Re: [Xen-devel] [PATCH RFC] x86/ACPI: allow CMOS RTC use even when ACPI says there is none

[Tim Deegan <tim@xxxxxxx>]

At 12:41 +0100 on 26 Jun (1403782897), Jan Beulich wrote:
> >>> On 26.06.14 at 13:12, <tim@xxxxxxx> wrote:
> > At 11:28 +0100 on 26 Jun (1403778496), Jan Beulich wrote:
> >> HP is setting the ACPI_FADT_NO_CMOS_RTC flag on newer systems,
> >> regardless of whether they're being booted from UEFI. Add a command
> >> line option to allow probing for a working RTC in that case.
> > 
> > I like this general direction.  I would add a printk in the case where
> > we are probing, since the probe will stall boot for >= 1 sec.
> 
> Right now we stall for up to 1 sec already; since the probing
> doesn't happen without being asked for, I don't view it as a
> problem that now we stall for up to 2 sec.

Ack.

> > Maybe also worth checking that the times returned on the two passes
> > actually make sense (i.e. that the difference in RTC times matches the
> > difference in NOW() to within +/- 10ms).
> 
> I had considered this too, but intentionally dropped the idea, so that
> odd behavior when running in nested virtualization scenarios can be
> avoided.

I guess so.  We'd have to be running inside a hypervisor that has the
same buggy tables as these HP machines, but OTOH anything that passes
the tests you already have looks quite a lot like an RTC. :)

Tim.

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