xen/arm: introduce guest_handle_for_field()

ARM port of c/s bb54458: "introduce guest_handle_for_field()"

This helper turns a field of a GUEST_HANDLE into a GUEST_HANDLE.

Signed-off-by: Christopher Clark <[email protected]>
Reviewed-by: Paul Durrant <[email protected]>
Reviewed-by: Stefano Stabellini <[email protected]>
===
v3: Added Stefano's Reviewed-by
v2: Added Paul's Reviewed-by

Author: dozylynx

xen/include/asm-arm/guest_access.h

@@ -63,6 +63,9 @@ int access_guest_memory_by_ipa(struct domain *d, paddr_t ipa, void *buf,
     _y;                                                     \
 })
 
+#define guest_handle_for_field(hnd, type, fld)          \
+    ((XEN_GUEST_HANDLE(type)) { &(hnd).p->fld })
+
 #define guest_handle_from_ptr(ptr, type)        \
     ((XEN_GUEST_HANDLE_PARAM(type)) { (type *)ptr })
 #define const_guest_handle_from_ptr(ptr, type)  \