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

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Date:	Tue, 18 May 2010 13:29:36 -0700 (PDT)
From:	Dan Magenheimer <dan.magenheimer@...cle.com>
To:	"H. Peter Anvin" <hpa@...or.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

> From: H. Peter Anvin [mailto:hpa@...or.com]
> Subject: Re: [PATCH] x86: Export tsc related information in sysfs
> 
> On 05/18/2010 12:16 PM, Dan Magenheimer wrote:
> > 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.

Yes... that still puts your opinion at odds with tglx/etc.
All of the cases I am concerned with ARE performance analysis
uses, not wall time uses.

> Furthermore, vsyscalls are user space from a CPU perspective.

Yes, understood, a minor semantic issue.  From a kernel perspective
vsyscalls are kernelspace, so IIUC this is OK with tglx/etc.

Since vsyscall shouldn't be using rdtsc when the kernel
doesn't trust TSC, it doesn't matter if CR4.TSD is enabled when
the kernel doesn't trust TSC.

I'm still not sure if you are in favor of optionally emulating
PL3 rdtsc instructions or not?  I thought my proposal was
just filling out some details of your proposal and suggesting
a default.

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