[PATCH V5 00/25] Live Update [restart] : fork mode?

[Zheng Chuan]

Hi, Steve
I have saw the discussion about the fork+exec mode below:
https://www.mail-archive.com/qemu-devel@nongnu.org/msg815956.html

And I am still very curious and I want to discuss about the possibility to 
support both fork+exec and exec in cpr framework.

1.Why
fork+exec could have some advantages and also drawbacks versus execvp() 
directly.
Advantages
i) fork+exec give the chance to fallback to original process even after we do 
exec which is important for workload seamless if any error happens.
ii) smaller downtime since we could remove the vm_start() downtime out of the 
frozen window.
Drawbacks
i）need more codes to handle including fork，address/ports conflict between 
parent and child.
ii）more complex life cycle management for the two processes.

2.How
The cpr framework is flexible and scalable, and maybe we can make use of most 
codes to support both execvp and fork+exec mode.
However, we may need to do more work compared to execvp method.
i) do fork mode in a thread like migration thread
ii) make parent and child talk to each other through socket or anonymous pipe
iii）make use of sharing mechanism of fds in cpr framework including memfd, vfio 
and devices fds
iv）deal with the conflict about the socket address and port like vnc （do by 
reuse port and pass the different args by cprexec）
v) do life cycle managements for two qemu processes and need parent exit and 
reconnection for the child at last by the management service

Please tell me if I am missing something important:)

-- 
Regards.
Chuan