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

[Jürgen Groß]

On 11.12.20 11:13, Thomas Gleixner wrote:
> On Fri, Dec 11 2020 at 07:17, Jürgen Groß wrote:
>> On 11.12.20 00:20, boris.ostrovsky at oracle.com 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.
>>
>> This binding to cpu0 was introduced with commit 97253eeeb792d61ed2
>> and I have no reason to believe the underlying problem has been
>> eliminated.
> 
>      "The kernel-side VCPU binding was not being correctly set for newly
>       allocated or bound interdomain events.  In ARM guests where 2-level
>       events were used, this would result in no interdomain events being
>       handled because the kernel-side VCPU masks would all be clear.
> 
>       x86 guests would work because the irq affinity was set during irq
>       setup and this would set the correct kernel-side VCPU binding."
> 
> I'm not convinced that this is really correctly analyzed because affinity
> setting is done at irq startup.
> 
>                  switch (__irq_startup_managed(desc, aff, force)) {
> 	        case IRQ_STARTUP_NORMAL:
> 	                ret = __irq_startup(desc);
>                          irq_setup_affinity(desc);
> 			break;
> 
> which is completely architecture agnostic. So why should this magically
> work on x86 and not on ARM if both are using the same XEN irqchip with
> the same irqchip callbacks.

I think this might be related to _initial_ cpu binding of events and
changing the binding later. This might be handled differently in the
hypervisor.


Juergen
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