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

* Daniel P. Berrange (berrange(a)redhat.com) wrote: > 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. > > Also OpenStack already has a way to pass guest information about the > required network setup, via cloud-init, so it would not be interested > in any thing that used the QEMU guest agent to configure network > manager. Which is really just another example of why this does not > belong anywhere in libvirt or lower. The decision to use NM is a > policy decision that will always be wrong for a non-negligble set > of use cases and as such does not belong in libvirt or QEMU. It is > the job of higher level apps to make that kind of policy decision. This is exactly my worry though; why should every higher level management system have it's own way of communicating network config for hotpluggable devices. You shoudln't need to reconfigure a VM to move it between them. This just makes it hard to move it between management layers; there needs to be some standardisation (or abstraction) of this; if libvirt isn't the place to do it, then what is?