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

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.

Second, when launching QEMU we need to be able to specify the desired version against the NIC backend. So, consider host A, initially supporting "ovsdpdk-v1". When libvirt launches the VM it will specify 'ovsdpgk-v1' as the desired version string to use. Now some time later you add features X, Y & Z to a new release of DPDK and install this on host B. Host B is able to support two versions 'ovsdppk-v1' and 'ovsdpdk-v2'. When libvirt launches a VM on host B, it'll pick 'ovsdpgk-v2' by default, since that's the newest. When libvirt migrates a VM from host A, however, it will request the old version 'ovsdpdk-v1' in order to ensure compatibility. Similarly when launching a new VM on host B, libvirt could choose to use 'ovsdpdk-v1' as the version, in order to enable migration to the olver host A, if desired. This is exactly the way QEMU machine types work, hiding the existance of 100's low level settings / default values, that a mgmt app would otherwise have to worry about.

Regards, Daniel