xen/arm: Introduce CONFIG_PARTIAL_EMULATION and "partial-emulation" cmd option

There can be situations when the registers cannot be emulated to their full
functionality. This can be due to the complexity involved. In such cases, one
can emulate those registers as RAZ/WI for example. We call them as partial
emulation.

Some registers are non-optional and as such there is nothing preventing an OS
from accessing them.
Instead of injecting undefined exception (which might result in crashing a
guest), one may want to prefer a partial emulation to let the guest running
(in some cases accepting the fact that it might result in unwanted behavior).

A suitable example of this (as seen in subsequent patches) is emulation of
DBGDTRTX_EL0 (on Arm64) and DBGDTRTXINT(on Arm32). These non-optional
registers can be emulated as RAZ/WI and they can be enclosed within
CONFIG_PARTIAL_EMULATION.

Further, "partial-emulation" command line option allows us to
enable/disable partial emulation at run time. While CONFIG_PARTIAL_EMULATION
enables support for partial emulation at compile time (i.e. adds code for
partial emulation), this option may be enabled or disabled by Yocto or other
build systems. However if the build system turns this option on, users
can use scripts like Imagebuilder to generate uboot-script which will append
"partial-emulation=true" to xen command line to turn on the partial emulation.
Thus, it helps to avoid rebuilding xen.

By default, "CONFIG_PARTIAL_EMULATION=y" and "partial-emulation=false".
This is done so that Xen supports partial emulation. However, customers are
fully aware when they enable partial emulation. It's important to note that
enabling such support might result in unwanted/non-spec compliant behavior.

Added a note in SUPPORT.md to clarify the security support for partial
emulation.

Signed-off-by: Ayan Kumar Halder <ayan.kumar.halder@amd.com>
Signed-off-by: Michal Orzel <michal.orzel@amd.com>
Acked-by: Julien Grall <jgrall@amazon.com>

SUPPORT.md

@@ -102,6 +102,15 @@ Extension to the GICv3 interrupt controller to support MSI.
 
     Status: Experimental
 
+### ARM/Partial Emulation
+
+Enable partial emulation of registers, otherwise considered unimplemented,
+that would normally trigger a fault injection.
+
+    Status: Supported, with caveats
+
+Only the following system registers are security supported:
+
 ### ARM Scalable Vector Extension (SVE/SVE2)
 
 Arm64 domains can use Scalable Vector Extension (SVE/SVE2).