Hello Al, I just debugged a kvmclock bug, so I claim to have some knowledge in this area now, but please take my answer with a grain of doubt.
On Monday 12 September 2011 15:21:25 al pat wrote: > Still seeking your guidance on this. Appreciate any pointers you may have. You have to distiguish between the real-time-clock (RTC), which in hardware is a battery powered clock running even when your PC is powered off. Since it's slow to access, most Linux distributions read out its value once during boot using "hwclock --hctosys --utc" and than don't care about that clock any more until shutdown, when they write back the system time to the RTC using "... --systohc ...". During runtime, other methods are used for time keeping: Either by counting regular interrupts, using the ACPI-PM clock, or the High Performance Event Timer (HPET), or the Time Stamp Counter (TSC) register, or ...; see /sys/devices/system/clocksource/clocksource0/available_clocksource for a list of available clock sources. For virtual machines there is an additional clock source named "kvmclock", which uses the host clock and the TSC: The host exports its current system time (plus some configurable offset) and a snapshot value of TSC register when doing so. Than the guest can interpolate the current time by using the exported_system_time + scale * (current_TSC_value-snapshot_TSC_value). This kvmclock doesn't have anything to do with the RTC clock as far as I know. Now to your problem: You should check the value of /sys/devices/system/clocksource/clocksource0/current_clocksource in your guest. If it is somethong other than kvmclock, you should if using "hwclock --hctosys --utc" re-synchronizes your guest clock to the host. Sincerely Philipp -- Philipp Hahn Open Source Software Engineer h...@univention.de Univention GmbH Linux for Your Business fon: +49 421 22 232- 0 Mary-Somerville-Str.1 D-28359 Bremen fax: +49 421 22 232-99 http://www.univention.de/ ---------------------------------------------------------------------------- Treffen Sie Univention auf der IT&Business vom 20. bis 22. September 2011 auf dem Gemeinschaftsstand der Open Source Business Alliance in Stuttgart in Halle 3 Stand 3D27-7.
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