linux-kernel - Re: [patch 9/9] Make use of the Master Timer

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Date:	Fri, 2 Feb 2007 08:04:29 +0100
From:	Andi Kleen <ak@...e.de>
To:	Jiri Bohac <jbohac@...e.cz>
Cc:	linux-kernel@...r.kernel.org, Vojtech Pavlik <vojtech@...e.cz>,
	ssouhlal@...ebsd.org, arjan@...radead.org, tglx@...utronix.de,
	johnstul@...ibm.com, zippel@...ux-m68k.org, andrea@...e.de,
	Recent@...e.de, Addresses@...e.de
Subject: Re: [patch 9/9] Make use of the Master Timer

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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