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AWS rebooted PV instances to implement the Vixen shim ( https://lists.xenproject.org/archives/html/xen-devel/2018-01... ) from my understanding.

I do not believe that HVM instances (which would be closer to your KVM infrastructure) needed to be rebooted.

I had an HVM instance that had forced downtime.

I had 35 HVM instances that didn’t have forced downtime. There are other reasons a machine could need to be brought out of service.

Seconded, I have about 320 HVM instances and have received only a handful of reboot requests.