[PATCH v4 12/20] genirq: Document vcpu_info usage for percpu_devid interrupts

[Thomas Gleixner]

On Fri, 20 Oct 2017, Christoffer Dall wrote:

Please Cc lkml when changing genirq core code.

> It is currently unclear how to set the VCPU affinity for a percpu_devid
> interrupt , since the Linux irq_data structure describes the state for
> multiple interrupts, one for each physical CPU on the system.  Since
> each such interrupt can be associated with different VCPUs or none at
> all, associating a single VCPU state with such an interrupt does not
> capture the necessary semantics.
> 
> The implementers of irq_set_affinity are the Intel and AMD IOMMUs, and
> the ARM GIC irqchip.  The Intel and AMD callers do not appear to use
> percpu_devid interrupts, and the ARM GIC implementation only checks the
> pointer against NULL vs. non-NULL.
> 
> Therefore, simply update the function documentation to explain the
> expected use in the context of percpu_devid interrupts, allowing future
> changes or additions to irqchip implementers to do the right thing.
> 
> This allows us to set the VCPU affinity for the virtual timer interrupt
> in KVM/ARM, which is a percpu_devid (PPI) interrupt.

I have a hard time to understand how adding the comment allows you to set
VCPU affinity ....

Thanks,

	tglx