linux-kernel - Re: [PATCH 1/5] Add a global synchronization point for pvclock

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Message-ID: <4BCC4006.8000501@redhat.com>
Date:	Mon, 19 Apr 2010 14:35:34 +0300
From:	Avi Kivity <avi@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	Glauber Costa <glommer@...hat.com>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	Zachary Amsden <zamsden@...hat.com>
Subject: Re: [PATCH 1/5] Add a global synchronization point for pvclock

On 04/19/2010 01:59 PM, Peter Zijlstra wrote:
>
>> So what do we need?  test for both TSC_RELIABLE and NONSTOP_TSC?  IMO
>> TSC_RELIABLE should imply NONSTOP_TSC.
>>      
> Yeah, I think RELIABLE does imply NONSTOP and CONSTANT, but NONSTOP&&
> CONSTANT does not make RELIABLE.
>    

The manual says:

> 16.11.1 Invariant TSC
>
> The time stamp counter in newer processors may support an enhancement, 
> referred
> to as invariant TSC. Processor’s support for invariant TSC is indicated by
> CPUID.80000007H:EDX[8].
> The invariant TSC will run at a constant rate in all ACPI P-, C-. and 
> T-states. This is
> the architectural behavior moving forward. On processors with 
> invariant TSC
> support, the OS may use the TSC for wall clock timer services (instead 
> of ACPI or
> HPET timers). TSC reads are much more efficient and do not incur the 
> overhead
> associated with a ring transition or access to a platform resource.

and this maps to NONSTOP, so I think we're fine.

-- 
error compiling committee.c: too many arguments to function

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