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SubjectRe: [PATCH] x86: Support always running TSC on Intel CPUs
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
> frequency.
>
> 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
> C-states.
>
> 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?

-hpa


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