Hello, On Wednesday 11 January 2012 01:56:25 Zhang, Yang Z wrote: > > -----Original Message----- > > From: Paolo Bonzini [mailto:pbonz...@redhat.com] > > Sent: Tuesday, January 10, 2012 5:25 PM > > > > >> Also, I'm not sure if the update in progress flag still works. > > >> Clients are supposed to wait for UIP=0 before reading the RTC, and an > > >> update is supposed to be at least 220 microseconds away when UIP=0. > > > > > > Hardware need a period time to update clock and it would not provide > > > the right value during the update. So it uses UIP to notify the > > > software doesn't believe the value if the UIP is set. For emulation, > > > you can read RTC at any time and it always gives you the right value. > > > So there is no need to emulate UIP. > > > > This is incorrect, for two reasons. First, the UIP is in the spec, and > > we have to implement it. Second, reading the clock is not atomic, and > > waiting for UIP=0 gives you 220 microseconds during which you know that > > the read will appear atomic. > > For a simulator, we need to follow the spec strictly and simulate hardware > as precisely as possible. But QEMU is a generic machine emulator and > virtualizer. It's not a hardware simulator. If there is an easy way we can > provide the same function, why we chose the complicated one? Also, is there > an actual case that break with my patch?
FYI:
But you must not break existing implementations (of any (closed-source) OS),
which depend on that behaviour of the RTC. Have a look at get_cmos_time() of
the Xen hypervisor for example (that is the one I have been looking at at the
past few hours), which explicitliy waits for the falling edge of UIP to get
sub-second precision. This would break if you no longer simulate UIP.
Sincerely
Philipp
--
Philipp Hahn Open Source Software Engineer h...@univention.de
Univention GmbH Linux for Your Business fon: +49 421 22 232- 0
Mary-Somerville-Str.1 D-28359 Bremen fax: +49 421 22 232-99
http://www.univention.de/
signature.asc
Description: This is a digitally signed message part.
