linux-kernel - Re: [PATCH] [5/20] x86: Introduce nsec

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Date:	Thu, 3 Jan 2008 13:55:44 +0100
From:	Andi Kleen <ak@...e.de>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH] [5/20] x86: Introduce nsec_barrier()

On Thursday 03 January 2008 11:47:54 Ingo Molnar wrote:
> 
> * Andi Kleen <ak@...e.de> wrote:
> 
> > nsec_barrier() is a new barrier primitive that stops RDTSC speculation 
> > to avoid races with timer interrupts on other CPUs.
> > 
> > Add it to all architectures. Except for x86 it is a nop right now. I 
> > only tested x86, but it's a very simple change.
> > 
> > On x86 it expands either to LFENCE (for Intel CPUs) or MFENCE (for AMD 
> > CPUs) which stops RDTSC on all currently known microarchitectures that 
> > implement SSE. On CPUs without SSE there is generally no RDTSC 
> > speculation.
> 
> i've picked up your rdtsc patches into x86.git but have simplified it: 
> there's no nsec_barrier() anymore - rdtsc() is always synchronous. 
> MFENCE/LFENCE is fast enough. Open-coding such barriers almost always 
> leads to needless trouble. Please check the next x86.git tree.

That's most likely wrong unless you added two barriers -- the barriers
are strictly need to be before and after RDTSC.

I still think having the open barrier is the better approach here.

It's also useful for performance measurements because it allows
a cheap way to measure a specific region with RDTSC

-Andi
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