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

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.