linux-kernel - Re: [PATCH 1/3] Add prctl commands PR_GET_TSC and PR_SET

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Message-ID: <48025F1B.10701@zytor.com>
Date:	Sun, 13 Apr 2008 12:29:31 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Andi Kleen <andi@...stfloor.org>
CC:	Erik Bosman <ejbosman@...vu.nl>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	Arjan van de Ven <arjan@...radead.org>,
	Andrea Arcangeli <andrea@...share.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3] Add prctl commands PR_GET_TSC and PR_SET_TSC

Andi Kleen wrote:
> Erik Bosman <ejbosman@...vu.nl> writes:
>> I'm using it for deterministic replay.
> 
> Ok that should be in the changelog.
> 
> BTW x86 CPUs are not fully deterministic. e.g. there are a few errata that
> can lead to differing EFLAGS (generally for instructions with undefined flags 
> output) based on random internal pipe line conditions.
> 

I think you have to define "x86 CPUs" more tightly for that.

> There's also RDPMC, but by default the kernel does not enable that
> for ring 3. And if you go for oddities there are the random number
> generator instructions on VIA CPUs which will obviously not 
> be repeatable.

There has been calls for an RDPMC counter which exposes true CPU cycles 
(varying with frequency, as opposed to wall time.)  And anything I/O -- 
including the RNG -- is obviously off.

I think what Erik is trying to do is to make it possible to disable as 
many of these in the kernel as possible; I/O is easy, it's off by 
default; RDTSC and RDPMC can be disabled in the kernel, and I think even 
XSTORE can be disabled.

	-hpa
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