Re: [Xen-devel] [PATCH 3/5] RTC: Add UIP(update in progress) check logic

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Tue, 2012-03-06 at 03:45 -0500, Jan Beulich wrote:
> >>> On 06.03.12 at 01:26, "Zhang, Yang Z" <yang.z.zhang@xxxxxxxxx> wrote:
> > Actually, in the draft patch, I also think it is unnecessary. And I don't 
> > find any real use mode to use RTC in this way.
> > But someone points out that the RTC datasheet allows us to check the update 
> > cycle by UIP. So we need to support it.
> 
> Doesn't Xen itself make use of this (and would hence be affected [non-
> fatally] when running nested)? See xen/arch/x86/time.c:get_cmos_time().

Yes, looks like it. I imagine this is a pretty common idiom for reading
the RTC (why else would Xen do it that way...) so I think it is fair to
assume that other guest OSes also rely on the UIP bit.

Ian.



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