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Date: Thu, 12 Jan 2012 10:26:17 +0100
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Subject: Re: [Qemu-devel] [PATCH 3/3] stop the periodic RTC update timer
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To: "Zhang, Yang Z" <yang.z.zhang@intel.com>
Cc: "aliguori@us.ibm.com" <aliguori@us.ibm.com>, "kvm@vger.kernel.org" <kvm@vger.kernel.org>, Marcelo Tosatti <mtosatti@redhat.com>, "Shan,
	Haitao" <haitao.shan@intel.com>, "qemu-devel@nongnu.org" <qemu-devel@nongnu.org>, "avi@redhat.com" <avi@redhat.com>

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