quoted

 But I think that a clock source can be expected to be
monotonic anyway, which Xen's interpolation mechanism doesn't
guarantee across multiple CPUs. (I'm actually beginning to think that
this might also be the reason for certain test suites occasionally reporting
timeouts to fire early.)
  

Does the kernel expect the tsc clocksource to be completely monotonic
across cpus?  Any form of cpu-local clocksource is going to have this
problem; I wonder if clocksources can really only be useful if they're
always referring to a single system-wide time reference - seems like a
bit of a limitation.

I suppose so - the clock source's rating gets set to zero if it can be predicted
that the TSCs aren't all synchronized, which should pretty much exclude the
use of this clock source for any other than fallback if there's really nothing
else available.

Jan