If you upgrade Debian Stretch to linux-image-4.9.0-7 (kernel version 4.9.110-1), and you have a paravirtualized VPS, your system will not boot. There is a known and verified workaround: add pti=off to your kernel command line. You may also write support to ask about a PV to HVM conversion. If you do decide to convert to HVM, make sure to remove the workaround pti=off when the conversion is done.

The Debian bug report is Bug 903821.