[tip:irq/core] genirq: Document vcpu_info usage for percpu_devid interrupts

[tip-bot for Christoffer Dall <tipbot@zytor.com>]

Commit-ID:  250a53d6fcd86012935d1cf71eb2e3d6e88c412c
Gitweb:     https://git.kernel.org/tip/250a53d6fcd86012935d1cf71eb2e3d6e88c412c
Author:     Christoffer Dall <cdall@linaro.org>
AuthorDate: Fri, 27 Oct 2017 10:34:33 +0200
Committer:  Thomas Gleixner <tglx@linutronix.de>
CommitDate: Sun, 29 Oct 2017 16:18:37 +0100

genirq: Document vcpu_info usage for percpu_devid interrupts

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.

Signed-off-by: Christoffer Dall <cdall@linaro.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Marc Zyngier <marc.zyngier@arm.com>
Cc: kvm@vger.kernel.org
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will.deacon@arm.com>
Cc: Eric Auger <eric.auger@redhat.com>
Cc: kvmarm@lists.cs.columbia.edu
Cc: linux-arm-kernel@lists.infradead.org
Link: https://lkml.kernel.org/r/1509093281-15225-13-git-send-email-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 e667912..c65f282 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