Re: [Xen-devel] [PATCH] x86: Don't honour ACPI indicating absence of CMOS RTC

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 17.06.14 at 16:58, <andrew.cooper3@xxxxxxxxxx> wrote:
> This reverts f74556693 "x86: honor ACPI indicating absence of CMOS RTC"
> 
> Certain HP Gen8 BIOSes have started setting this bit despite an RTC CMOS 
> being present and working.
> 
> Their reasonsing is to prevent EFI-booted OSes from playing with the CMOS,
> combined with the erroneous assumption that the only OSes using legacy boot
> are too old to know about ACPI v5 and therefore to understand this bit.

Which implies you can boot from EFI on those systems, which is
precisely what the panic message says you ought to do. Why do
you not boot via EFI in the first place?

> As a result, the change being reverted prevented modern Xen from booting on
> modern HP hardware, despite older Xen working perfectly fine on the same
> hardware.

Still, the flag has a purpose, and HP assigning a slightly non-standard
meaning to it doesn't mean we should outright revert that change.
Options I could live with (short of them revisiting the bogus assumption)
are
- a command line option suppressing the mandated behavior,
- a DMI quirk suppressing the mandated behavior,
- code probing for a functional CMOS RTC gating the panic()
  invocation.

Jan


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