xen: arm: comment on why 64-bit xen_pfn_t is safe even on 32 bit

#ifndef __ASSEMBLY__

/* Explicitly size integers that represent pfns in the interface with

 * Xen so that we can have one ABI that works for 32 and 64 bit guests. */

 * Xen so that we can have one ABI that works for 32 and 64 bit guests.

 * Note that this means that the xen_pfn_t type may be capable of

 * representing pfn's which the guest cannot represent in its own pfn

 * type. However since pfn space is controlled by the guest this is

 * fine since it simply wouldn't be able to create any sure pfns in

 * the first place.

 */

typedef uint64_t xen_pfn_t;

#define PRI_xen_pfn "llx"

typedef uint64_t xen_ulong_t;