Re: [Xen-devel] [PATCH 05/12] xen/pvclock: add monotonicity check

[Jeremy Fitzhardinge <jeremy@goop.org>]

On 10/15/09 18:32, john stultz wrote:
>>> 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().
>>>       
>> I understand but you are now essentially emulating a
>> reliable platform timer with a potentially unreliable
>> (but still high resolution) per-CPU timer AND probably
>> delivering that result to userland.
>>
>> Read_tsc should only be used if either CONSTANT_TSC
>> or TSC_RELIABLE is true, so read_tsc is guaranteed
>> to be monotonically-strictly-increasing by hardware
>> (and enforced for CONSTANT_TSC by check_tsc_warp
>> at boot).
>>     
> Ideally, yes, only perfect TSCs should be used.
>
> But in reality, its a big performance win for folks who can get away
> with just slightly offset TSCs.
>   

What monotonicity guarantees do we make to usermode, for both syscall
and vsyscall gettimeofday and clock_gettime?

Though its not clear to me how usermode would even notice very small
amounts of cross-thread/cpu non-monotonicity anyway.  It would need make
sure that it samples the time and stores it to some globally visible
place atomically (with locks, compare-and-swap, etc), which is going to
be pretty expensive.  And if its going to all that effort it may as well
do its own monotonicity checking/adjustments if its all that important.

(I can think of plenty of ways of doing it incorrectly, where you'd get
apparent non-monotonicity regardless of the quality of the time source.)

    J