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

On 11/24/2016 01:33 PM, Yuanhan Liu wrote: > 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. I'm also interested to know the reason.

In any case, I think this is something that can/should be managed by the management tool, which should disable it in cmd parameters. Kevin, do you agree?

Cheers, Maxime