tcg: Note requirement on atomic direct jump patching

Signed-off-by: NSergey Fedorov <serge.fdrv@gmail.com>
Signed-off-by: NSergey Fedorov <sergey.fedorov@linaro.org>
Reviewed-by: NAlex Bennée <alex.bennee@linaro.org>
Message-Id: <1461341333-19646-12-git-send-email-sergey.fedorov@linaro.org>
Signed-off-by: NRichard Henderson <rth@twiddle.net>

include/exec/exec-all.h

@@ -230,6 +230,7 @@ static inline void tlb_flush_by_mmuidx(CPUState *cpu, ...)
     || defined(__sparc__) || defined(__aarch64__) \
     || defined(__s390x__) || defined(__mips__) \
     || defined(CONFIG_TCG_INTERPRETER)
+/* NOTE: Direct jump patching must be atomic to be thread-safe. */
 #define USE_DIRECT_JUMP
 #endif