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

[Thomas Gleixner]

On Sat, 21 Oct 2017, Christoffer Dall wrote:

> On Fri, Oct 20, 2017 at 01:56:16PM +0200, Thomas Gleixner wrote:
> > On Fri, 20 Oct 2017, Christoffer Dall wrote:
> > 
> > Please Cc lkml when changing genirq core code.
> > 
> 
> ok, will do so in the future.
> 
> > > 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 ....
> > 
> 
> Obviously everything already works without adding the comment, however
> it is not entirely clear how to use the vcpu_into for percpu_devid
> interrupts, and this patch aims to make that clear, as it is the first
> user of irq_set_vcpu_affinity for a percpu_devid interrupt which
> actually uses the vcpu_info pointer.
> 
> Would you rather that we did not update the documentation or what do you
> suggest?

Documentation is welcome of course. I just want a changelog which is not
misleading. i.e. it says:

   Update the function documentation .....

   This allow us to set ....

Which reads like: Updating the documentation allows us to set ...

See?

Thanks,

	tglx