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

[Keir Fraser <keir.fraser@xxxxxxxxxxxxx>]

On 28/09/2009 22:01, "Keir Fraser" <keir.fraser@xxxxxxxxxxxxx> wrote:

> 
> 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 should add, not only is the algorithm stupid and slow, but it doesn't even
check for exactly what RELIABLE_TSC guarantees -- constant-rate TSCs. This
would be useless on a single-CPU system, for example, or perhaps more
practically a single-socket system where all TSCs skewed together due to
package-wide power management. In the latter case TSCs would not skew
relative to each other, even though they could 'skew' relative to wallclock
(represented in Xen by the platform timer).

 -- Keir



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