Bug#534978: clock drift in Xen domU with clocksource=xen
Package: linux-image-2.6.26-2-686-bigmem
Version: 2.6.26-15lenny3
Severity: important
I'm running this kernel in a Xen domU using the xen clocksource:
# cat /sys/devices/system/clocksource/clocksource0/current_clocksource
xen
The dom0 is running linux-image-2.6.26-2-xen-686 (same version 2.6.26-15lenny3),
also with the xen clocksource, and an NTP client.
My understanding of the documentation is that the domU's wall clock should
be based on information passed (in shared memory) by the hypervisor, which
in turn gets clock updates from dom0.
I'm observing that the domU's clock runs fast relative to dom0 and the rest
of the world.
Rebooting the domU causes its clock to be reset to the correct time.
Moreover, I've tried running Xen's mini-os.gz (not in Debian's binary
packages of Xen, I built it from the extras/mini-os directory of the
xen-3 source package) as another domU on the same system, and it
printed correct timestamps. From this I deduce that the hypervisor's
notion of time is correct, and that the problem must lie in how
the domU kernel uses the information from the hypervisor.
So far I haven't observed the 'clocksource/0: Time went backwards' error
message mentioned at http://wiki.debian.org/Xen . I know I could switch
the domU to the "jiffies" clocksource and run NTP in it, but that's only
a workaround.
"xm info" on the dom0 reports:
release : 2.6.26-2-xen-686
version : #1 SMP Thu May 28 18:35:28 UTC 2009
machine : i686
nr_cpus : 1
nr_nodes : 1
cores_per_socket : 1
threads_per_core : 1
cpu_mhz : 2399
hw_caps : bfebfbff:00000000:00000000:00000080:00000400
total_memory : 2559
free_memory : 32
node_to_cpu : node0:0
xen_major : 3
xen_minor : 2
xen_extra : -1
xen_caps : xen-3.0-x86_32p
xen_scheduler : credit
xen_pagesize : 4096
platform_params : virt_start=0xf5800000
xen_changeset : unavailable
cc_compiler : gcc version 4.3.1 (Debian 4.3.1-2)
cc_compile_by : waldi
cc_compile_domain : debian.org
cc_compile_date : Sat Jun 28 15:25:00 UTC 2008
xend_config_format : 4
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