Re: [Qemu-devel] [PATCH V2 3/3] virtio-net: announce self by guest

[Amit Shah]

On (Wed) 11 Jun 2014 [10:50:04], Jason Wang wrote:
> On 06/10/2014 06:10 PM, Michael S. Tsirkin wrote:
> > On Tue, Jun 10, 2014 at 11:50:33AM +0530, Amit Shah wrote:
> >> On (Tue) 20 May 2014 [14:01:44], Jason Wang wrote:
> >>> It's hard to track all mac addresses and their configurations (e.g
> >>> vlan or ipv6) in qemu. Without this information, it's impossible to
> >>> build proper garp packet after migration. The only possible solution
> >>> to this is let guest (who knows all configurations) to do this.
> >>>
> >>> So, this patch introduces a new readonly config status bit of virtio-net,
> >>> VIRTIO_NET_S_ANNOUNCE which is used to notify guest to announce
> >>> presence of its link through config update interrupt.When guest has
> >>> done the announcement, it should ack the notification through
> >>> VIRTIO_NET_CTRL_ANNOUNCE_ACK cmd. This feature is negotiated by a new
> >>> feature bit VIRTIO_NET_F_ANNOUNCE (which has already been supported by
> >>> Linux guest).
> >>>
> >>> During load, a counter of announcing rounds is set so that after the vm is
> >>> running it can trigger rounds of config interrupts to notify the guest to 
> >>> build
> >>> and send the correct garps.
> >> Live migration is supposed to be transparent to guests.
> >>
> >> Doing things this way makes the guest involved in live migration.
> >> It's not desirable.
> > I'm not sure there's a problem.
> > As long as guest doesn't use networking, it does not
> > need to be involved. If guest does want to use networking,
> > it needs to be involved, but then it's accessing the
> > device anyway.
> >
> >> For networking, this may well be not possible.
> >> Are there any ways of doing this w/o involving the guest that have
> >> been considered?
> >>
> > Since we don't know guest addresses, this looks like the only way to me.

I was afraid of that.

> Yes and this method were also used by Xen and HyperV.

Well there's precedent, but that didn't stop us from trying for
something better in the past ;-)

                Amit