target-arm/helper.c: Don't flush the TLB if SCTLR is rewritten unchanged

Linux makes a habit of writing the same value to the SCTLR that it
already holds. In a sample boot of the kernel to a shell prompt
it wrote the SCTLR with the value it already held 325465 times,
and wrote different values just 3 times.

Skip flushing the TLB if the SCTLR value isn't actually being changed;
this speeds up my sample boot by 3-5%.
Reported-by: NLaurent Desnogues <laurent.desnogues@gmail.com>
Reviewed-by: NLaurent Desnogues <laurent.desnogues@gmail.com>
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>
Message-id: 1399560029-19007-1-git-send-email-peter.maydell@linaro.org

target-arm/helper.c

@@ -2081,6 +2081,13 @@ static void sctlr_write(CPUARMState *env, const ARMCPRegInfo *ri,
 {
     ARMCPU *cpu = arm_env_get_cpu(env);
 
+    if (env->cp15.c1_sys == value) {
+        /* Skip the TLB flush if nothing actually changed; Linux likes
+         * to do a lot of pointless SCTLR writes.
+         */
+        return;
+    }
+
     env->cp15.c1_sys = value;
     /* ??? Lots of these bits are not implemented.  */
     /* This may enable/disable the MMU, so do a TLB flush.  */