From mboxrd@z Thu Jan 1 00:00:00 1970 From: Paolo Bonzini Subject: Re: [PATCH 3/3] stop the periodic RTC update timer Date: Thu, 12 Jan 2012 10:26:17 +0100 Message-ID: <4F0EA739.1040707@redhat.com> References: <20120111131004.GA18178@amt.cnet> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Cc: Marcelo Tosatti , "qemu-devel@nongnu.org" , "avi@redhat.com" , "aliguori@us.ibm.com" , "Zhang, Xiantao" , "Shan, Haitao" , "kvm@vger.kernel.org" To: "Zhang, Yang Z" Return-path: Received: from mail-we0-f174.google.com ([74.125.82.174]:62708 "EHLO mail-we0-f174.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753197Ab2ALJ0U (ORCPT ); Thu, 12 Jan 2012 04:26:20 -0500 Received: by werm1 with SMTP id m1so1176680wer.19 for ; Thu, 12 Jan 2012 01:26:19 -0800 (PST) In-Reply-To: Sender: kvm-owner@vger.kernel.org List-ID: On 01/12/2012 01:00 AM, Zhang, Yang Z wrote: >> Regarding the UIP bit, a guest could read it in a loop and wait >> for the value to change. But you can emulate it in >> cmos_ioport_read by reading the host time, that is, return 1 >> during 244us, 0 for remaining of the second, and have that in >> sync with update-cycle-ended interrupt if its enabled. > > Yes. Guest may use the loop to read RTC, but the point is the guest > is waiting for the UIP changed to 0. If this bit always equal to 0 , > guest will never go into the loop. For real RTC, this may wrong, > because the RTC cannot give you the valid value during the update > cycle. But the virtual RTC doesn't' need this logic, whenever you > read it, it will always return the right value to you. The point is not _correctness_. It is _atomicity_. Paolo