Re: [PATCH for-4.21 02/10] x86/HPET: disable unused channels

["Roger Pau Monné" <roger.pau@citrix.com>]

On Thu, Oct 16, 2025 at 09:31:42AM +0200, Jan Beulich wrote:
> Keeping channels enabled when they're unused is only causing problems:
> Extra interrupts harm performance, and extra nested interrupts could even
> have caused worse problems.
> 
> Note that no explicit "enable" is necessary - that's implicitly done by
> set_channel_irq_affinity() once the channel goes into use again.
> 
> Along with disabling the counter, also "clear" the channel's "next event",
> for it to be properly written by whatever the next user is going to want
> (possibly avoiding too early an IRQ).
> 
> Further, along the same lines, don't enable channels early when starting
> up an IRQ. This similarly should happen no earlier than from
> set_channel_irq_affinity() (here: once a channel goes into use the very
> first time). This eliminates a single instance of
> 
> (XEN) [VT-D]INTR-REMAP: Request device [0000:00:1f.0] fault index 0
> (XEN) [VT-D]INTR-REMAP: reason 25 - Blocked a compatibility format interrupt request
> 
> during boot. (Why exactly there's only one instance, when we use multiple
> counters and hence multiple IRQs, I can't tell. My understanding would be
> that this was due to __hpet_setup_msi_irq() being called only after
> request_irq() [and hence the .startup handler], yet that should have
> affected all channels.)
> 
> Fixes: 3ba523ff957c ("CPUIDLE: enable MSI capable HPET for timer broadcast")
> Signed-off-by: Jan Beulich <jbeulich@suse.com>

Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>

> ---
> A window still remains for IRQs to be caused by stale comparator values:
> hpet_attach_channel() is called ahead of reprogram_hpet_evt_channel().
> Should we also write the comparator to "far into the future"?

I think we can possibly live with this to avoid doing an extra MMIO
access?

Thanks, Roger.