linux-kernel - Re: Use of barriers in pvclock ABI

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Message-ID: <489FE56E.1080707@redhat.com>
Date:	Mon, 11 Aug 2008 09:08:30 +0200
From:	Gerd Hoffmann <kraxel@...hat.com>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
CC:	Glauber de Oliveira Costa <glommer@...il.com>,
	Avi Kivity <avi@...ranet.com>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	kvm-devel <kvm-devel@...ts.sourceforge.net>
Subject: Re: Use of barriers in pvclock ABI

  Hi,

> However, the pvclock_clocksource_read() implementation is
> over-engineered, because it checks for an odd version and uses very
> strong rmb() barriers (which generates either an "lfence" or "lock add
> $0, (%esp)").
> 
> If we're happy to guarantee as an ABI issue that the record will never
> be updated cross-cpu, then we can make the barriers simply barrier() and
> just check for (src->version != dst->version).
> 
> Is that OK with you, or do you want to leave open the possibility of
> doing cross-cpu time updates?

Due to the TSC being involved here I don't expect cross-cpu time updates
will ever happen.  IMHO it is fine to change that.

cheers,
  Gerd

-- 
http://kraxel.fedorapeople.org/xenner/
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