Re: [PATCH] hvm/hpet: Correctly gate the virtual HPET on HVM_PARAM_HPET_ENABLE

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

On 15/01/15 15:34, Jan Beulich wrote:
>>>> On 15.01.15 at 15:40, <andrew.cooper3@citrix.com> wrote:
>> c/s 3f8e22de7 "x86 hvm: Allow HPET to be configured as a per-domain config
>> option" introduced the parameter to conditionally enable the HPET.
>>
>> However, having the check in hpet_range() does not have the intended effect.
>> As currently implemented, when the HPET is disabled, the range is not 
>> claimed
>> and an ioreq is forwarded to qemu, which implements an HPET itself.
>>
>> Properly disable the HPET by always claiming the range, dropping writes and
>> reading ~0.
> Hmm, while the patch certainly does what you describe above, is that
> really correct? There could be something else at that address when
> the HPET is disabled. Therefore I would rather think that qemu should
> be told to also not make a HPET available when the option is off.

Without out wiring up northbridge chipset reigsters from Qemu to Xen to
control where components such as the HPET appear, I think we can
reasonably assume that there will be nothing there.

Whether the HPET is present or not, the range ought to reserved in the
E820 and not free for arbitrary reuse by the OS.

~Andrew