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

[Yuanhan Liu <yuanhan.liu@linux.intel.com>]

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.

commit 362ca39639ae871806be5ae97d55e1cbb14afd92
Author: mweglicx <michalx.weglicki@intel.com>
Date:   Thu Apr 14 17:40:06 2016 +0100

    Update relevant artifacts to add support for DPDK 16.04.

    Following changes are applied:
     - INSTALL.DPDK.md: CONFIG_RTE_BUILD_COMBINE_LIBS step has been
       removed because it is no longer present in DPDK configuration
       (combined library is created by default),
     - INSTALL.DPDK.md: VHost Cuse configuration is updated,
     - netdev-dpdk.c: Link speed definition is changed in DPDK and
       netdev_dpdk_get_features is updated accordingly,
     - netdev-dpdk.c: TSO and checksum offload has been disabled for
       vhostuser device.
     - .travis/linux-build.sh: DPDK version is updated and legacy
       flags have been removed in configuration.

    Signed-off-by: Michal Weglicki <michalx.weglicki@intel.com>
    Signed-off-by: Panu Matilainen <pmatilai@redhat.com>
    Acked-by: Daniele Di Proietto <diproiettod@vmware.com>

	--yliu