Message-ID: <48A05169.2090303@redhat.com>
Date: Mon, 11 Aug 2008 16:49:13 +0200
From: Gerd Hoffmann <kraxel@redhat.com>
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To: Glauber Costa <glommer@gmail.com>
CC: Jeremy Fitzhardinge <jeremy@goop.org>, Avi Kivity <avi@qumranet.com>,
       Marcelo Tosatti <mtosatti@redhat.com>,
       Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
       kvm-devel <kvm-devel@lists.sourceforge.net>
Subject: Re: Use of barriers in pvclock ABI
References: <489CA3DA.1090400@goop.org> <489FE56E.1080707@redhat.com> <5d6222a80808110718i6a600858v7bdb5e08054ebefa@mail.gmail.com>
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Glauber Costa wrote:
> On Mon, Aug 11, 2008 at 4:08 AM, Gerd Hoffmann <kraxel@redhat.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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