From: Andi Kleen <ak@suse.de>
To: Jiri Bohac <jbohac@suse.cz>
Subject: Re: [patch 9/9] Make use of the Master Timer
Date: Fri, 2 Feb 2007 08:04:29 +0100
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On Thursday 01 February 2007 15:29, Jiri Bohac wrote:

> If I do:
> 	rdtscll(a)
> 	...
> 	rdtscll(b)
> is it guaranteed that (b > a) ?

It's not architecturally -- unless you have a barrier.

On P4 the micro architecture guarantees it, but there the barrier in
get_cycles_sync is  patched away. On other x86-64s it is generally needed.

The effect can be also seen between CPUs.

> Because of the __vxtime.cpu[cpu].tsc_invalid flag. We may be

You can still precompute it for the HPET etc. case.
They are already slow, but saving a condition there might be still
worth it.

-Andi
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