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

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Message-ID: <48A05169.2090303@redhat.com>
Date:	Mon, 11 Aug 2008 16:49:13 +0200
From:	Gerd Hoffmann <kraxel@...hat.com>
To:	Glauber Costa <glommer@...il.com>
CC:	Jeremy Fitzhardinge <jeremy@...p.org>,
	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

Glauber Costa wrote:
> On Mon, Aug 11, 2008 at 4:08 AM, Gerd Hoffmann <kraxel@...hat.com> wrote:
>> 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.
> 
> Okay for guest vcpu, but what about physical cpus?
> 
> IIRC, the checks are there, and so strict, to account for the
> possiblity of the vcpu to be migrated to another cpu in the middle of
> the
> clock reading.

This is about the check in pvclock_get_time_values() that it got a
consistent snapshot.  Dropping that is fine.

pvclock_clocksource_read() will still notice when being migrated to
another pcpu in the middle of the clock reading.

cheers,
  Gerd

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