cpu-defs.h: Document CPUIOTLBEntry 'addr' field

The 'addr' field in the CPUIOTLBEntry struct has a rather non-obvious
use; add a comment documenting it (reverse-engineered from what
the code that sets it is doing).

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20180611125633.32755-2-peter.maydell@linaro.org

include/exec/cpu-defs.h

@@ -127,6 +127,15 @@ QEMU_BUILD_BUG_ON(sizeof(CPUTLBEntry) != (1 << CPU_TLB_ENTRY_BITS));
  * structs into one.)
  */
 typedef struct CPUIOTLBEntry {
+    /*
+     * @addr contains:
+     *  - in the lower TARGET_PAGE_BITS, a physical section number
+     *  - with the lower TARGET_PAGE_BITS masked off, an offset which
+     *    must be added to the virtual address to obtain:
+     *     + the ram_addr_t of the target RAM (if the physical section
+     *       number is PHYS_SECTION_NOTDIRTY or PHYS_SECTION_ROM)
+     *     + the offset within the target MemoryRegion (otherwise)
+     */
     hwaddr addr;
     MemTxAttrs attrs;
 } CPUIOTLBEntry;