xen/arm64: Document the register mapping aarch64 <-> aarch32

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>
Acked-by: Ian Campbell <ian.campbell@citrix.com>

diff --git a/xen/include/asm-arm/arm64/processor.h b/xen/include/asm-arm/arm64/processor.h
index 3a9c0cde0ffd1c86c1094828582caf21c9d811dd..fef35a5aef0248e8dfbf1a9a8dc82ec855887a3e 100644 (file)
--- 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*/);