linux-kernel - RE: [PATCH] x86: Support always running TSC on Intel CPUs

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Message-ID: <7E82351C108FA840AB1866AC776AEC464291B456@orsmsx505.amr.corp.intel.com>
Date:	Mon, 17 Nov 2008 16:16:09 -0800
From:	"Pallipadi, Venkatesh" <venkatesh.pallipadi@...el.com>
To:	"H. Peter Anvin" <hpa@...or.com>
CC:	Ingo Molnar <mingo@...e.hu>, Thomas Gleixner <tglx@...utronix.de>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH] x86: Support always running TSC on Intel CPUs

 

>-----Original Message-----
>From: H. Peter Anvin [mailto:hpa@...or.com] 
>Sent: Monday, November 17, 2008 4:14 PM
>To: Pallipadi, Venkatesh
>Cc: Ingo Molnar; Thomas Gleixner; linux-kernel
>Subject: Re: [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

All C-states higher than C1. 

Thanks,
Venki--
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