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

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

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);