RE: [Xen-devel] [PATCH 0/2] range timer support

[Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>]

> here is my two cents just for your reference. I have tried range timer
> in several places:
> <snip>
> - HVM virtual period timer: this timer allow expires behind deadline a
> bit, because it has logic to handling missing ticks. so the range can
> be [deadline, deadline + period/n]. to be more conservative, it is
> safe to add up limit of 1ms. the 1ms comes from the commodity OS
> frequency, e.g. Linux kernel has 1000/250/100 HZ, and windows has 64
> HZ, 1ms should meet the up limit of 1000HZ.

A reminder that hvm HPET usage by some Linux kernels has a
problem where delivery of a tick sooner than expected can
cause the guests apparent time to advance too quickly.
Dave Winchell submitted a patch for this some time back
that didn't get accepted (for various reasons) and I'm
not sure if this range timer implementation will make it
better or worse.

But I'm pretty sure that if ticks are delivered at:

T+0d+3s, T+1d+2s, T+2d+1s

where d=1/Hz and s=slop, the problem will be observed.

Dave, can you comment?

Dan

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