KVM: Introduce direct MSI message injection for in-kernel irqchips

Currently, MSI messages can only be injected to in-kernel irqchips by
defining a corresponding IRQ route for each message. This is not only
unhandy if the MSI messages are generated "on the fly" by user space,
IRQ routes are a limited resource that user space has to manage
carefully.

By providing a direct injection path, we can both avoid using up limited
resources and simplify the necessary steps for user land.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Avi Kivity <avi@redhat.com>
diff --git a/arch/x86/kvm/Kconfig b/arch/x86/kvm/Kconfig
index 1a7fe86..a28f338 100644
--- a/arch/x86/kvm/Kconfig
+++ b/arch/x86/kvm/Kconfig
@@ -36,6 +36,7 @@
 	select TASKSTATS
 	select TASK_DELAY_ACCT
 	select PERF_EVENTS
+	select HAVE_KVM_MSI
 	---help---
 	  Support hosting fully virtualized guest machines using hardware
 	  virtualization extensions.  You will need a fairly recent