argo: Introduce the Kconfig option to govern inclusion of Argo

Defines CONFIG_ARGO when enabled. Default: disabled.

When the Kconfig option is enabled, the Argo hypercall implementation
will be included, allowing use of the hypervisor-mediated interdomain
communication mechanism.

Argo is implemented for x86 and ARM hardware platforms.

Availability of the option depends on EXPERT and Argo is currently an
experimental feature.

Signed-off-by: Christopher Clark <[email protected]>
Acked-by: Jan Beulich <[email protected]>

===
Jan's ack only stands for committing this together with at least one patch
actually using the CONFIG_ARGO symbol.

v3 added Jan's Ack
v2 #01 feedback, Jan: replace def_bool/prompt with bool
v1 #02 feedback, Jan: default Kconfig off, use EXPERT, fix whitespace

xen/common/Kconfig

@@ -200,6 +200,25 @@ config LATE_HWDOM
 
 	  If unsure, say N.
 
+config ARGO
+	bool "Argo: hypervisor-mediated interdomain communication" if EXPERT = "y"
+	---help---
+	  Enables a hypercall for domains to ask the hypervisor to perform
+	  data transfer of messages between domains.
+
+	  This allows communication channels to be established that do not
+	  require any shared memory between domains; the hypervisor is the
+	  entity that each domain interacts with. The hypervisor is able to
+	  enforce Mandatory Access Control policy over the communication.
+
+	  If XSM_FLASK is enabled, XSM policy can govern which domains may
+	  communicate via the Argo system.
+
+	  This feature does nothing if the "argo" boot parameter is not present.
+	  Argo is disabled at runtime by default.
+
+	  If unsure, say N.
+
 menu "Schedulers"
 	visible if EXPERT = "y"