[RFC PATCH v2 11/19] genirq: Document vcpu_info usage for per-CPU interrupts

[cdall@linaro.org (Christoffer Dall)]

It is currently unclear how to set the VCPU affinity for an interrupt
which is of the per-CPU kind, since the Linux irq_data structure
describes the state for multiple interrupts when the interrupt is a
per-CPU interrupt, 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
per-CPU 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 per-CPU 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 per-CPU (PPI) interrupt.

Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Christoffer Dall <cdall@linaro.org>
---
 kernel/irq/manage.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/kernel/irq/manage.c b/kernel/irq/manage.c
index 5624b2d..050b9f6 100644
--- a/kernel/irq/manage.c
+++ b/kernel/irq/manage.c
@@ -381,7 +381,8 @@ int irq_select_affinity_usr(unsigned int irq)
 /**
  *	irq_set_vcpu_affinity - Set vcpu affinity for the interrupt
  *	@irq: interrupt number to set affinity
- *	@vcpu_info: vCPU specific data
+ *	@vcpu_info: vCPU specific data or pointer to a percpu array of vCPU
+ *	            specific data for per-CPU interrupts
  *
  *	This function uses the vCPU specific data to set the vCPU
  *	affinity for an irq. The vCPU specific data is passed from
-- 
2.9.0