[PATCH v3 12/20] genirq: Document vcpu_info usage for percpu_devid interrupts

[Christoffer Dall]

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.

Cc: Thomas Gleixner <tglx at linutronix.de>
Cc: Marc Zyngier <marc.zyngier at arm.com>
Signed-off-by: Christoffer Dall <cdall at 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 573dc52..2b2c94f 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 percpu_devid 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