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

On Tue, Jun 17, 2014 at 04:12:32PM +0100, Jan Beulich wrote:
> >>> 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?

You can swap between legacy and EFI in the BIOS. Customers can choose
either option.

> > 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.

That is probably the best option. The DMI is going to get large
as it would probably include the G8 family.
> Jan
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