[libvirt] [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