x86: Add support for Linux guests on an ACRN hypervisor

ACRN is an open-source hypervisor maintained by The Linux Foundation. It
is built for embedded IOT with small footprint and real-time features.
Add ACRN guest support so that it allows Linux to be booted under the
ACRN hypervisor. This adds only the barebones implementation.

 [ bp: Massage commit message and help text. ]

Co-developed-by: Jason Chen CJ <jason.cj.chen@intel.com>
Signed-off-by: Jason Chen CJ <jason.cj.chen@intel.com>
Signed-off-by: Zhao Yakui <yakui.zhao@intel.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: x86-ml <x86@kernel.org>
Link: https://lkml.kernel.org/r/1559108037-18813-3-git-send-email-yakui.zhao@intel.com
diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index c9ab090..8a95c50 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -835,6 +835,16 @@
 	  cell. You can leave this option disabled if you only want to start
 	  Jailhouse and run Linux afterwards in the root cell.
 
+config ACRN_GUEST
+	bool "ACRN Guest support"
+	depends on X86_64
+	help
+	  This option allows to run Linux as guest in the ACRN hypervisor. ACRN is
+	  a flexible, lightweight reference open-source hypervisor, built with
+	  real-time and safety-criticality in mind. It is built for embedded
+	  IOT with small footprint and real-time features. More details can be
+	  found in https://projectacrn.org/.
+
 endif #HYPERVISOR_GUEST
 
 source "arch/x86/Kconfig.cpu"