Re: [Qemu-devel] [V5 PATCH 0/4] Send gratuitous packets by guest

[Jason Wang]

On 03/27/2012 06:10 AM, Anthony Liguori wrote:
> On 03/16/2012 03:54 AM, Jason Wang wrote:
> > This an update of series that let guest and qemu to be co-operated to
> > send gratuitous packets when needed such as after migration, loadvm
> > and continuing.
> >
> > As it's hard for qemu to track the network configuration in guest such
> > as bondings, vlans or ipv6. So current gratuitous may not work under
> > those situations.
>
> Can you be more specific about the failure scenarios?

The failure can happen when:

- Guest does not use primary mac address. Current qemu only send rarp 
packets for primary mac address. This looks could be solved by iterating 
nic with mac address table, but their size are limited and guest could 
use all-uni/promisc mode to use more mac addresses. So it's almost 
impossible to track all addresses in qemu side.

- Guest have some network configuration such as 802.1Q vlan, in this 
case, we need to send tagged gratuitous packet which qemu can't handle.

The point is qemu does not know the network configuration in guest or 
how mac addresses are used, so it's better for us to let guest choose 
the correct way to do this if they can. If guest could not do the 
announcement, the rarp packets would be sent as in the past.
> Does this mean that migration cannot work today with guests using 
> ipv6?  I don't think just pushing this to the guest is an acceptable 
> solution in the short term.

I haven't check, but w/o this patch a ipv4 rarp would be sent even guest 
are using ipv6; w/ this patch, a ipv6 neighbor advertisement would be 
sent by guest which looks pretty reasonable.

Did you see any drawbacks of this method? Xen and hyper-v also let guest 
to send gratuitous packet for their para-virtualized network card.
> Are there scenarios we cannot handle no matter what in the host?  Does 
> this mean that for emulated drivers, we're completely out of luck?

A possible improvement but not a final solution is to send garp for all 
address in the mac table I think.
> Regards,
>
> Anthony Liguori