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

[Simon Gaiser <simon@xxxxxxxxxxxxxxxxxxxxxx>]

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).

Simon

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