Re: [PATCH] x86: Support always running TSC on Intel CPUs

From: H. Peter Anvin
Date: Mon Nov 17 2008 - 19:14:23 EST

Venki Pallipadi wrote:
Add support for CPUID_0x80000007_Bit8 on Intel CPUs as well. This bit means
that the TSC is invariant with C/P/T states and always runs at constant

With Intel CPUs, we have 3 classes
* CPUs where TSC runs at constant rate and does not stop n C-states
* CPUs where TSC runs at constant rate, but will stop in deep C-states
* CPUs where TSC rate will vary based on P/T-states and TSC will stop in deep

To cover these 3, one feature bit (CONSTANT_TSC) is not enough. So, add a
second bit (NOSTOP_TSC). CONSTANT_TSC indicates that the TSC runs at
constant frequency irrespective of P/T-states, and NOSTOP_TSC indicates
that TSC does not stop in deep C-states.

What is the definition of a "deep" C-state? C2? C3? C4?

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