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…