linux-kernel - Re: [PATCH] x86: Export tsc related information in sysfs

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Message-ID: <4BF2E9DB.3000202@zytor.com>
Date:	Tue, 18 May 2010 12:26:19 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Dan Magenheimer <dan.magenheimer@...cle.com>
CC:	Peter Zijlstra <peterz@...radead.org>,
	Andi Kleen <andi@...stfloor.org>,
	Arjan van de Ven <arjan@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Venkatesh Pallipadi <venki@...gle.com>,
	Ingo Molnar <mingo@...e.hu>, chris.mason@...cle.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86: Export tsc related information in sysfs

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

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