Re: dpdk/vpp and cross-version migration for vhost

[Kevin Traynor <ktraynor@redhat.com>]

On 11/24/2016 12:47 PM, Maxime Coquelin wrote:
> 
> 
> 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.

I can't remember off hand, added Mark K or Michal W who should be able
to shed some light on it.

> 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?

I think best to find out the reason first. Because if no reason to
disable in the code, then no need to debate!

> 
> Cheers,
> Maxime