KVM: PPC: Document KVM_INTERRUPT ioctl

This adds some documentation for the KVM_INTERRUPT special cases that
PowerPC now implements.
Signed-off-by: Alexander Graf <agraf@suse.de>

Documentation/kvm/api.txt

@@ -320,13 +320,13 @@ struct kvm_translation {
 4.15 KVM_INTERRUPT
 
 Capability: basic
-Architectures: x86
+Architectures: x86, ppc
 Type: vcpu ioctl
 Parameters: struct kvm_interrupt (in)
 Returns: 0 on success, -1 on error
 
 Queues a hardware interrupt vector to be injected.  This is only
-useful if in-kernel local APIC is not used.
+useful if in-kernel local APIC or equivalent is not used.
 
 /* for KVM_INTERRUPT */
 struct kvm_interrupt {
@@ -334,8 +334,37 @@ struct kvm_interrupt {
 	__u32 irq;
 };
 
+X86:
+
 Note 'irq' is an interrupt vector, not an interrupt pin or line.
 
+PPC:
+
+Queues an external interrupt to be injected. This ioctl is overleaded
+with 3 different irq values:
+
+a) KVM_INTERRUPT_SET
+
+  This injects an edge type external interrupt into the guest once it's ready
+  to receive interrupts. When injected, the interrupt is done.
+
+b) KVM_INTERRUPT_UNSET
+
+  This unsets any pending interrupt.
+
+  Only available with KVM_CAP_PPC_UNSET_IRQ.
+
+c) KVM_INTERRUPT_SET_LEVEL
+
+  This injects a level type external interrupt into the guest context. The
+  interrupt stays pending until a specific ioctl with KVM_INTERRUPT_UNSET
+  is triggered.
+
+  Only available with KVM_CAP_PPC_IRQ_LEVEL.
+
+Note that any value for 'irq' other than the ones stated above is invalid
+and incurs unexpected behavior.
+
 4.16 KVM_DEBUG_GUEST
 
 Capability: basic