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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> 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.

> 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.

> Thirdly, since get_cmos_time() can be called more than once, I think
> that just leaving cmos_rtc_probe == 0 could make us panic on the second
> invocation.

Very good point, this definitely needs fixing. I guess I'll just amend
the condition with a system_state check.

Jan


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