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

[Jeremy Fitzhardinge]

On 10/15/09 06:27, Dan Magenheimer wrote:
> 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).
>   

read_tsc clearly isn't expected to produce absolutely globally monotonic
results; if it were then the check wouldn't be necessary.

    J
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