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

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Date:	Tue, 18 May 2010 09:40:47 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	Andi Kleen <andi@...stfloor.org>,
	Arjan van de Ven <arjan@...radead.org>,
	Dan Magenheimer <dan.magenheimer@...cle.com>,
	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 04:58 AM, Peter Zijlstra wrote:
> On Tue, 2010-05-18 at 13:25 +0200, Andi Kleen wrote:
>> On Tue, May 18, 2010 at 11:58:18AM +0200, Peter Zijlstra wrote:
>>> On Sun, 2010-05-16 at 22:06 -0700, Arjan van de Ven wrote:
>>>> look we're not disabling ring 3 tsc. We could, but we don't.
>>>
>>> Maybe we should.
>>
>> That would kill the vsyscall too. Remember it's running in ring 3.
>>
>> That is in theory you could disable it on systems where the vsyscall
>> doesn't use it, but then you would likely break huge amounts of software, 
>> unless you emulate it.
> 
> Well, software shouldn't use it, so breaking it sounds like a fine
> idea ;-)
> 
> Also, a slow emulation is an incentive to actually do the right thing.

The problem is that you throw the baby (vsyscall) out with the bathwater
(user rdtsc).

	-hpa

-- 
H. Peter Anvin, Intel Open Source Technology Center
I work for Intel.  I don't speak on their behalf.

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