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

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Date:	Tue, 18 May 2010 13:34:44 -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 01:29 PM, Dan Magenheimer wrote:
> 
> 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.
> 

That is correct.

> 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.

I'm not in favor of emulating rdtsc instructions.  I would consider
letting them SIGILL (actually SIGSEGV since RDTSC #GP in userspace) when
the TSC is unavailable, though.

It's not clear to me that it's possible, though, since that also affects
RDTSCP.

	-hpa


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