From: Philipp Hahn <hahn@univention.de>
To: qemu-devel@nongnu.org
Subject: Re: [Qemu-devel] [PATCH 3/3] stop the periodic RTC update timer
Date: Wed, 11 Jan 2012 08:25:38 +0100 [thread overview]
Message-ID: <201201110825.41752.hahn@univention.de> (raw)
In-Reply-To: <A9667DDFB95DB7438FA9D7D576C3D87E0240C7@SHSMSX102.ccr.corp.intel.com>
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Hello,
On Wednesday 11 January 2012 01:56:25 Zhang, Yang Z wrote:
> > -----Original Message-----
> > From: Paolo Bonzini [mailto:pbonzini@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 hahn@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/
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next prev parent reply other threads:[~2012-01-11 7:26 UTC|newest]
Thread overview: 18+ messages / expand[flat|nested] mbox.gz Atom feed top
2012-01-06 7:37 [Qemu-devel] [PATCH 3/3] stop the periodic RTC update timer Zhang, Yang Z
2012-01-06 17:26 ` Jan Kiszka
2012-01-09 7:10 ` Zhang, Yang Z
2012-01-09 8:19 ` Paolo Bonzini
2012-01-10 6:37 ` Zhang, Yang Z
2012-01-10 9:24 ` Paolo Bonzini
2012-01-11 0:56 ` Zhang, Yang Z
2012-01-11 7:24 ` Paolo Bonzini
2012-01-12 0:51 ` Zhang, Yang Z
2012-01-11 7:25 ` Philipp Hahn [this message]
2012-01-11 13:10 ` Marcelo Tosatti
2012-01-12 0:00 ` Zhang, Yang Z
2012-01-12 9:26 ` Paolo Bonzini
2012-01-12 10:20 ` Marcelo Tosatti
2012-01-12 10:43 ` Paolo Bonzini
2012-01-12 9:59 ` Marcelo Tosatti
2012-01-12 10:03 ` Marcelo Tosatti
2012-01-12 10:12 ` Zhang, Yang Z
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