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

[Marcelo Tosatti]

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.