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

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Daniel P. Berrange (berrange@redhat.com) wrote:
> On Wed, Apr 22, 2015 at 06:12:25PM +0100, Dr. David Alan Gilbert wrote:
> > * Daniel P. Berrange (berrange@redhat.com) wrote:
> > > On Wed, Apr 22, 2015 at 06:01:56PM +0100, Dr. David Alan Gilbert wrote:
> > > > * Daniel P. Berrange (berrange@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?
> 
> Hmm, yes, actually hotplug would be a problem with that.
> 
> A even simpler idea would be to just keep things real dumb and simply
> use the same MAC address for both NICs. Once you put them in a bond
> device, the kernel will be copying the MAC address of the first NIC
> into the second NIC anyway, so unless I'm missing something, we might
> as well just use the same MAC address for both right away. That makes
> it easy for guest to discover NICs in the same set and works with
> hotplug trivially.

I bet you need to distinguish the two NICs though; you'd want the bond
to send all the traffic through the real NIC during normal use;
and how does the guest know when it sees the hotplug of the 1st NIC in the pair
that this is a special NIC that it's about to see it's sibbling arrive.

Dave

> 
> Regards,
> Daniel
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--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK