time does not move forward in HVM guests

[Olaf Hering <olaf@aepfle.de>]

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In my testing with sysbench in a HVM domU running a linux-4.4 based
pvops kernel on a xen-4.7 based dom0 the time does not move forward
properly:

There (URL below) is basically code like this:
  clock_gettime(CLOCK_MONOTONIC, a)
  do_work
  clock_gettime(CLOCK_MONOTONIC, b)
  diff_time(a,b)

All 'do_work' does is writing zeros to a block of memory.
clock_getres(CLOCK_MONOTONIC) indicates a resolution of 1ns.
If 'do_work' takes like 100ns or less: a==b. I think this is something
that should not happen. In case of vcpu overcommit this happens also
when 'do_work' takes around 800ns. At some point I have also seen cases
of time going backward. I can not reproduce this anymore, might have
been bugs in my code or the domU.cfg changed.

A workaround is booting the domU kernel with 'clocksource=tsc nohz=off highres=off'.

Why does this happen? Are the expectations too high?


Olaf


 https://github.com/olafhering/sysbench/compare/master...pv
 bash autogen.sh
 make -j
 bash mem.1K.on.sh

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