RTC synchronization (11-minute mode) doesn't work on KVM

Solution Verified - Updated

Environment

  • Red Hat Enterprise Linux 8
  • KVM virtual machines

Issue

  • The kernel does not synchronize the RTC to the system clock. It should set the RTC about every 11 minutes.
  • Chrony continues to synchronize time correctly, however, RTC does not get in sync even after 11 minutes as expected.

Resolution

  • Update the kernel to version 4.18.0-477.10.1.el8_8 or later.
    For information on the advisory, and where to find the updated files, follow the link below:

https://access.redhat.com/errata/RHSA-2023:2951

  • As a workaround, kvmclock can be disabled by adding "no-kvmclock" to the kernel parameters, which enables the original x86 set_wallclock() used by sync_cmos_clock().

Root Cause

  • The KVM module performed the real-time clock (RTC) synchronization less frequently than intended. As a consequence, the system time in VMs hosted on RHEL 8 sometimes did not correctly reflect the system time on the host. This update fixes the RTC scheduling in KVM, which prevents the described problem from occurring.
  • The kvmclock driver overrides the x86-specific set_wallclock() function with a stub returning ENODEV, which causes sync_cmos_clock() to fail, but sync_rtc_clock() doesn't do anything due to missing CONFIG_RTC_SYSTOHC.

Diagnostic Steps

[root@rhel8 ~]# cat /proc/cmdline 
BOOT_IMAGE=(hd0,msdos1)/vmlinuz-4.18.0-372.26.1.el8_6.x86_64 root=/dev/mapper/rhel-root ro crashkernel=auto resume=/dev/mapper/rhel-swap rd.lvm.lv=rhel/root rd.lvm.lv=rhel/swap rhgb quiet no-kvmclock
  • Manually modify the RTC clock:
[root@rhel8 ~]# timedatectl 
               Local time: Thu 2022-10-27 20:46:42 IDT
           Universal time: Thu 2022-10-27 17:46:42 UTC
                 RTC time: Thu 2022-10-27 17:46:42
[...]

[root@rhel8 ~]# hwclock --set --date '10 sec'

[root@rhel8 ~]# timedatectl 
               Local time: Thu 2022-10-27 20:46:59 IDT
           Universal time: Thu 2022-10-27 17:46:59 UTC
                 RTC time: Thu 2022-10-27 17:47:09
[...]
  • Wait at least 11 minutes:
[root@rhel8 ~]# sleep 11m
  • The RTC time was corrected:
[root@rhel8 ~]# timedatectl 
               Local time: Thu 2022-10-27 21:02:37 IDT
           Universal time: Thu 2022-10-27 18:02:37 UTC
                 RTC time: Thu 2022-10-27 18:02:37
[...]
SBR
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