CentOS server reboot automatically

My virtual server is CentOS 6, it reboot itself randomly without any clear errors. Only see following messages in /var/log/messages:

Jul 3 00:27:17 server kernel: Speculative Store Bypass: Vulnerable
Jul 3 00:27:17 server kernel: FEATURE SPEC_CTRL Not Present
Jul 3 00:27:17 server kernel: Spectre V2 : Mitigation: Full retpoline
Jul 3 00:27:17 server kernel: Spectre V2 : WARNING: module ‘parasite_loader’ built without retpoline-enabled compiler, may affect Spectre v2 mitigation

This was because the virtual server kernel get updated but the host wasn’t patched to mitigate Spectre And Meltdown. To resolve the issue temporarily, you can switch the kernel to previous version by updating the /etc/grub.conf to use the old kernel version instead. You need to reboot the server after this change.

You could also disable it by adding these kernel flags in /etc/grub.conf

spectre_v2=off nopti