include/exec: Introduce TARGET_PAGE_ENTRY_EXTRA

Allow the target to cache items from the guest page tables.

Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

include/exec/cpu-defs.h

@@ -163,6 +163,15 @@ typedef struct CPUTLBEntryFull {
 
     /* @lg_page_size contains the log2 of the page size. */
     uint8_t lg_page_size;
+
+    /*
+     * Allow target-specific additions to this structure.
+     * This may be used to cache items from the guest cpu
+     * page tables for later use by the implementation.
+     */
+#ifdef TARGET_PAGE_ENTRY_EXTRA
+    TARGET_PAGE_ENTRY_EXTRA
+#endif
 } CPUTLBEntryFull;
 
 /*