xen/arm: mpu: Introduce choice between MMU and MPU

There are features in the forthcoming patches which are dependent on
MPU. For eg fixed start address.
Also, some of the Xen features (eg STATIC_MEMORY) will be selected
by the MPU configuration.

Thus, this patch introduces a choice between MMU and MPU for the type
of memory management system. By default, MMU is selected.
MPU is now gated by UNSUPPORTED.

Update SUPPORT.md to state that the support for MPU is experimental.
Also updated CHANGELOG.md as well.

Signed-off-by: Ayan Kumar Halder <ayan.kumar.halder@amd.com>
Reviewed-by: Luca Fancellu <luca.fancellu@arm.com>
Acked-by: Julien Grall <jgrall@amazon.com>
Acked-by: Oleksii Kurochko <oleksii.kurochko@gmail.com>

CHANGELOG.md

@@ -12,6 +12,8 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/)
    - Prefer ACPI reboot over UEFI ResetSystem() run time service call.
 
 ### Added
+ - On Arm:
+   - Experimental support for Armv8-R.
 
 ### Removed
  - On x86:

SUPPORT.md

@@ -40,6 +40,7 @@ supported in this document.
 
     Status, Xen in AArch64 mode: Supported
     Status, Xen in AArch32 mode: Tech Preview
+    Status, Xen in Armv8-R: Experimental
     Status, Cortex A57 r0p0-r1p1: Supported, not security supported
     Status, Cortex A77 r0p0-r1p0: Supported, not security supported
 

xen/arch/arm/Kconfig

@@ -58,10 +58,25 @@ config PADDR_BITS
 	default 40 if ARM_PA_BITS_40
 	default 48 if ARM_64
 
+choice
+	prompt "Memory management system"
+	default MMU
+	help
+	  User can choose between the different forms of memory management system.
+
 config MMU
-	def_bool y
+	bool "MMU"
 	select HAS_PMAP
 	select HAS_VMAP
+	help
+	  Select it if you plan to run Xen on A-profile Armv7+
+
+config MPU
+	bool "MPU" if UNSUPPORTED
+	help
+	  Memory Protection Unit (MPU). Select if you plan to run Xen on ARMv8-R
+	  systems supporting EL2. (UNSUPPORTED)
+endchoice
 
 source "arch/Kconfig"
 

xen/arch/arm/platforms/Kconfig

@@ -1,5 +1,5 @@
 choice
-	prompt "Platform Support"
+	prompt "Platform Support" if MMU
 	default ALL_PLAT
 	help
 	Choose which hardware platform to enable in Xen.