Re: [PATCH] Skip tsc synchronization checks if CONSTANT_TSC bit isset.

From: Ingo Molnar
Date: Wed Oct 22 2008 - 15:26:54 EST

* Alok Kataria <akataria@xxxxxxxxxx> wrote:

> Skip tsc synchronization checks if CONSTANT_TSC bit is set.
> From: Alok N Kataria <akataria@xxxxxxxxxx>
> TSC synchronization checks between CPU's bail out even if we see a
> distortion of a single cycle. This makes the TSC mostly unsuable in a
> virtualized environment.
> The CONSTANT_TSC bit tells us if the hardware exports a constant TSC,
> we can use this bit to trust the hardware and skip the TSC sync checks
> at bootup.

the sync check is there to check the _offset_ between CPUs. CONSTANT_TSC
is not a guarantee that the TSC will be coherent across all CPUs.

so this patch is fundamentally wrong.

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