Re: [PATCH 1/3] Add prctl commands PR_GET_TSC and PR_SET_TSC

["H. Peter Anvin" <hpa@zytor.com>]

Andi Kleen wrote:
> Erik Bosman <ejbosman@cs.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