Re: [dpdk-dev] dpdk/vpp and cross-version migration for vhost

["Daniel P. Berrange" <berrange@redhat.com>]

On Fri, Dec 09, 2016 at 05:45:13PM +0100, Maxime Coquelin wrote:
> 
> 
> On 12/09/2016 03:42 PM, Daniel P. Berrange wrote:
> > On Fri, Dec 09, 2016 at 02:35:58PM +0100, Maxime Coquelin wrote:
> > > ++Daniel for libvirt
> > > 
> > > On 11/24/2016 07:31 AM, Yuanhan Liu wrote:
> > > > > > > > > > > As version here is an opaque string for libvirt and qemu,
> > > > > > > > > > > > > > > > > > anything can be used - but I suggest either a list
> > > > > > > > > > > > > > > > > > of values defining the interface, e.g.
> > > > > > > > > > > > > > > > > > any_layout=on,max_ring=256
> > > > > > > > > > > > > > > > > > or a version including the name and vendor of the backend,
> > > > > > > > > > > > > > > > > > e.g. "org.dpdk.v4.5.6".
> > > > > > > > 
> > > > > > > > The version scheme may not be ideal here. Assume a QEMU is supposed
> > > > > > > > to work with a specific DPDK version, however, user may disable some
> > > > > > > > newer features through qemu command line, that it also could work with
> > > > > > > > an elder DPDK version. Using the version scheme will not allow us doing
> > > > > > > > such migration to an elder DPDK version. The MTU is a lively example
> > > > > > > > here? (when MTU feature is provided by QEMU but is actually disabled
> > > > > > > > by user, that it could also work with an elder DPDK without MTU support).
> > > > > > > > 
> > > > > > > > 	--yliu
> > > > > > 
> > > > > > OK, so does a list of values look better to you then?
> > > > Yes, if there are no better way.
> > > > 
> > > > And I think it may be better to not list all those features, literally.
> > > > But instead, using the number should be better, say, features=0xdeadbeef.
> > > > 
> > > > Listing the feature names means we have to come to an agreement in all
> > > > components involved here (QEMU, libvirt, DPDK, VPP, and maybe more
> > > > backends), that we have to use the exact same feature names. Though it
> > > > may not be a big deal, it lacks some flexibility.
> > > > 
> > > > A feature bits will not have this issue.
> > > 
> > > I initially thought having key/value pairs would be more flexible, and
> > > could allow migrating to another application if compatible (i.e. from
> > > OVS to VPP, and vice versa...) without needing synchronization between
> > > the applications.
> > > 
> > > But Daniel pointed me out that it would add a lot of complexity on
> > > management tool side, as it would need to know how to interpret these
> > > key/value pairs. I think his argument is very valid.
> > > 
> > > So maybe the best way would be the version string, letting the
> > > application (OVS-DPDK/VPP/...) specify which version it is
> > > compatible with.
> > > For the downsides, as soon as a new feature is supported in vhost-user
> > > application, the new version will not be advertised as compatible with
> > > the previous one, even if the user disables the feature in Qemu (as
> > > pointed out by Yuanhan).
> > 
> > We need two distinct capabilities in order to make this work properly.
> > 
> > First, libvirt needs to be able to query the list of (one or more)
> > supported versions strings for a given host.
> 
> Shouldn't be the role of OpenStack/Neutron? IIUC, libvirt knows nothing
> about OVS.

If libvirt doesn't know about it, then libvirt can't do any migration
checks upfront. Nova will have todo a check against supported version
strings before triggering migrate in libvirt.  That's probably fine
from libvirt POV.


Regards,
Daniel
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