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
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