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.
DPDK v16.04 added support for vHost User TSO; as such, by default, TSO is
advertised to guest devices as an available feature during feature negotiation
with QEMU.
However, while the vHost user backend sets up the majority of the mbuf fields
that are required for TSO, there is still a reliance on the associated DPDK
application (i.e. in this case OvS-DPDK) to set the remaining flags and/or
offsets. Since OvS-DPDK doesn't currently provide that functionality, it is
necessary to explicitly disable TSO; otherwise, undefined behaviour will ensue.