Re: [Xen-devel] [RFC] [PATCH] use "reliable" tsc properly when available, but verify

[Keir Fraser <keir.fraser@xxxxxxxxxxxxx>]

On 28/09/2009 21:19, "Dan Magenheimer" <dan.magenheimer@xxxxxxxxxx> wrote:

> So we trust but verify... if the cpuid bit is set
> we assume a reliable tsc, but use the Linux boottime
> check_tsc_warp algorithm to periodically verify that
> the tsc hasn't skewed.  If it has, we fall back to Xen
> managing (and periodically writing) the TSC.
> 
> The check_tsc_warp algorithm is CPU-intensive, so
> we test it on a decaying schedule, at 1sec, 2sec,
> 4sec, 8sec, 16sec, 32sec, etc.

Surely it should be sufficient to check TSCs for consistency across all CPUs
periodically, and against the chosen platform timer, and ensure none are
drifting? An operation which would not require us to loop for 2ms and would
provide rather more useful information than an ad-hoc multi-CPU
race-to-update-a-shared-variable-an-arbitrary-and-large-number-of-times.

I wouldn't take anything like this algorithm.

 -- Keir



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