Re: [PATCH v3] drivers: hv: vmbus: Use kthread for vmbus interrupts on PREEMPT_RT

[Jan Kiszka <jan.kiszka@siemens.com>]

On 18.03.26 10:08, Sebastian Andrzej Siewior wrote:
> On 2026-03-17 12:55:15 [+0100], Jan Kiszka wrote:
>> Point is that a task that was interrupted by a potentially threaded
>> interrupt keeps this flag longer that it needs it. And that is
>> apparently harmless, but fairly confusing.
> 
> correct. My only concern would be a shared handler where the second is
> not threaded.

The vmbus irqs are not shared (beyond what sysvec_hyperv_callback does).

> 
>>>> With that in mind, the new logic here is no different from the one the
>>>> kernel used before. If both are not doing what they should, we likely
>>>> want to add a generic reset of hardirq_threaded to the IRQ exit path(s).
>>>
>>> The difference is that you expect that _everyone_ calling this driver
>>> has everything else threaded. This might not be the case. That is why
>>> this should be in core knowing what is called if threaded, use in driver
>>> after explicit killing that flag afterwards since you don't know what
>>> can follow or add a generic threaded infrastructure here. 
>>
>> This driver is different, unfortunately. I'm not sure if we can / want
>> to thread everything that the platform interrupt does on x86. So far,
>> only the last part of it - vmbus handling - is threaded. On arm64, the
>> irq is exclusive (see vmbus_percpu_isr), thus everything can be and is
>> threaded.
> 
> No, it is a percpu interrupt which are not forced-threaded.

It is threaded now due to my patch.

> 
>>>>> Couldn't the whole logic be integrated into the IRQ code? Then we could
>>>>> have mask/ unmask if supported/ provided and threaded interrupts. Then
>>>>> sysvec_hyperv_reenlightenment() could use a proper threaded interrupt
>>>>> instead apic_eoi() + schedule_delayed_work(). 
>>>>>
>>>>
>>>> Again, you are thinking x86-only. We need a portable solution.
>>>
>>> well, ARM could use a threaded interrupt, too.
>>
>> For a reason we didn't explore in details, per-CPU interrupts aren't
>> threaded. See older version of this patch
>> (https://lore.kernel.org/lkml/005a01dc9d30$a40515e0$ec0f41a0$@zohomail.com/)
>> where I thought I only had to fix x86, but arm64 was needing care as well.
> 
> Per-CPU are usually timers or other things which are not threaded and
> have their own thing for the "second" port and I only remember MCE using
> a workqueue for notification.

And the hv vmbus now provides a case where threading could be useful, at
least for arm64. For x86, we would have to check if the first half of
sysvec_hyperv_callback (mshv_handler) wants threading as well / would
support that.

Jan

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