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

[Maxime Coquelin]

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