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

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 17/06/14 16:12, Jan Beulich wrote:
>>>> On 17.06.14 at 16:58, <andrew.cooper3@citrix.com> 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,

Indeed you can

>  which is
> precisely what the panic message says you ought to do. Why do
> you not boot via EFI in the first place?

Inertia mostly.  The current state of play of needing experimental
patches against grub2 and pvops is suboptimal, and XenServer management
decided that our efforts were better spent elsewhere.

Fundamentally though, that patch caused a boot regression on HP DL580
Gen 8 systems, and an unknown number of other systems.

>
>> 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,

I have no indication on the extent of the issue, but it could be as much
as all HP Gen8 and 9 systems.  I suspect this option wouldn't scale.

> - code probing for a functional CMOS RTC gating the panic()
>   invocation.

If the firmware claims no RTC, is it even safe to probe?  There is
nothing I am aware of preventing the IO port space being reused by
something else if the firmware has told the OS that there genuinely
isn't an RTC there.

~Andrew