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Date: Tue, 18 May 2010 12:26:19 -0700
From: "H. Peter Anvin" <hpa@zytor.com>
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To: Dan Magenheimer <dan.magenheimer@oracle.com>
CC: Peter Zijlstra <peterz@infradead.org>, Andi Kleen <andi@firstfloor.org>,
       Arjan van de Ven <arjan@infradead.org>,
       Thomas Gleixner <tglx@linutronix.de>,
       Venkatesh Pallipadi <venki@google.com>, Ingo Molnar <mingo@elte.hu>,
       chris.mason@oracle.com, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] x86: Export tsc related information in sysfs
References: <1273887635-27610-1-git-send-email-venki@google.com>> <87tyq9mqrz.fsf@basil.nowhere.org>> <35aa841b-e151-424d-b1c1-0c03dbcae5cc@default>> <20100515121424.38f5b389@infradead.org>> <e07ac2f3-4a82-41d9-aa41-49a94435e7b7@default>> <20100515224305.17a04022@infradead.org>> <alpine.LFD.2.00.1005161113450.3199@localhost.localdomain>> <5adf3fee-0f7b-4039-b13f-619640cc4b88@default>> <20100516220638.2baf315d@infradead.org> <1274176698.5605.7358.camel@twins>> <20100518112509.GD22675@basil.fritz.box> <1274183935.5605.7726.camel@twins>> <4BF2C30F.6030502@zytor.com> <1274201536.5605.8347.camel@twins> <4BF2C881.60807@zytor.com> <1dc4abc2-6455-4b95-90f6-c86bf56ff39a@default 4BF2E0A2.6030209@zytor.com dd1c0508-5570-4e65-89c0-4037eb210646@default> <3479976f-5c91-4bcb-8542-3de93bee268a@default>
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On 05/18/2010 12:16 PM, Dan Magenheimer wrote:
> 
> Oops, hit send too soon.
> 
> And the reason I expect tglx/arjan/andi/mingo to disagree is because
> their position is that there is NO safe use for rdtsc in userspace EVER!
> Whereas your position stated earlier:
> 
>> There are restricted uses of the TSC in userspace which are still
>> useful
>> (mainly involving performance analysis and/or CPU-locked processes).
> 
> says there are.
> 
> While the engineer in me agrees with tglx/arjan/andi/mingo, the
> realist in me agrees with you.

I should have added "that are not related to wall time" to the statement
above.

Furthermore, vsyscalls are user space from a CPU perspective.

	-hpa