Messages in this thread | | | Date | Wed, 14 Oct 2009 22:24:00 -0700 | From | Jeremy Fitzhardinge <> | Subject | Re: [Xen-devel] [PATCH 05/12] xen/pvclock: add monotonicity check |
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On 10/14/09 20:26, Dan Magenheimer wrote: > As long as we are going through the trouble of making > this monotonic, shouldn't it be monotonically increasing > (rather than just monotonically non-decreasing)? The > rdtsc instruction and any suitably high-precision > hardware timer will never return the same value > on subsequent uses so this might be a reasonable > precedent to obey. E.g. > > + return ret > xen_clocksource.cycle_last ? > + ret : ++xen_clocksource.cycle_last; >
No, cycle_last isn't updated on every read, only on timer ticks. This test doesn't seem to be intended to make sure that every clocksource_read is globally monotonic, but just to avoid some boundary conditions in the timer interrupt. I just copied it directly from read_tsc().
J
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