Re: [XEN PATCH] x86/hpet: Disable legacy replacement mode after IRQ test if not needed

[Jan Beulich <jbeulich@xxxxxxxx>]

On 24.07.2023 11:48, Simon Gaiser wrote:
> Jan Beulich:
>> On 18.07.2023 23:51, Simon Gaiser wrote:
>>> Roger Pau Monné:
>>>> On Tue, Jul 18, 2023 at 02:26:03PM +0200, Simon Gaiser wrote:
>>>>> As far as I understand the HPET legacy mode is not required on systems
>>>>> with ARAT after the timer IRQ test.
>>>>
>>>> What's the relation with ARAT here?
>>>>
>>>> It would seem to me that keeping legacy replacement enabled should
>>>> only be done when opt_hpet_legacy_replacement > 0, and the currently
>>>> modified block is already in a opt_hpet_legacy_replacement < 0 gated
>>>> chunk.
>>>
>>> I was concerned that on systems without ARAT cpuidle might rely on HPET
>>> legacy mode being available. See _disable_pit_irq and lapic_timer_init.
>>> But now that I stared at this again, I think that condition isn't
>>> actually needed. If we reach that code we know that we have no working
>>> PIT, but HPET is working. So _disable_pit_irq which is run after
>>> check_timer (__start_xen first calls check_timer via smp_prepare_cpus
>>> and only later disable_pit_irq via do_initcalls) will setup HPET
>>> broadcast, which should succeed since HPET worked previously.
>>>
>>> So I guess we can just drop the condition (please double check, that
>>> code is quite tangled and I'm not familiar with it).
>>
>> What you want to respect instead though is opt_hpet_legacy_replacement.
> 
> Can you please explain what behavior you expect? As Roger pointed out
> this code only runs with opt_hpet_legacy_replacement < 0 so the user
> didn't make an explicit choice.

Oh, I'm sorry. Please disregard my comment then.

Jan