From mboxrd@z Thu Jan 1 00:00:00 1970 From: Keir Fraser Subject: Re: [PATCH] x86: don't write_tsc() non-zero values on CPUs updating only the lower 32 bits Date: Fri, 15 Apr 2011 18:28:09 +0100 Message-ID: References: Mime-Version: 1.0 Content-Type: text/plain; charset="US-ASCII" Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Sender: xen-devel-bounces@lists.xensource.com Errors-To: xen-devel-bounces@lists.xensource.com To: Dan Magenheimer , Jan Beulich Cc: xen-devel@lists.xensource.com, "winston.l.wang" List-Id: xen-devel@lists.xenproject.org On 15/04/2011 15:34, "Dan Magenheimer" wrote: >>> Agreed. In fact, maybe it should be asserted in write_tsc? >> >> We still write_tsc on CPU physical hot-add. > > Hmmm... IIRC the testing that Intel was doing for hot-add was > not for processors that were actually electrically hot-plugged > but only for processors that were powered-on at the same > time as all other processors but left offline until needed > (e.g. for capacity-on-demand). For this situation, writing > to tsc is still the wrong approach. I don't think we finished > the discussion about electrically hot-plugged processors > because they didn't exist... don't know if they do yet either. > IIRC I had proposed an unnamed boot parameter that said > "this machine may add unsynchronized processors post-boot" > and disallow hot-add processors if not specified (or if > not specified AND a run-time check of a hot-add processor > shows non-synchronization). Well, I think the case I'm thinking of is electrical hot-plug. Not sure. Either way I doubt anyone is actually using the feature. -- Keir