Re: [virtio-dev] Re: [PATCH v4 2/2] virtio_net: Extend virtio to use VF datapath when available

["Samudrala, Sridhar" <sridhar.samudrala@intel.com>]

On 3/2/2018 12:44 PM, Siwei Liu wrote:
> On Fri, Mar 2, 2018 at 12:10 PM, Michael S. Tsirkin <mst@redhat.com> wrote:
> > On Fri, Mar 02, 2018 at 11:52:27AM -0800, Samudrala, Sridhar wrote:
> > > On 3/2/2018 11:41 AM, Michael S. Tsirkin wrote:
> > > > On Fri, Mar 02, 2018 at 07:26:25AM -0800, Alexander Duyck wrote:
> > > > > The design limits things to a 1:1 relationship since we just have the
> > > > > child and backup pointers, but I don't think I am seeing exception
> > > > > handling to prevent us from overwriting the child pointers so there
> > > > > may be a leak there.
> > > > >
> > > > > Thanks.
> > > > >
> > > > > - Alex
> > > > In fact maintaining a list in that case would be nicer, and
> > > > just use an arbitrary one.
> > > > E.g. one can see how a user wanting to swap device 1 for device 2
> > > > might first add device 2 with same MAC then drop device 1.
> > > It should be possible to swap VF1 with VF2 by
> > > 1.- enabling virtio link
> > > 2.- unplugging VF1
> > > 3.- plugging VF2
> > > 4.- disabling virtio link
> > True, but it isn't hard to avoid breakage if user
> > swapped steps 2 and 3. No need to make it more
> > fragile that it has to be.
> The migration case, VF2 is associated with another PF on another
> machine (destination), I wonder how it is possible.
>
> Even with local plugging of VF2 on the same PF, the MAC address
> requirement (VF1's == VF2's) would fail the MAC address assignment on
> VF2.
I didn't include updating the MAC filter step in the above sequence.
So definitely plugging 2 VFs with the same MAC address will be an issue.

Here is the more complete sequence of steps that are required to
enable live migration.

Source Hypervisor
- Bring up the virtio link
- Hot Unplug VF from the VM
- Delete FDB entry for the MAC on the Bridge associated with virtio/tap
- Remove the MAC filter associated with the VF
- Migrate VM to destination

Destination Hypervisor (after migration is completed)
- Set MAC filter for the VF
- Hot Plug VF to the VM
- Bring down the virtio link

Thanks
Sridhar