linux-kernel - Re: Disabling x86 System Management Mode

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Message-ID: <46253CE0.6020209@free.fr>
Date:	Tue, 17 Apr 2007 23:32:16 +0200
From:	John Sigler <linux.kernel@...e.fr>
To:	Andi Kleen <andi@...stfloor.org>
CC:	linux-kernel@...r.kernel.org, linux.kernel@...e.fr
Subject: Re: Disabling x86 System Management Mode

Andi Kleen wrote:

> Modern x86 CPUs execute code out of order and in parallel.

I am aware of the (apparent) non-deterministic nature of
superscalar out-of-order speculative execution.

> The reordering window can be quite large and the CPU can execute code
> speculatively. This can add large errors to RDTSC when the 
> instruction is not executed where you think it is. One way around 
> this is to synchronize it -- using CPUID --

Your terminology is surprising. CPUID is commonly referred to as a
/serializing/ instruction.

> but that also adds latency and makes the measurement less precise.

Why would adding known latency make the measurement less precise?

What I needed was a block of code with consistent latency, whatever
the actual number.

Regards.
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