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

[Jan Beulich <jbeulich@suse.com>]

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.

Jan