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[Qemu-devel] [PATCH] target-arm/helper.c: Don't flush the TLB if SCTLR i

From: Peter Maydell
Subject: [Qemu-devel] [PATCH] target-arm/helper.c: Don't flush the TLB if SCTLR is rewritten unchanged
Date: Thu, 8 May 2014 15:40:29 +0100

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: Laurent Desnogues <address@hidden>
Signed-off-by: Peter Maydell <address@hidden>
I believe there are kernel patches in the works to avoid being
quite so profligate with SCTLR writes, but there are still a
lot of older kernels out in the world, so this is worth having IMHO.

 target-arm/helper.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/target-arm/helper.c b/target-arm/helper.c
index 3be917c..417161e 100644
--- a/target-arm/helper.c
+++ b/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.  */

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