Subject: Re: [patch 27/30] xen/events: Only force affinity mask for percpu interrupts

From: Thomas Gleixner <[email-masked]>
Date: Fri, 11 Dec 2020 13:37:10 +0100

On Fri, Dec 11 2020 at 13:10, Jürgen Groß wrote:
> On 11.12.20 00:20, [email-masked] wrote:
>> 
>> On 12/10/20 2:26 PM, Thomas Gleixner wrote:
>>> All event channel setups bind the interrupt on CPU0 or the target CPU for
>>> percpu interrupts and overwrite the affinity mask with the corresponding
>>> cpumask. That does not make sense.
>>>
>>> The XEN implementation of irqchip::irq_set_affinity() already picks a
>>> single target CPU out of the affinity mask and the actual target is stored
>>> in the effective CPU mask, so destroying the user chosen affinity mask
>>> which might contain more than one CPU is wrong.
>>>
>>> Change the implementation so that the channel is bound to CPU0 at the XEN
>>> level and leave the affinity mask alone. At startup of the interrupt
>>> affinity will be assigned out of the affinity mask and the XEN binding will
>>> be updated.
>> 
>> 
>> If that's the case then I wonder whether we need this call at all and instead bind at startup time.
>
> After some discussion with Thomas on IRC and xen-devel archaeology the
> result is: this will be needed especially for systems running on a
> single vcpu (e.g. small guests), as the .irq_set_affinity() callback
> won't be called in this case when starting the irq.

That's right, but not limited to ARM. The same problem exists on x86 UP.
So yes, the call makes sense, but the changelog is not really useful.
Let me add a comment to this.

Thanks,

        tglx