REGRESSION: relocating a Debian/bullseye guest is losing network connection

[Waldemar Brodkorb <mail@waldemar-brodkorb.de>]

Hi,

we use z/VM's Live Guest Relocation for our Debian/GNU Linux
servers. Since following commit this is broken:

d4560150cb47daba50b0d080550ca4bbe80cf3c3 is the first bad commit
commit d4560150cb47daba50b0d080550ca4bbe80cf3c3
Author: Julian Wiedmann <jwi@linux.ibm.com>
Date:   Thu Feb 28 18:59:39 2019 +0100

     s390/qeth: call dev_close() during recovery


With Debian/buster and Linux 4.18.x this works fine.
With Debian/bullseye and Linux 5.10.x this does not work anymore.
The reason for that is that all configured routes are lost after
relocating the guest.
This looks to me like a major regression.

The network is configured via /etc/network/interfaces and executing
systemctl restart networking fixes the issue. This can be done via a 
udev rule, but this looks like a rather incomplete workaround to us.

All manually added routing information will be lost anyway.

And I might not imagine what happens to any firewall connection
tables or ipvs connection tracking information in case of a Live
Guest Relocation.

So is there any kernel level solution for this you can think of?

Thanks for any advice and comments,

best regards
 Waldemar