[PATCH 2/4] xen/arm64: Document the register mapping aarch64 <-> aarch32

[Julien Grall <julien.grall@citrix.com>]

The mapping between aarch64 and aarch32 has not been chosen in random.
It's based on D1.20.1 in ARM DDI 0487A.d.

The section is not obvious to find in the spec, so make it clear for the
anyone else.

Signed-off-by: Julien Grall <julien.grall@citrix.com>
---
 xen/include/asm-arm/arm64/processor.h | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/xen/include/asm-arm/arm64/processor.h b/xen/include/asm-arm/arm64/processor.h
index 3a9c0cd..fef35a5 100644
--- a/xen/include/asm-arm/arm64/processor.h
+++ b/xen/include/asm-arm/arm64/processor.h
@@ -15,7 +15,12 @@
 /* On stack VCPU state */
 struct cpu_user_regs
 {
-    /*         Aarch64       Aarch32 */
+    /*
+     * The mapping AArch64 <-> AArch32 is based on D1.20.1 in ARM DDI
+     * 0487A.d.
+     *
+     *         AArch64       AArch32
+     */
     __DECL_REG(x0,           r0/*_usr*/);
     __DECL_REG(x1,           r1/*_usr*/);
     __DECL_REG(x2,           r2/*_usr*/);
-- 
2.1.4