Re: [PATCH v3 for-4.21 1/9] x86/HPET: deal with unused channels

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Thu, Oct 23, 2025 at 05:49:57PM +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. However, on all Intel hardware I looked at
> closely, a 0->1 transition of the enable bit causes an immediate IRQ.
> Hence disabling channels isn't a good idea there. Set a "long" timeout
> instead.
> 
> Along with that 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 doesn't need to happen earlier than from
> set_channel_irq_affinity() (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@xxxxxxxx>
> Release-Acked-by: Oleksii Kurochko<oleksii.kurochko@xxxxxxxxx>

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

Thanks, Roger.