Re: [Qemu-devel] [PATCH 3/3] stop the periodic RTC update timer

[Zhang, Yang Z]

> -----Original Message-----
> From: Marcelo Tosatti [mailto:address@hidden
> Sent: Thursday, January 12, 2012 6:03 PM
> To: Zhang, Yang Z
> Cc: address@hidden; address@hidden; address@hidden; Zhang,
> Xiantao; Shan, Haitao; address@hidden
> Subject: Re: [PATCH 3/3] stop the periodic RTC update timer
> 
> On Thu, Jan 12, 2012 at 07:59:06AM -0200, Marcelo Tosatti wrote:
> > On Thu, Jan 12, 2012 at 12:00:06AM +0000, Zhang, Yang Z wrote:
> > > > -----Original Message-----
> > > > From: Marcelo Tosatti [mailto:address@hidden
> > > >
> > > > 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.
> >
> > Can't it wait a change from 0 to 1?
> 
> The point is the guest can use the hardware as it pleases, not only as is 
> suggested
> in the hardware documentation.
You are right. I will add it to next version.

best regards
yang