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.