Re: [virtio-dev] [PATCH v4] content: Introduce VIRTIO_NET_F_STANDBY feature

[Siwei Liu <loseweigh@gmail.com>]

On Tue, Oct 2, 2018 at 5:43 AM Michael S. Tsirkin <mst@redhat.com> wrote:
>
> On Tue, Oct 02, 2018 at 01:42:09AM -0700, Siwei Liu wrote:
> > The VF's MAC can be updated by PF/host on the fly at any time. One can
> > start with a random MAC but use group ID to pair device instead. And
> > only update MAC address to the real one when moving MAC filter around
> > after PV says OK to switch datapath.
> >
> > Do you see any problem with this design?
>
> Isn't this what I proposed:
>         Maybe we can
>         start VF with a temporary MAC, then change it to a final one when guest
>         tries to use it. It will work but we run into fact that MACs are
>         currently programmed by mgmnt - in many setups qemu does not have the
>         rights to do it.
>
> ?
>
> If yes I don't see a problem with the interface design, even though
> implementation wise it's more work as it will have to include management
> changes.

I thought we discussed this design a while back:
https://www.spinics.net/lists/netdev/msg512232.html

... plug in a VF with a random MAC filter programmed in prior, and
initially use that random MAC within guest. This would require:
a) not relying on permanent MAC address to do pairing during the
initial discovery, e.g. use the failover group ID as in this
discussion
b) host to toggle the MAC address filter: which includes taking down
the tap device to return the MAC back to PF, followed by assigning
that MAC to VF using "ip link ... set vf ..."
c) notify guest to reload/reset VF driver for the change of hardware MAC address
d) until VF reloads the driver it won't be able to use the datapath,
so very short period of network outage is (still) expected

though I still don't think this design can elimnate downtime. However,
it looks like as of today the MAC matching still haven't addressed the
datapath switching and error handling in a clean way. As said, for
SR-IOV live migration on iSCSI root disk there will be a lot of
dancing parts going along the way, reliable network connectity and
dedicated handshakes are critical to this kind of setup.

-Siwei

>
> --
> MST
>
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