make it work well for RHEL guests. Others will work with less integration.

-- 
MST