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

On Thu, Nov 24, 2016 at 09:30:49AM +0000, Kevin Traynor wrote: > > On 11/24/2016 06:31 AM, Yuanhan Liu wrote: >> > > On Tue, Nov 22, 2016 at 04:53:05PM +0200, Michael S. Tsirkin wrote: >>>>> > >>>> You keep assuming that you have the VM started first and >>>>> > >>>> figure out things afterwards, but this does not work. >>>>> > >>>> >>>>> > >>>> Think about a cluster of machines. You want to start a VM in >>>>> > >>>> a way that will ensure compatibility with all hosts >>>>> > >>>> in a cluster. >>>> > >>> >>>> > >>> I see. I was more considering about the case when the dst >>>> > >>> host (including the qemu and dpdk combo) is given, and >>>> > >>> then determine whether it will be a successfull migration >>>> > >>> or not. >>>> > >>> >>>> > >>> And you are asking that we need to know which host could >>>> > >>> be a good candidate before starting the migration. In such >>>> > >>> case, we indeed need some inputs from both the qemu and >>>> > >>> vhost-user backend. >>>> > >>> >>>> > >>> For DPDK, I think it could be simple, just as you said, it >>>> > >>> could be either a tiny script, or even a macro defined in >>>> > >>> the source code file (we extend it every time we add a >>>> > >>> new feature) to let the libvirt to read it. Or something >>>> > >>> else. >>> > >> >>> > >> There's the issue of APIs that tweak features as Maxime >>> > >> suggested. >> > > >> > > Yes, it's a good point. >> > > >>> > >> Maybe the only thing to do is to deprecate it, >> > > >> > > Looks like so. >> > > >>> > >> but I feel some way for application to pass info into >>> > >> guest might be benefitial. >> > > >> > > The two APIs are just for tweaking feature bits DPDK supports before >> > > any device got connected. It's another way to disable some features >> > > (the another obvious way is to through QEMU command lines). >> > > >> > > IMO, it's bit handy only in a case like: we have bunch of VMs. Instead >> > > of disabling something though qemu one by one, we could disable it >> > > once in DPDK. >> > > >> > > But I doubt the useful of it. It's only used in DPDK's vhost example >> > > after all. Nor is it used in vhost pmd, neither is it used in OVS. > > > > rte_vhost_feature_disable() is currently used in OVS, lib/netdev-dpdk.c Hmmm. I must have checked very old code ... > > > > netdev_dpdk_vhost_class_init(void) > > { > > static struct ovsthread_once once = OVSTHREAD_ONCE_INITIALIZER; > > > > /* This function can be called for different classes. The > > initialization > > * needs to be done only once */ > > if (ovsthread_once_start(&once)) { > > rte_vhost_driver_callback_register(&virtio_net_device_ops); > > rte_vhost_feature_disable(1ULL << VIRTIO_NET_F_HOST_TSO4 > > | 1ULL << VIRTIO_NET_F_HOST_TSO6 > > | 1ULL << VIRTIO_NET_F_CSUM); I saw the commit introduced such change, but it tells no reason why it was added.