Control: tag -1 moreinfo On Mon, 2016-10-03 at 04:33 -0400, Gedalya wrote: > Package: src:linux > Version: 4.7.5-1 > Severity: normal > > Dear Maintainer, > > When booting the host with linux 3.16, it looks like kvm-clock > provides guests with time as adjusted by ntpd. [...] > When booting with linux 4.6/4.7, [...] > ntpd measures on the guest the same drift as on the host. > This gives me the impression that on the later kernels, kvm-clock > provides a raw, unadjusted time. > > > On this particular setup I have the host running a stratum-1 NTP > server, hooked up to a GPS device. > The guests sync against it and other servers. > However I observe the same behavior on several other servers running > in various locations. > On those running linux 3.16, the guests' measured drift hovers around > zero, and on those running 4.6 or 4.7, the drift is around the same > as on the host. [...] Presumably you consider the behaviour on 4.6/4.7 to be undesirable. From what I can see, the host side may attempt to keep the pvclock synchronised with its real time but it is not recommended for guests to rely on this. This synchronisation is now optional and controlled by the kvmclock_periodic_sync module parameter, *but* it defaults to Y so I don't see why the actual behaviour has changed. First, check that this module parameter isn't somehow being turned off: cat /sys/module/kvm/parameters/kvmclock_periodic_sync Assuming it's on, check that the host is sending clock updates to the guest. You can trace that by running the following commands in the host as root, while the guest is running: cd /sys/kernel/debug/tracing echo function > current_tracer echo kvmclock_update_fn > set_ftrace_filter echo 1 > tracing_on cat trace_pipe The update interval is 5 minutes so you'll need to wait at least that long to be sure it's not happening. On my KVM host running 4.7 I saw this after a few minutes: kworker/3:0-23291 [003] .... 360386.076593: kvmclock_update_fn <-process_one_work Ben. -- Ben Hutchings Every program is either trivial or else contains at least one bug
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