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

* Daniel P. Berrange (berrange(a)redhat.com) wrote: > On Wed, Apr 22, 2015 at 06:01:56PM +0100, Dr. David Alan Gilbert wrote: > > * Daniel P. Berrange (berrange(a)redhat.com) 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. > > > > > > 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. > > > > I disagree; there should be a way for the admin not to have to do this manually; > > however it should interact well with existing management stuff. > > > > At the simplest, something that marks the two NICs in a discoverable way > > so that they can be seen that they're part of a set; with just that ID system > > then an installer or setup tool can notice them and offer to put them into > > a bond automatically; I'd assume it would be possible to add a rule somewhere > > that said anything with the same ID would automatically be added to the bond. > > I didn't mean the admin would literally configure stuff manually. I really > just meant that the guest OS itself should decide how it is done, whether > NetworkManager magically does the right thing, or the person building the > cloud disk image provides a magic udev rule, or $something else. I just > don't think that the QEMU guest agent should be involved, as that will > definitely trample all over other things that manage networking in the > guest. OK, good, that's about the same level I was at. > I could see this being solved in the cloud disk images by using > cloud-init metadata to mark the NICs as being in a set, or perhaps there > is some magic you could define in SMBIOS tables, or something else again. > A cloud-init based solution wouldn't need any QEMU work, but an SMBIOS > solution might. Would either of these work with hotplug though? I guess as the VM starts off with the pair of NICs, then when you remove one and add it back after migration then you don't need any more information added; so yes cloud-init or SMBIOS would do it. (I was thinking SMBIOS stuff in the way that you get device/slot numbering that NIC naming is sometimes based off). What about if we hot-add a new NIC later on (not during migration); a normal hot-add of a NIC now turns into a hot-add of two new NICs; how do we pass the information at hot-add time to provide that?